A Risk Assessment Framework for Seed Degeneration: Informing an Integrated Seed Health Strategy for Vegetatively Propagated Crops

Pathogen buildup in vegetative planting material, termed seed degeneration, is a major problem in many low-income countries. When smallholder farmers use seed produced on-farm or acquired outside certified programs, it is often infected. We introduce a risk assessment framework for seed degeneration, evaluating the relative performance of individual and combined components of an integrated seed health strategy. The frequency distribution of management performance outcomes was evaluated for models incorporating biological and environmental heterogeneity, with the following results. (1) On-farm seed selection can perform as well as certified seed, if the rate of success in selecting healthy plants for seed production is high; (2) when choosing among within-season management strategies, external inoculum can determine the relative usefulness of 'incidence-altering management' (affecting the proportion of diseased plants/seeds) and 'rate-altering management' (affecting the rate of disease transmission in the field); (3) under severe disease scenarios, where it is difficult to implement management components at high levels of effectiveness, combining management components can be synergistic and keep seed degeneration below a threshold; (4) combining management components can also close the yield gap between average and worst-case scenarios. We also illustrate the potential for expert elicitation to provide parameter estimates when empirical data are unavailable. [Formula: see text] Copyright © 2017 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Establishing alighting preferences and species transmission differences for Pea seed-borne mosaic virus aphid vectors

Pea seed-borne mosaic virus (PSbMV) infection causes a serious disease of field pea (Pisum sativum) crops worldwide. The PSbMV transmission efficiencies of five aphid species previously found landing in south-west Australian pea crops in which PSbMV was spreading were studied. With plants of susceptible pea cv. Kaspa, the transmission efficiencies of Aphis craccivora, Myzus persicae, Acyrthosiphon kondoi and Rhopalosiphum padi were 27%, 26%, 6% and 3%, respectively. Lipaphis erysimi did not transmit PSbMV in these experiments. The transmission efficiencies found for M. persicae and A. craccivora resembled earlier findings, but PSbMV vector transmission efficiency data were unavailable for A. kondoi, R. padi and L. erysimi. With plants of partially PSbMV resistant pea cv. PBA Twilight, transmission efficiencies of M. persicae, A. craccivora and R. padi were 16%, 12% and 1%, respectively, reflecting putative partial resistance to aphid inoculation. To examine aphid alighting preferences over time, free-choice assays were conducted with two aphid species representing efficient (M. persicae) and inefficient (R. padi) vector species. For this, alatae were set free on multiple occasions (10-15 repetitions each) amongst PSbMV-infected and mock-inoculated pea or faba bean (Vicia faba) plants. Following release, non-viruliferous R. padi alatae exhibited a general preference for PSbMV-infected pea and faba bean plants after 30min-4h, but preferred mock-inoculated plants after 24h. In contrast, non-viruliferous M. persicae alatae alighted on mock-inoculated pea plants preferentially for up to 48h following their release. With faba bean, M. persicae preferred infected plants at the front of assay cages, but mock-inoculated ones their backs, apparently due to increased levels of natural light there. When preliminary analyses were performed to detect PSbMV-induced changes in the volatile organic compound profiles of pea and faba bean plants, higher numbers of volatiles representing a range of compound groups (such as aldehydes, ketones and esters) were found in the headspaces of PSbMV-infected than of mock-inoculated pea or faba bean plants. This indicates PSbMV induces physiological changes in these hosts which manifest as altered volatile emissions. These alterations could be responsible for the differences in alighting preferences. Information from this study enhances understanding of virus-vector relationships in the PSbMV-pea and faba bean pathosystems.

Forecasting model for Pea seed-borne mosaic virus epidemics in field pea crops in a Mediterranean-type environment

An empirical model was developed to forecast Pea seed-borne mosaic virus (PSbMV) incidence at a critical phase of the annual growing season to predict yield loss in field pea crops sown under Mediterranean-type conditions. The model uses pre-growing season rainfall to calculate an index of aphid abundance in early-August which, in combination with PSbMV infection level in seed sown, is used to forecast virus crop incidence. Using predicted PSbMV crop incidence in early-August and day of sowing, PSbMV transmission from harvested seed was also predicted, albeit less accurately. The model was developed so it provides forecasts before sowing to allow sufficient time to implement control recommendations, such as having representative seed samples tested for PSbMV transmission rate to seedlings, obtaining seed with minimal PSbMV infection or of a PSbMV-resistant cultivar, and implementation of cultural management strategies. The model provides a disease forecast risk indication, taking into account predicted percentage yield loss to PSbMV infection and economic factors involved in field pea production. This disease risk forecast delivers location-specific recommendations regarding PSbMV management to end-users. These recommendations will be delivered directly to end-users via SMS alerts with links to web support that provide information on PSbMV management options. This modelling and decision support system approach would likely be suitable for use in other world regions where field pea is grown in similar Mediterranean-type environments.

Pea seed-borne mosaic virus Pathosystem Drivers under Mediterranean-Type Climatic Conditions: Deductions from 23 Epidemic Scenarios

Drivers of Pea seed-borne mosaic virus (PSbMV) epidemics in rainfed field pea crops were examined under autumn to spring growing conditions in a Mediterranean-type environment. To collect aphid occurrence and PSbMV epidemic data under a diverse range of conditions, 23 field pea data collection blocks were set up over a 6-year period (2010 to 2015) at five locations in the southwest Australian grain-growing region. PSbMV infection levels in seed sown (0.1 to 13%), time of sowing (22 May to 22 June), and cultivar (Kaspa or PBA Twilight) varied with location and year. Throughout each growing season, rainfall data were collected, leaf and seed samples were tested to monitor PSbMV incidence in the crop and transmission from harvested seed, and sticky traps were used to monitor flying aphid numbers. Winged migrant Acyrthosiphon kondoi, Lipaphis erysimi, Myzus persicae, and Rhopalosiphum padi were identified in green tile traps in 2014 and 2015. However, no aphid colonization of field pea plants ever occurred in the blocks. The deductions made from collection block data illustrated how the magnitude of PSbMV spread prior to flowering is determined by two primary epidemic drivers: (i) PSbMV infection incidence in the seed sown, which defines the magnitude of virus inoculum source for within-crop spread by aphids, and (ii) presowing rainfall that promotes background vegetation growth which, in turn, drives early-season aphid populations and the time of first arrival of their winged migrants to field pea crops. Likely secondary epidemic drivers included wind-mediated PSbMV plant-to-plant contact transmission and time of sowing. PSbMV incidence at flowering time strongly influenced transmission rate from harvested seed to seedlings. The data collected are well suited for development and validation of a forecasting model that informs a Decision Support System for PSbMV control in field pea crops.

Pea seed-borne mosaic virus in Field Pea: Widespread Infection, Genetic Diversity, and Resistance Gene Effectiveness

From 2013 to 2015, incidences of Pea seed-borne mosaic virus (PSbMV) infection were determined in semi-leafless field pea (Pisum sativum) crops and trial plots growing in the Mediterranean-type environment of southwest Australia. PSbMV was found at incidences of 2 to 51% in 9 of 13 crops, 1 to 100% in 20 of 24 cultivar plots, and 1 to 57% in 14 of 21 breeding line plots. Crops and plots of 'PBA Gunyah', 'Kaspa', and 'PBA Twilight' were frequently PSbMV infected but none of PSbMV resistance gene sbm1-carrying 'PBA Wharton' plants were infected. In 2015, 14 new PSbMV isolates obtained from these various sources were sequenced and their partial coat protein (CP) nucleotide sequences analyzed. Sequence identities and phylogenetic comparison with 39 other PSbMV partial CP nucleotide sequences from GenBank demonstrated that at least three PSbMV introductions have occurred to the region, one of which was previously unknown. When plants of 'Greenfeast' and PBA Gunyah pea (which both carry resistance gene sbm2) and PBA Wharton and 'Yarrum' (which carry sbm1) were inoculated with PSbMV pathotype P-2 isolate W1, resistance was overcome in a small proportion of plants of each cultivar, showing that resistance-breaking variants were likely to be present. An improved management effort by pea breeders, advisors, and growers is required to diminish infection of seed stocks, avoid sbm gene resistance being overcome in the field, and mitigate the impact of PSbMV on seed yield and quality. A similar management effort is likely to be needed in field pea production elsewhere in the world.

Pea seed-borne mosaic virus: Stability and Wind-Mediated Contact Transmission in Field Pea

Pea seed-borne mosaic virus (PSbMV) stability in sap and its contact transmission between field pea plants were investigated in glasshouse experiments. When infective leaf sap was kept at room temperature and inoculated to plants in the absence of abrasive, it was still highly infective after 6 h and low levels of infectivity remained after 30 h. PSbMV was transmitted from infected to healthy plants by direct contact when leaves were rubbed against each other. It was also transmitted when intertwining healthy and PSbMV-infected plants were blown by a fan to simulate wind. When air was blown on plants kept at 14 to 20°C, contact transmission of PSbMV occurred consistently and the extent of transmission was enhanced when plants were dusted with diatomaceous earth prior to blowing. In contrast, when plants were kept at 20 to 30°C, blowing rarely resulted in transmission. No passive contact transmission occurred when healthy and infected plants were allowed to intertwine together. This study demonstrates that PSbMV has the potential to be transmitted by contact when wind-mediated wounding occurs in the field. This may play an important role in the epidemiology of the virus in field pea crops, especially in situations where contact transmission expands initial crop infection foci before aphid arrival.

Future Scenarios for Plant Virus Pathogens as Climate Change Progresses

Knowledge of how climate change is likely to influence future virus disease epidemics in cultivated plants and natural vegetation is of great importance to both global food security and natural ecosystems. However, obtaining such knowledge is hampered by the complex effects of climate alterations on the behavior of diverse types of vectors and the ease by which previously unknown viruses can emerge. A review written in 2011 provided a comprehensive analysis of available data on the effects of climate change on virus disease epidemics worldwide. This review summarizes its findings and those of two earlier climate change reviews and focuses on describing research published on the subject since 2011. It describes the likely effects of the full range of direct and indirect climate change parameters on hosts, viruses and vectors, virus control prospects, and the many information gaps and deficiencies. Recently, there has been encouraging progress in understanding the likely effects of some climate change parameters, especially over the effects of elevated CO2, temperature, and rainfall-related parameters, upon a small number of important plant viruses and several key insect vectors, especially aphids. However, much more research needs to be done to prepare for an era of (i) increasingly severe virus epidemics and (ii) increasing difficulties in controlling them, so as to mitigate their detrimental effects on future global food security and plant biodiversity.

Potato spindle tuber viroid: Stability on Common Surfaces and Inactivation With Disinfectants

The length of time Potato spindle tuber viroid (PSTVd) remained infective in extracted tomato leaf sap on common surfaces and the effectiveness of disinfectants against it were investigated. When sap from PSTVd-infected tomato leaves was applied to eight common surfaces (cotton, wood, rubber tire, leather, metal, plastic, human skin, and string) and left for various periods of time (5 min to 24 h) before rehydrating the surface and rubbing onto healthy tomato plants, PSTVd remained infective for 24 h on all surfaces except human skin. It survived best on leather, plastic, and string. It survived less well after 6 h on wood, cotton, and rubber and after 60 min on metal. On human skin, PSTVd remained infective for only 30 min. In general, rubbing surfaces contaminated with dried infective sap directly onto leaves caused less infection than when the sap was rehydrated with distilled water but overall results were similar. The effectiveness of five disinfectant agents at inactivating PSTVd in sap extracts was investigated by adding them to sap from PSTVd-infected leaves before rubbing the treated sap onto leaves of healthy tomato plants. Of the disinfectants tested, 20% nonfat dried skim milk and a 1:4 dilution of household bleach (active ingredient sodium hypochlorite) were the most effective at inactivating PSTVd infectivity in infective sap. When reverse-transcription polymerase chain reaction was used to test the activity of the five disinfectants against PSTVd in infective sap, it detected PSTVd in all instances except in sap treated with 20% nonfat dried skim milk. This study highlights the stability of PSTVd in infective sap and the critical importance of utilizing hygiene practices such as decontamination of clothing, tools, and machinery, along with other control measures, to ensure effective management of PSTVd and, wherever possible, its elimination in solanaceous crops.

Potato virus Y: Contact Transmission, Stability, Inactivation, and Infection Sources

In glasshouse experiments, two isolates of Potato virus Y 'O' strain (PVY^O) were transmitted from infected to healthy potato plants by direct contact when leaves were rubbed against each other, when cut surfaces of infected tubers were rubbed onto leaves, and to a limited extent, when blades contaminated with infective sap were used to cut healthy potato tubers. However, no tuber-to-tuber transmission occurred when blades were used to cut healthy tubers after cutting infected tubers. When leaf sap from potato plants infected with two PVY^O isolates was kept at room temperature, it was highly infective for 6 to 7 h and remained infectious for up to 28 h. Also, when sap from infected leaves with one isolate was applied to five surfaces (cotton, hessian, metal, rubber vehicle tire, and wood) and left to dry for up to 24 h before each surface was rubbed onto healthy tobacco plants, PVY^O remained infective for 24 h on tire and metal, 6 h on cotton and hessian, and 3 h on wood. The effectiveness of disinfectants at inactivating this isolate was evaluated by adding them to sap from infected leaves which was then rubbed onto healthy tobacco plants. None of the plants became infected when bleach (42 g/liter sodium hypochlorite, diluted 1:4) or Virkon-S (potassium peroxymonosulfate 50% wt/wt, diluted to 1%) was used. A trace of infection remained after using nonfat milk powder (20% wt/vol). PVY infection sources were studied in 2011-2012 in the main potato growing regions of southwest Australia. In tests on >17,000 potato leaf samples, PVY was detected at low levels in seed (4/155) and ware (6/51) crops. It was also detected in volunteer potatoes from a site with a previous history of PVY infection in a seed crop. None of the 15 weed species tested were PVY infected. Plants of Solanum nigrum were symptomlessly infected with PVY^O after sap inoculation, and no seed transmission was detected (>2,500 seeds). This study demonstrates PVY^O can be transmitted by contact and highlights the need to include removal of volunteer potatoes and other on-farm hygiene practices (decontaminating tools, machinery, clothing, etc.) in integrated disease management strategies for PVY in potato crops.

Polymyxa graminis Isolates from Australia: Identification in Wheat Roots and Soil, Molecular Characterization, and Wide Genetic Diversity

Polymyxa graminis is an obligate parasite of roots and an important vector of viruses that damage cereal crops in different parts of the world. In 2011 and 2012, P. graminis was identified infecting 11 wheat root samples from three widely dispersed locations in southwest Australia. Its presence was detected by polymerase chain reaction (PCR) and confirmed by DNA sequencing of the transcribed regions of its ribosomal RNA genes (rDNA) and observing sporosori of characteristic morphology and size in stained wheat roots. Also, when soil samples were collected from two locations where P. graminis was found and wheat bait plants grown in them, P. graminis was detected in their roots by PCR. Ribosomal DNA sequences of six southwest Australian isolates were obtained from wheat roots, and one northeast Australian isolate from barley roots. When these seven P. graminis sequences were compared with others from GenBank by phylogenetic analysis, three southwest Australian isolates were classified as P. graminis f. sp. temperata (ribotypes Ia and Ib), and three as f. sp. tepida (ribotypes IIa and IIb). P. graminis f. sp. temperata and tepida both occur in temperate growing regions of other continents and are associated with transmission of soil-borne viruses to cereal crops. The P. graminis isolate from northeast Australia was sufficiently distinct from the five existing sequence groups for it to be placed into a newly proposed grouping, ribotype VI, which also included an isolate from tropical West Africa. However, when randomly collected wheat leaf samples from 39 field crops from 27 widely dispersed locations, 21 individual wheat plant samples collected from low lying areas within 21 fields at 11 locations, and wheat bait plants growing in five soil samples from two locations were tested by reverse transcription (RT)-PCR for the presence of Soil-borne wheat mosaic virus, Soil-borne cereal mosaic virus, Wheat spindle streak mosaic virus, and furoviruses in general, no virus infection was detected. These findings suggest at least three P. graminis introductions into Australia, and the occurrence of f. sp. temperata (ribotype I) and f. sp. tepida (ribotype II) suggests that, if not already present, soil-borne cereal viruses are likely to become established should they become introduced to the continent in the future.

Black Pod Syndrome of Lupinus angustifolius Is Caused by Late Infection with Bean yellow mosaic virus

Black pod syndrome (BPS) causes devastating losses in Lupinus angustifolius (narrow-leafed lupin) crops in Australia, and infection with Bean yellow mosaic virus (BYMV) was suggested as a possible cause. In 2011, an end-of-growing-season survey in which L. angustifolius plants with BPS were collected from six locations in southwestern Australia was done. Tissue samples from different positions on each of these symptomatic plants were tested for BYMV and generic potyvirus by enzyme-linked immunosorbent assay and reverse-transcription polymerase chain reaction (RT-PCR). Detection was most reliable when RT-PCR with generic potyvirus primers was used on tissue taken from the main stem of the plant just below the black pods. Partial coat protein nucleotide sequences from eight isolates from BPS-symptomatic L. angustifolius plants all belonged to the BYMV general phylogenetic group. An initial glasshouse experiment revealed that mechanical inoculation of L. angustifolius plants with BYMV after pods had formed caused pods to turn black. This did not occur when the plants were inoculated before this growth stage (at first flowering) because BYMV infection caused plant death. A subsequent experiment in which plants were inoculated at eight different growth stages confirmed that BPS was only induced when L. angustifolius plants were inoculated after first flowering, when pods had formed. Thus, BYMV was isolated from symptomatic L. angustifolius survey samples, inoculated to and maintained in culture hosts, inoculated to healthy L. angustifolius test plants inducing BPS, and then successfully reisolated from them. As such, Koch's postulates were fulfilled for the hypothesis that late infection with BYMV causes BPS in L. angustifolius plants.

Hardenbergia mosaic virus: crossing the barrier between native and introduced plant species

Hardenbergia mosaic virus (HarMV), genus Potyvirus, belongs to the bean common mosaic virus (BCMV) potyvirus lineage found only in Australia. The original host of HarMV, Hardenbergia comptoniana, family Fabaceae, is indigenous to the South-West Australian Floristic Region (SWAFR), where Lupinus spp. are grown as introduced grain legume crops, and exist as naturalised weeds. Two plants of H. comptoniana and one of Lupinus cosentinii, each with mosaic and leaf deformation symptoms, were sampled from a small patch of disturbed vegetation at an ancient ecosystem-recent agroecosystem interface. Potyvirus infection was detected in all three samples by ELISA and RT-PCR. After sequencing on an Illumina HiSeq 2000, three complete and two nearly complete HarMV genomes from H. comptoniana and one complete HarMV genome from L. cosentinii were obtained. Phylogenetic analysis which compared (i) the four new complete genomes with the three HarMV genomes on Genbank (two of which were identical), and (ii) coat protein (CP) genes from the six new genomes with the 38 HarMV CP sequences already on Genbank, revealed that three of the complete and one of the nearly complete new genomes were in HarMV clade I, one of the complete genomes in clade V and one nearly complete genome in clade VI. The complete HarMV genome from L. cosentinii differed by only eight nucleotides from one of the HarMV clade I genomes from a nearby H. comptoniana plant, with only one of these nucleotide changes being non-synonymous. Pairwise comparison between all the complete HarMV genomes revealed nucleotide identities ranging between 82.2% and 100%. Recombination analysis revealed evidence of two recombination events amongst the six complete genomes. This study provides the first report of HarMV naturally infecting L. cosentinii and the first example for the SWAFR of virus emergence from a native plant species to invade an introduced plant species.

First Report of Wheat mosaic virus Infecting Wheat in Western Australia

In eastern Australia, there have been several as yet unconfirmed reports of Wheat mosaic virus (WMoV) infecting wheat (3). WMoV, previously known as High plains virus (HPV), is transmitted by the wheat curl mite (WCM, Aceria tosichella). It is often found in mixed infections with Wheat streak mosaic virus (WSMV), also transmitted by WCM (2,3). WSMV was first identified in Australia in 2003 (3). In October 2012, stunted wheat plants with severe yellow leaf streaking were common in a field experiment near Corrigin in Western Australia consisting of nine wheat cultivars. These symptoms were also common in two commercial crops of wheat cv. Mace near Kulin. Leaf samples (one per plant) from each location were tested by ELISA using specific antiserum to WMoV (syn. HPV 17200, Agdia, Elkhart, IN). At the field experiment, 20 leaf samples were collected at random from each wheat plot (4 replicates) and tested individually by ELISA. WMoV incidence was 5% for cv. Yipti, 16% for cvs Emu Rock, Wyalkatchem and Mace, 22% for cvs. Corack, Fortune, Calingiri, and Magenta, and 55% for cv. Cobra. From the two commercial wheat crops, 100 leaf samples were collected at random from each and tested by ELISA. WMoV incidence was 2 and 4%. In addition, 50 leaf samples of Hordeum leporinum (barley grass) and 20 of Lolium rigidum (annual ryegrass) were collected and tested by ELISA. WMoV incidence was 2% in H. leporinum, but 0% in L. rigidum. Infected H. leporinum plants were symptomless. Symptomatic wheat leaf samples from both sites were tested by RT-PCR using WMoV specific primers designed from its RNA3 sequence (1). The PCR products (339 bp) were sequenced and lodged in GenBank (Accession Nos KC337341 and KC337342). WMoV isolates from Corrigin (WA-CG12) and Kulin (WA-KU12) had identical sequences. When the nucleic acid sequences of WA-CG12 and WA-KU12 were compared with those of the three other WMoV isolates on GenBank, they had 100% nucleotide sequence identity with a Nebraska isolate (U60141), and 99.7% identity to two United States sweet corn isolates (AY836524 and AY836525). Ten symptomatic wheat plants were collected from each location, transplanted into pots and leaf samples tested individually for WMoV and WSMV (07048, Loewe, Germany) by ELISA. All were infected with both viruses and infested with WCM. WCM-infested glumes (>10 WCM/glume) were placed on the leaf sheaths of 60 wheat plants cv. Calingiri (35 with WA-CG12 and 25 with WA-KU12) and 13 sweet corn plants cv. Snow Gold (WA-CG12 only). In addition, 20 wheat and 10 sweet corn plants were left without infested glumes to be uninoculated controls. All 60 WCM-inoculated wheat plants became stunted with severe leaf streaking. When leaf samples from each plant were tested by ELISA 18 to 30 days later, both viruses were detected. WMoV was detected in all 13 WCM-inoculated sweet corn plants and WSMV in two of them. Plants with WMoV alone initially had short chlorotic leaf streaks that subsequently combined, causing broad streaks. These are typical WMoV symptoms for sweet corn (1). No symptoms developed and no virus was detected in any of the uninoculated wheat or sweet corn control plants. The WMoV nucleotide sequence obtained from an infected sweet corn plant was identical to those of WA-CG12 and WA-KU12. To our knowledge, this is the first confirmed report of WMoV presence in Australia. References: (1) B. S. M. Lebas et al. Plant Dis. 89:1103, 2005. (2) D. Navia et al. Exp. Appl. Acarol. 59:95, 2013. (3) J. M. Skare et al. Virology 347:343, 2006.

Zucchini yellow mosaic virus: Contact Transmission, Stability on Surfaces, and Inactivation with Disinfectants

In glasshouse experiments, Zucchini yellow mosaic virus (ZYMV) was transmitted from infected to healthy zucchini (Cucurbita pepo) plants by direct contact when leaves were rubbed against each other, crushed, or trampled, and, to a lesser extent, on ZYMV-contaminated blades. When sap from zucchini plants infected with three ZYMV isolates was kept at room temperature for up to 6 h, it infected healthy plants readily. Also, when sap from ZYMV-infected leaves was applied to seven surfaces (cotton, plastic, leather, metal, rubber vehicle tire, rubber-soled footwear, and human skin) and left for up to 48 h before the ZYMV-contaminated surface was rubbed onto healthy zucchini plants, ZYMV remained infective for 48 h on tire, 24 h on plastic and leather, and up to 6 h on cotton, metal, and footwear. On human skin, ZYMV remained infective for 5 min only. The effectiveness of 13 disinfectants at inactivating ZYMV was evaluated by adding them to sap from ZYMV-infected leaves which was then rubbed on to healthy zucchini plants. None of the plants became infected when nonfat dried milk (20%, wt/vol) or bleach (sodium hypochlorite at 42 g/liter, diluted 1:4) were used. When ZYMV-infected pumpkin leaves were trampled by footwear and then used to trample healthy plants, all plants became infected; however, when contaminated footwear was dipped in a footbath containing bleach (sodium hypochlorite at 42 g/liter, diluted 1:4) before trampling, none became infected. This study demonstrates that ZYMV can be transmitted by contact and highlights the need for on-farm hygiene practices (decontaminating tools, machinery, clothing, and so on) to be included in integrated disease management strategies for ZYMV in cucurbit crops.

Detection of Viruses in Sweetpotato from Honduras and Guatemala Augmented by Deep-Sequencing of Small-RNAs

Sweetpotato (Ipomoea batatas) plants become infected with over 30 RNA or DNA viruses in different parts of the world but little is known about viruses infecting sweetpotato crops in Central America, the center of sweetpotato domestication. Small-RNA deep-sequencing (SRDS) analysis was used to detect viruses in sweetpotato in Honduras and Guatemala, which detected Sweet potato feathery mottle virus strain RC and Sweet potato virus C (Potyvirus spp.), Sweet potato chlorotic stunt virus strain WA (SPCSV-WA; Crinivirus sp.), Sweet potato leaf curl Georgia virus (Begomovirus sp.), and Sweet potato pakakuy virus strain B (synonym: Sweet potato badnavirus B). Results were confirmed by polymerase chain reaction and sequencing of the amplicons. Four viruses were detected in a sweetpotato sample from the Galapagos Islands. Serological assays available to two of the five viruses gave results consistent with those obtained by SRDS, and were negative for six additional sweetpotato viruses tested. Plants coinfected with SPCSV-WA and one to two other viruses displayed severe foliar symptoms of epinasty and leaf malformation, purpling, vein banding, or chlorosis. The results suggest that SRDS is suitable for use as a universal, robust, and reliable method for detection of plant viruses, and especially useful for determining virus infections in crops infected with a wide range of unrelated viruses.

First Report of Alfalfa mosaic virus Infecting Tedera (Bituminaria bituminosa (L.) C.H. Stirton var. albomarginata and crassiuscula) in Australia

Tedera (Bituminaria bituminosa (L.) C.H. Stirton vars albomarginata and crassiuscula) is being established as a perennial pasture legume in southwest Australia because of its drought tolerance and ability to persist well during the dry summer and autumn period. Calico (bright yellow mosaic) leaf symptoms occurred on occasional tedera plants growing in genetic evaluation plots containing spaced plants at Newdegate in 2007 and Buntine in 2010. Alfalfa mosaic virus (AlMV) infection was suspected as it often causes calico in infected plants (1,2) and infects perennial pasture legumes in local pastures (1,3). Because AlMV frequently infects Medicago sativa (alfalfa) in Australia and its seed stocks are commonly infected (1,3), M. sativa buffer rows were likely sources for spread by aphids to healthy tedera plants. When leaf samples from plants with typical calico symptoms from Newdegate (2007) and Buntine (2010) were tested by ELISA using poyclonal antisera to AlMV, Bean yellow mosaic virus (BYMV) and Cucumber mosaic virus (CMV), only AlMV was detected. When leaf samples from 864 asymptomatic spaced plants belonging to 34 tedera accessions growing at Newdegate and Mount Barker in 2010 were tested by ELISA, no AlMV, BYMV, or CMV were detected, despite presence of M. sativa buffer rows. A culture of AlMV isolate EW was maintained by serial planting of infected seed of M. polymorpha L. (burr medic) and selecting seed-infected seedlings (1,3). Ten plants each of 61 accessions from the local tedera breeding program were grown at 20°C in an insect-proof air conditioned glasshouse. They were inoculated by rubbing leaves with infective sap containing AlMV-EW or healthy sap (five plants each) using Celite abrasive. Inoculations were always done two to three times to the same plants. When both inoculated and tip leaf samples from each plant were tested by ELISA, AlMV was detected in 52 of 305 AlMV-inoculated plants belonging to 36 of 61 accessions. Inoculated leaves developed local necrotic or chlorotic spots or blotches, or symptomless infection. Systemic invasion was detected in 20 plants from 12 accessions. Koch's postulates were fulfilled in 12 plants from nine accessions (1 to 2 of 5 plants each), obvious calico symptoms developing in uninoculated leaves, and AlMV being detected in symptomatic samples by ELISA, inoculation of sap to diagnostic indicator hosts (2) and RT-PCR with AlMV CP gene primers. Direct RT-PCR products were sequenced and lodged in GenBank. When complete nucleotide CP sequences (666 nt) of two isolates from symptomatic tedera samples and two from alfalfa (Aq-JX112758, Hu-JX112759) were compared with that of AlMV-EW, those from tedera and EW were identical (JX112757) but had 99.1 to 99.2% identities to the alfalfa isolates. JX112757 had 99.4% identity with Italian tomato isolate Y09110. Systemically infected tedera foliage sometimes also developed vein clearing, mosaic, necrotic spotting, leaf deformation, leaf downcurling, or chlorosis. Later-formed leaves sometimes recovered, but plant growth was often stunted. No infection was detected in the 305 plants inoculated with healthy sap. To our knowledge, this is the first report of AlMV infecting tedera in Australia or elsewhere. References: (1) B. A. Coutts and R. A. C. Jones. Ann. Appl. Biol. 140:37, 2002. (2) E. M. J. Jaspars and L. Bos. Association of Applied Biologists, Descriptions of Plant Viruses No. 229, 1980. (3) R. A. C. Jones. Aust. J. Agric. Res. 55:757, 2004.

Influence of climate change on plant disease infections and epidemics caused by viruses and bacteria

Abstract

This review is motivated by (i) the magnitude of the threat to world food security and diversity of natural vegetation posed by viral and bacterial pathogens of plants at a time of accelerating climate change; and (ii) the inadequate attention given to this subject by earlier reviews on climate change and plant disease. It starts by providing background information on current climate change predictions, the increasing worldwide importance of viral and bacterial diseases, critical features of their pathosystems and the general influence of environmental factors upon them. It then develops comprehensive climatic and biological frameworks and uses them to determine the likely influences of direct and indirect climate change parameters on the many different host, vector and pathogen parameters that represent the diversity of viral and bacterial pathosystems. This approach proved a powerful way to identify the relevant international research data available and many information gaps where research is needed in the future. The analysis suggested that climate change is likely to modify many critical viral and bacterial epidemic components in different ways, often resulting in epidemic enhancement but sometimes having the opposite effect, depending on the type of pathosystem and circumstances. With vector-borne pathosystems and new encounter scenarios, the complication of having to consider the effects climate change parameters on diverse types of vectors and the emergence of previously unknown pathogens added important additional variables. The increasing difficulties in controlling damaging plant viral and bacterial epidemics predicted to arise from future climate instability warrants considerable research effort to safeguard world food security and biodiversity.

Genetic variability of the coat protein sequence of pea seed-borne mosaic virus isolates and the current relationship between phylogenetic placement and resistance groups

Nucleotide sequences of complete or partial coat protein (CP) genes were determined for 11 isolates of pea seed-borne mosaic virus (PSbMV) from Australia and one from China, and compared with known sequences of 20 other isolates. On phylogenetic analysis, the isolates from Australia and China grouped into 2 of 3 clades. Clade A contained three sub-clades (Ai, Aii and Aiii), Australian isolates were in Ai or Aiii, and the Chinese isolate in Aii. Clade A contained isolates in pathotypes P-1, P-2 and U-2; clade B, one isolate in P-2; and clade C, only isolates in P-4.

Minimising losses caused by Zucchini yellow mosaic virus in vegetable cucurbit crops in tropical, sub-tropical and Mediterranean environments through cultural methods and host resistance

Between 2006 and 2009, 10 field experiments were done at Kununurra, Carnarvon or Medina in Western Australia (WA) which have tropical, sub-tropical and Mediterranean climates, respectively. These experiments investigated the effectiveness of cultural control measures in limiting ZYMV spread in pumpkin, and single-gene resistance in commercial cultivars of pumpkin, zucchini and cucumber. Melon aphids (Aphis gossypii) colonised field experiments at Kununurra; migrant green peach aphids (Myzus persicae) visited but did not colonise at Carnarvon and Medina. Cultural control measures that diminished ZYMV spread in pumpkin included manipulation of planting date to avoid exposing young plants to peak aphid vector populations, deploying tall non-host barriers (millet, Pennisetum glaucum) to protect against incoming aphid vectors and planting upwind of infection sources. Clustering of ZYMV-infected pumpkin plants was greater without a 25m wide non-host barrier between the infection source and the pumpkin plants than when one was present, and downwind compared with upwind of an infection source. Host resistance gene zym was effective against ZYMV isolate Knx-1 from Kununurra in five cultivars of cucumber. In zucchini, host resistance gene Zym delayed spread of infection (partial resistance) in 2 of 14 cultivars but otherwise did not diminish final ZYMV incidence. Zucchini cultivars carrying Zym often developed severe fruit symptoms (8/14), and only the two cultivars in which spread was delayed and one that was tolerant produced sufficiently high marketable yields to be recommended when ZYMV epidemics are anticipated. In three pumpkin cultivars with Zym, this gene was effective against isolate Cvn-1 from Carnarvon under low inoculum pressure, but not against isolate Knx-1 under high inoculum pressure, although symptoms were milder and marketable yields greater in them than in cultivars without Zym. These findings allowed additional cultural control recommendations to be added to the existing Integrated Disease Management strategy for ZYMV in vegetable cucurbits in WA, but necessitated modification of its recommendations over deployment of cultivars with resistance genes.

Molecular Genetic Characterization of Olpidium virulentus Isolates Associated with Big-Vein Diseased Lettuce Plants

Lettuce plants showing symptoms of lettuce big-vein disease were collected from fields in the Perth Metropolitan region of southwest Australia. When root extracts from each plant were tested by polymerase chain reaction (PCR) using primers specific to the rDNA internal transcribed spacer (ITS) region of Olpidium brassicae and O. virulentus, only O. virulentus was detected in each of them. The nucleotide sequences of the complete rDNA ITS regions of isolates from five of the root samples and 10 isolates of O. virulentus from Europe and Japan showed 97.9 to 100% identities. However, with the six O. brassicae isolates, their identities were only 76.9 to 79.4%. On phylogenetic analysis of the complete rDNA-ITS region sequences of the five Australian isolates and 10 others, the Australian isolates fitted within two clades of O. virulentus (I and II), and within clade I into two of its four subclades (Ia and Id). Japanese isolates had greatest sequence diversity fitting into both clades and into all of clade I subclades except Ib, while European isolates were restricted to subclades Ib and Id. When the partial rDNA-ITS region sequences of two additional southwest Australian isolates, four from Europe, and four from the Americas were included in the analyses, the Australian isolates were within O. virulentus subclades Ia and Id, the European isolates within subclade Ic, and the American isolates within subclades Ia and Ib. These findings suggest that there may have been at least three separate introductions of O. virulentus into the isolated Australian continent since plant cultivation was introduced following its colonization by Europeans. They also have implications regarding numbers of different introductions to other isolated regions. Lettuce big-vein associated virus and Mirafiori lettuce big-vein virus were both detected when symptomatic lettuce leaf tissue samples corresponding to the root samples from southwest Australia were tested using virus-specific primers in reverse transcription-PCR, so presence of both viruses was associated with O. virulentus occurrence.

Quantifying effects of seedborne inoculum on virus spread, yield losses, and seed infection in the pea seed-borne mosaic virus-field pea pathosystem

Field experiments examined the effects of sowing field pea seed with different amounts of infection with Pea seed-borne mosaic virus (PSbMV) on virus spread, seed yield, and infection levels in harvested seed. Plots were sown with seed with actual or simulated seed transmission rates of 0.3 to 6.5% (2005) or 0.1 to 8% (2006), and spread was by naturally occurring migrant aphids. Plants with symptoms and incidence increased with the amount of primary inoculum present. When final incidence reached 97 to 98% (2005) and 36% (2006) in plots sown with 6.5 to 8% infected seed, yield losses of 18 to 25% (2005) and 13% (2006) resulted. When incidence reached 48 to 76% in plots sown with 1.1-2 to 2% initial infection, seed yield losses were 15 to 21% (2005). Diminished seed weight and seed number both contributed to the yield losses. When the 2005 data for the relationships between percent incidence and yield or yield gaps were plotted, 81 to 84% of the variation was explained by final incidence and, for each 1% increase, there was a yield decline of 7.7 to 8.2 kg/ha. Seed transmission rates in harvested seed were mostly greater than those in the seed sown when climatic conditions favored early virus spread (1 to 17% in 2005) but smaller when they did not (0.2 to 2% in 2006). In 2007, sowing infected seed at high seeding rate with straw mulch and regular insecticide application resulted in slower spread and smaller seed infection than sowing at standard seeding rate without straw mulch or insecticide. When data for the relationship between final percent incidence and seed transmission in harvested seed were plotted (all experiments), 95 to 99% of the variation was explained by PSbMV incidence. A threshold value of <0.5% seed infection was established for sowing in high-risk zones.

Molecular Characterization of Sweet potato feathery mottle virus (SPFMV) Isolates from Easter Island, French Polynesia, New Zealand, and Southern Africa

Strains of Sweet potato feathery mottle virus (SPFMV; Potyvirus; Potyviridae) infecting sweet-potato (Ipomoea batatas) in Oceania, one of the worlds' earliest sweetpotato-growing areas, and in southern Africa were isolated and characterized phylogenetically by analysis of the coat protein (CP) encoding sequences. Sweetpotato plants from Easter Island were co-infected with SPFMV strains C and EA. The EA strain isolates from this isolated location were related phylogenetically to those from Peru and East Africa. Sweetpotato plants from French Polynesia (Tahiti, Tubuai, and Moorea) were co-infected with SPFMV strains C, O, and RC in different combinations, whereas strains C and RC were detected in New Zealand. Sweetpotato plants from Zimbabwe were infected with strains C and EA and those from Cape Town, South Africa, with strains C, O, and RC. Co-infections with SPFMV strains and Sweet potato virus G (Potyvirus) were common and, additionally, Sweet potato chlorotic fleck virus (Carlavirus) was detected in a sample from Tahiti. Taken together, occurrence of different SPFMV strains was established for the first time in Easter Island, French Polynesia, and New Zealand, and new strains were detected in Zimbabwe and the southernmost part of South Africa. These results from the Southern hemisphere reflect the anticipated global distribution of strains C, O, and RC but reveal a wider distribution of strain EA than was known previously.

Role of recombination in the evolution of host specialization within bean yellow mosaic virus

Seven complete genomes and 64 coat protein gene sequences belonging to Bean yellow mosaic virus (BYMV) isolates from different continents were examined for evidence of genetic recombination using six different recombination-detection programs. In the seven complete genomes and a single complete genome of the related virus Clover yellow vein virus (ClYVV), evidence for eight recombination patterns was found by four or more programs, giving firm evidence of their presence, and five additional recombination patterns were detected by three or fewer programs, giving tentative evidence of their occurrence. When the nucleotide sequences of 64 BYMV and one ClYVV coat protein genes were analyzed, three firm recombination patterns were detected in 21 isolates (32%). With another six isolates (9%), tentative evidence was found for three further recombination patterns. Of the 19 firm or tentative recombination patterns detected within and between strain groups of BYMV, and with ClYVV, 12 involved a generalist group of isolates as a parent but none of the other BYMV groups acted as parents more than six times. These findings suggest that recombination played an important role in the evolution of BYMV strain groups that specialize in infecting particular groups of domesticated plants.

Phylogenetic Analysis of Bean yellow mosaic virus Isolates from Four Continents: Relationship Between the Seven Groups Found and Their Hosts and Origins

Genetic diversity of Bean yellow mosaic virus (BYMV) was studied by comparing sequences from the coat protein (CP) and genome-linked viral protein (VPg) genes of isolates from four continents. CP sequences compared were those of 17 new isolates and 47 others already on the database, while the VPg sequences used were from four new isolates and 10 from the database. Phylogenetic analysis of the CP sequences revealed seven distinct groups, six polytypic and one monotypic. The largest and most genetically diverse polytypic group, which had intragroup diversity of 0.061 nucleotide substitutions per site, contained isolates from natural infections in eight host species. These original isolation hosts included both wild (four) and domesticated (four) species and were from monocotyledonous and dicotyledonous plant families, indicating a generalized natural host range strategy. Only one of the other five polytypic groups spanned both monocotyledons and dicotyledons, and all contained isolates from fewer species (one to four), all of which were domesticated and had lower intragroup diversity (0.019 to 0.045 nucleotide substitutions per site), indicating host specialization. Phylogenetic analysis of the fewer VPg sequences revealed three polytypic and two monotypic groupings. These groups also correlated with original natural isolation hosts, but the branch topologies were sometimes incongruous with those formed by CPs. Also, intragroup diversity was generally higher for VPgs than for CPs. A plausible explanation for the groups found when the 64 different CP sequences were compared is that the generalized group represents the original ancestral type from which the specialist host groups evolved in response to domestication of plants after the advent of agriculture. Data on the geographical origins of the isolates within each group did not reveal whether the specialized groups might have coevolved with their principal natural hosts where these were first domesticated, but this seems plausible.

An epidemiological model for externally sourced vector-borne viruses applied to Bean yellow mosaic virus in lupin crops in a Mediterranean-type environment

A hybrid mechanistic/statistical model was developed to predict vector activity and epidemics of vector-borne viruses spreading from external virus sources to an adjacent crop. The pathosystem tested was Bean yellow mosaic virus (BYMV) spreading from annually self-regenerating, legume-based pastures to adjacent crops of narrow-leafed lupin (Lupinus angustifolius) in the winter-spring growing season in a region with a Mediterranean-type environment where the virus persists over summer within dormant seed of annual clovers. The model uses a combination of daily rainfall and mean temperature during late summer and early fall to drive aphid population increase, migration of aphids from pasture to lupin crops, and the spread of BYMV. The model predicted time of arrival of aphid vectors and resulting BYMV spread successfully for seven of eight datasets from 2 years of field observations at four sites representing different rainfall and geographic zones of the southwestern Australian grainbelt. Sensitivity analysis was performed to determine the relative importance of the main parameters that describe the pathosystem. The hybrid mechanistic/statistical approach used created a flexible analytical tool for vector-mediated plant pathosystems that made useful predictions even when field data were not available for some components of the system.

Molecular Genetic Characterization of Sweet potato virus G (SPVG) Isolates from Areas of the Pacific Ocean and Southern Africa

Sweet potato virus G (SPVG, genus Potyvirus, family Potyviridae) was detected in sweetpotato (Ipomoea batatas) storage roots sold in the local markets and storage roots or cuttings sampled directly from farmers' fields. Using serological and molecular methods, the virus was detected for the first time in Java, New Zealand, Hawaii, Tahiti, Tubuai, Easter Island, Zimbabwe, and South Africa, and also in an imported storage root under post-entry quarantine conditions in Western Australia. In some specimens, SPVG was detected in mixed infection with Sweet potato feathery mottle virus (genus Potyvirus). The coat protein (CP) encoding sequences of SPVG were analyzed for 11 plants from each of the aforementioned locations and compared with the CP sequences of 12 previously characterized isolates from China, Egypt, Ethiopia, Spain, Peru, and the continental United States. The nucleotide sequence identities of all SPVG isolates ranged from 79 to 100%, and amino acid identities ranged from 89 to 100%. Isolates of the same strain of SPVG had nucleotide and amino acid sequence identities from 97 to 100% and 96 to 100%, respectively, and were found in sweetpotatoes from all countries sampled except Peru. Furthermore, a plant from Zimbabwe was co-infected with two clearly different SPVG isolates of this strain. In contrast, three previously characterized isolates from China and Peru were phylogenetically distinct and exhibited <90% nucleotide identity with any other isolate. So far, the highest genetic diversity of SPVG seems to occur among isolates in China. Distribution of SPVG within many sweetpotato growing areas of the world emphasizes the need to determine the economic importance of SPVG.

Finding Wheat streak mosaic virus in south-west Australia

Between 2003 and summer 2006, 33 659 samples of wheat and grasses were collected from diverse locations in south-west Australia and tested for presence of Wheat streak mosaic virus (WSMV), but none was detected. In April–early May 2006, 2840 random samples of volunteer wheat from 28 fields on 24 farms in 6 districts in the grainbelt were tested. WSMV was detected for the first time, the infected samples coming from three fields, one in the Hyden and two in the Esperance districts. In ‘follow-up’ surveys in May 2006 in the same two districts, 8983 samples of volunteer wheat or grasses were tested, and the virus was detected on further farms, two in the Hyden and four in the Esperance districts. Incidences of infection in volunteer wheat were 1–8%, but WSMV was not found in grasses. By September 2006, when 1769 samples from further visits were tested, WSMV was detected in wheat crops or volunteer wheat plants at 2/3 of the original farms, with infection also found at one of them in barley, volunteer oats, and barley grass (Hordeum sp.). When samples of the seed stocks originally used in 2005 to plant five of the fields containing infected volunteer wheat at the three original infected farms were tested, seed transmission of WSMV was detected in four of them (0.1–0.2% transmission rates). In August–October 2006, 16 436 samples were collected in a growing-season survey for WSMV in wheat trials and crops throughout the grainbelt. WSMV was detected in 33% of ‘variety’ trials, 18% of other trials, 13% of seed ‘increase’ crops, and 52% of commercial crops. Incidences of infection were <1–100% within individual crops, <1–17% in trials, and <1–3% in seed increase crops. WSMV-infected sites were concentrated in the low-rainfall zone (east) of the central grainbelt. This area received considerable summer rains in 2006, which allowed growth of a substantial ‘green ramp’ of volunteer cereals and grasses, favouring infection of subsequent wheat plantings. WSMV was also detected at low levels over a much wider area involving all rainfall zones, from Dongara in the north to Esperance in the south. All 26 122 samples collected in January–May 2006 and 515 with possible WSMV symptoms collected in August–October 2006 were also tested for High plains virus (HPV), but it was not detected.

Further studies on Pea seed-borne mosaic virus in cool-season crop legumes: responses to infection and seed quality defects

Field and glasshouse experiments (3 of each) were done during 2003–06 to determine the responses of a range of genotypes belonging to 13 species of cool-season crop legumes to infection with Pea seed-borne mosaic virus (PSbMV). Seed quality defects were determined and genotypes of some species were also tested for seed transmission of the virus. In field experiments, of 39 genotypes of field pea (Pisum sativum) evaluated, 15 were ranked as highly susceptible, 10 susceptible, 9 moderately resistant, and 5 resistant, while all 7 lupin species (Lupinus spp.) tested were resistant. In glasshouse sap and graft inoculations with PSbMV to genotypes not found infected in the field and 2 additional lupin species, no virus was detected in any of the 9 lupin species or in 5 field pea genotypes tested. Thus, the lupins all appeared to be non-hosts and the 5 field pea genotypes had resistance to the 2 PSbMV isolates used to inoculate them. All 14 genotypes of faba bean (Vicia faba) evaluated in the field were ranked highly susceptible, while 12 out of 16 lentil (Lens culinaris) genotypes were ranked as highly susceptible and 4 as susceptible. Chickpea (Cicer arietinum) genotypes were moderately resistant (50) or susceptible (7). Once infected, plant sensitivities (symptom severities) ranged from low in some field pea and most lentil genotypes to high in most faba bean genotypes. Chickpea genotypes all were ranked as moderately sensitive. Seed lots harvested from PSbMV-infected plants of field pea, faba bean, and chickpea all showed severe seed quality defects, but lentil was usually less affected. The predominant seed symptoms were necrotic rings and line markings on the seed coat, malformation, reduced size, and splitting. Kabuli chickpea types also showed darkening of the seed coat. Seed transmission of PSbMV was detected in faba bean (0.2%) and field pea (5–30%). When PSbMV infection foci were introduced into plots of lentil cv. Nugget, the virus spread to the lentil plants and decreased shoot dry weight by 23%, seed yield by 96%, and individual seed weight by 58%. Seed transmission of PSbMV (6%) was detected in seed from the infected lentil plants. In a survey for possible viral seed symptoms, all seed lots of kabuli chickpea (5) and field pea (70), and 10 of 18 of faba bean were affected, but none of the 23 of lentil. When seedlings from 16 faba bean and 7 field pea seed lots were tested for 3 viruses, neither Broad bean stain virus nor Broad bean true mosaic virus was detected, but PSbMV was found in 5 field pea seed lots at incidences of <1–14%. PSbMV was detected in commercial field pea seed stocks of cvv. Kaspa (33) and Parafield (12) at incidences of 0.5–47% and 0.3–30%, respectively. The implications of these findings in terms of genotype susceptiblility and sensitivity to PSbMV infection and their importance for the management of PSbMV in legume crops are discussed.

The epidemiology of Wheat streak mosaic virus in Australia: case histories, gradients, mite vectors, and alternative hosts

Wheat streak mosaic virus (WSMV) infection and infestation with its wheat curl mite (WCM; Aceria tosichella) vector were investigated in wheat crops at two sites in the low-rainfall zone of the central grainbelt of south-west Australia. In the 2006 outbreak, after a preceding wet summer and autumn, high WCM populations and total infection with WSMV throughout a wheat crop were associated with presence of abundant grasses and self-sown ‘volunteer’ wheat plants before sowing the field that became affected. Wind strength and direction had a major effect on WSMV spread by WCM to neighbouring wheat crops, the virus being carried much further downwind than upwind by westerly frontal winds. Following a dry summer and autumn in 2007, together with control of grasses and volunteer cereals before sowing and use of a different seed stock, no WSMV or WCM were found in the following wheat crop within the previously affected area or elsewhere on the same farm. In the 2007 outbreak, where the preceding summer and autumn were wet, a 40% WSMV incidence and WCM numbers that reached 4800 mites/ear at the margin of the wheat crop were associated with abundant grasses and volunteer wheat plants in adjacent pasture. WSMV incidence and WCM populations declined rapidly with increasing distance from the affected pasture. Also, wheat plants that germinated early had higher WSMV infection incidences than those that germinated later. The alternative WSMV hosts identified at these sites were volunteer wheat, annual ryegrass (Lolium rigidum), barley grass (Hordeum sp.), and wild oats (Avena fatua). In surveys outside the growing season at or near these two sites or elsewhere in the grainbelt, small burr grass (Tragus australianus), stink grass (Eragrostis cilianensis), and witch grass (Panicum capillare) were identified as additional alternative hosts.

Discussion paper: The naming of Potato virus Y strains infecting potato

Potato virus Y (PVY) strain groups are based on host response and resistance gene interactions. The strain groups PVY(O), PVY(C) and PVY(N) are well established for the isolates infecting potato in the field. A switch in the emphasis from host response to nucleotide sequence differences in the virus genomes, detection of isolates recombining sequences of different strains, and the need to recognize isolates that cause necrotic symptoms in potato tubers have led to the assignment of new acronyms, especially to isolates of the PVY(N) strain group. This discussion paper proposes that any newly found isolates should be described within the context of the original strain groups based on the original methods of distinguishing strains (i.e., tobacco and potato assays involving use of 'differential' potato cultivars). Additionally, sequence characterization of the complete genomes of isolates is highly recommended. However, it is acceptable to amend the names of PVY isolates with additional, specific codes to show that the isolate differs at the molecular, serological or phenotypic level from the typical strains within a strain group. The new isolates should preferably not be named using geographical, cultivar, or place-association designations. Since many new variants of PVY are being discovered, any new static classification system will be meaningless for the time being. A more systematic investigation and characterization of PVY from potato at the biological and molecular levels should eventually result in a biologically meaningful genetic strain concept.

Yield-limiting potential of Beet western yellows virus in Brassica napus

Losses in seed yield and quality caused by infection with Beet western yellows virus (BWYV) alone or in combination with direct feeding damage by Myzus persicae (green peach aphid) were quantified in field experiments with Brassica napus (canola, oilseed rape) in the ‘grainbelt’ region of south-western Australia. Plants infected with BWYV and infested with M. persicae were introduced into plots early to provide infection sources and spread BWYV to B. napus plants. Insecticides were applied as seed dressings and/or foliar applications to generate a wide range of BWYV incidences in plots. Colonisation by vector aphids and spread of BWYV infection were recorded in the plots of the different treatments. At sites A (Medina) and B (Badgingarra) in 2001, foliar insecticide applications were applied differentially at first, but, later, ‘blanket’ insecticide sprays were applied to all plots to exclude any direct feeding damage by aphids. When BWYV infection at sites A and B reached 96% and 100% of plants, it decreased seed yield by up to 46% and 37%, respectively. Also, variation in BWYV incidence explained 95% (site A) and 96% (site B) of the variation in yield gaps, where for each 1% increase in virus incidence there was a yield decrease of 12 (site A) and 6 (site B) kg/ha. At both sites, this yield decline was entirely because fewer seeds formed on infected plants. At site B, BWYV infection significantly diminished oil content of seeds (up to 3%), but significantly increased individual seed weight (up to 11%) and erucic acid content (up to 44%); significant increases in seed protein content (up to 6–11%) were recorded at both sites. In field experiments at sites B and C (Avondale) in 2002, insecticides were applied as seed dressings or foliar sprays. At site B, when BWYV incidence reached 98%, the overall yield loss caused by BWYV and direct M. persicae feeding damage combined was 50%. At site C, when BWYV incidence reached 97%, the overall combined yield decline caused by BWYV and direct feeding damage was 46%. This research under Australian conditions shows that, when aphids spread it to B. napus plantings such that many plants become infected at an early growth stage, BWYV has substantial yield-limiting potential in B. napus crops. Although the results represent a worst case scenario, the losses were greater than those reported previously in Europe and are cause for concern for the Australian B. napus industry. When applied at 525 g a.i./100 kg of seed, imidacloprid seed dressing controlled insecticide-resistant M. persicae and effectively suppressed spread of BWYV for 2.5 months and increased seed yield by 84% at site B and 88% at site C. Therefore, provided that mixing the insecticide with seed is sufficiently thorough, dressing seed with imidacloprid before sowing provides good prospects for control of BWYV and M. persicae in B. napus crops.

Occurrence of Beet western yellows virus and its aphid vectors in over-summering broad-leafed weeds and volunteer crop plants in the grainbelt region of south-western Australia

During the summer periods of 2000, 2001, and 2002, presence of Beet western yellows virus (BWYV) was assessed in tests on samples from at least 12 broad-leafed weed species and 5 types of volunteer crop plants growing in the grainbelt region of south-western Australia. In 2000, BWYV was detected in 2 of 35 sites in 2% of 1437 samples, whereas in 2001 and 2002 the corresponding figures were 3 of 108 sites in 0.04% of 8782 samples, and 1 of 30 sites in 0.08% of 2524 samples, respectively. The sites with infection were in northern, central, and southern grainbelt districts, and in high and medium rainfall zones. The hosts in which BWYV was detected were the weeds Citrullus lanatus (Afghan or wild melon), Conzya spp. (fleabane), Navarretia squarrosa (stinkweed), and Solanum nigrum (blackberry nightshade), and the volunteer crop plant Brassica napus (canola). Small populations of aphids were found over-summering at 28% (2000), 4% (2001), and 17% (2002) of sites, mostly infesting volunteer canola and Raphanus raphanistrum (wild radish). They occurred in high, medium, and low rainfall zones, but were only found in central and southern grainbelt districts. The predominant aphid species found was Brevicoryne brassicae, with Acyrthosiphon pisum, Brachycaudus helichrysi, Hyperomyzus lactucae, Lipaphis erysimi, Myzus persicae, and Uroleucon sonchi present occasionally. The importance of these findings in relation to the epidemiology and control of BWYV in the grainbelt is discussed.

Incidence and distribution of viruses infecting cucurbit crops in the Northern Territory and Western Australia

During 2003–04, a survey was done to determine the incidence and distribution of virus diseases infecting cucurbit crops growing in the field at Kununurra, Broome, and Carnarvon in north-western Australia, Perth in south-western Australia, and Darwin and Katherine in the Northern Territory. Overall, 43 cucurbit-growing farms and 172 crops of susceptible cultivars were sampled. From each crop, shoot samples were collected from plants chosen at random and from symptomatic plants. Shoot samples were sometimes also collected from potential alternative virus hosts (cucurbit volunteer plants and weeds). All samples were tested by enzyme-linked immunosorbent assay (ELISA) using antibodies to Cucumber mosaic virus (CMV), Papaya ringspot virus-cucurbit strain (PRSV), Squash mosaic virus (SqMV), Watermelon mosaic virus (WMV), and Zucchini yellow mosaic virus (ZYMV). Samples from one-third of the crops were also tested by tissue blot immunosorbent assay (TBIA) using generic luteovirus antibodies. Overall, 72% of farms and 56% of crops sampled were virus-infected. The growing areas with the highest incidences of virus infection were Darwin and Carnarvon, and those with the lowest incidences were Katherine and Perth. For WA, overall 78% of farms and 56% of crops were virus-infected, and in the NT the corresponding figures were 55% of farms and 54% of crops. Overall virus incidences in individual crops sometimes reached 100% infection. Crops of cucumber, melon, pumpkin, squash, and zucchini were all infected, with squash and zucchini being the most severely affected. The most prevalent viruses were ZYMV and PRSV, each being detected in 5 and 4 of 6 cucurbit-growing areas, respectively, with infected crop incidences of <1–100%. SqMV was detected in 2 cucurbit-growing areas, sometimes reaching high incidences (<1–60%). WMV and CMV were found in 3 and 4 of 6 cucurbit-growing areas, respectively, but generally at low incidences in infected crops (<1–8%). Infection with luteovirus was found in 3 growing areas but only occurred in 16% of crops. Beet western yellows virus was detected once but at least one other luteovirus was also present. Infection of individual crops by more than 1 virus was common, with up to 4 viruses found within the same crop. Virus-resistant pumpkin cultivars (6 crops) had little infection when adjacent virus-susceptible cucurbit crops had high virus incidences. Viruses were detected in cucurbit volunteer plants and weeds, suggesting that they may act as important reservoirs for spread to nearby cucurbit crops. In general, established cucurbit-growing farms in close proximity to others and with poor crop hygiene suffered most from virus epidemics, whereas isolated farms with large-sized crops or that had only recently started growing cucurbits had less infection. The extent of infection revealed in this survey, and the financial losses to growers resulting from virus-induced yield losses and high fruit rejection rates, are cause for concern for the Australian cucurbit industry.

Incidence and distribution of Barley yellow dwarf virus and Cereal yellow dwarf virus in over-summering grasses in a Mediterranean-type environment

During the summer periods of 2000 and 2001, incidences of infection with Barley yellow dwarf virus (BYDV) and Cereal yellow dwarf virus (CYDV) were determined in grass weeds and volunteer cereals surviving at isolated sites throughout the grainbelt of south-western Australia, which has a Mediterranean-type climate. Samples of Cynodon dactylon, Eragrostis curvula, Erharta calycina, Pennisetum clandestinum, and volunteer cereals (mostly wheat) were tested for BYDV (serotypes MAV, PAV and RMV) and CYDV (serotype RPV), and those of at least 19 other grass species were tested for BYDV only (serotypes PAV and MAV). In 2000, BYDV and/or CYDV were detected in 33% of 192 sites in 0.7% of 26 700 samples, and in 2001 the corresponding values were 19% of 176 sites and 0.5% of 21 953 samples. Infection was distributed relatively evenly throughout the different annual average rainfall zones of the grainbelt, but when sites were categorised according to actual rainfall for late spring to early autumn, the proportion of sites and samples infected increased where such rainfall exceeded 300 mm. In both summer sampling periods, the most abundant grass species were C. dactylon and E. curvula, with BYDV and/or CYDV being detected in 0.1–0.6% and 0.1–0.5% of samples, respectively. The corresponding incidences were 0–1% for Erharta calycina, 7–8% for P. clandestinum, and 0.2–2% for volunteer wheat. The most abundant species tested for BYDV only were Chloris truncata and Digitaria sanguinalis, with infection incidences of 0.2–0.7 and 0.2–0.3%, respectively. Chloris virgata (2–3%) and Urochloa panicoides (0.3–0.6%) were the only other infected species. Within individual sites and host species, the greatest incidences of CYDV were in P. clandestinum (23% in 2000 and 18% in 2001) and of BYDV in Chloris virgata (14% with PAV and 12% with MAV in 2000). Small populations of grass-infesting aphids were found over-summering at 26% (2000) and 3% (2001) of sites and occurred in all 3 annual rainfall zones. The predominant species was Hysteroneura setariae, but Rhopalosiphum maidis, R. padi, and Sitobion miscanthi occurred occasionally. Presence of over-summering BYDV, CYDV, and aphids in all rainfall zones has important implications for virus spread to cereal crops throughout the grainbelt.

Further studies on Carrot virus Y: hosts, symptomatology, search for resistance, and tests for seed transmissibility

Carrot virus Y (CarVY) was studied to provide information on its host range and symptoms, identify any alternative natural hosts and sources of host resistance in carrot germplasm, and determine whether it is seed-borne. Twenty-two species belonging to the Apiaceae were inoculated with CarVY by viruliferous aphids in the glasshouse. Systemic infection with CarVY developed in carrot itself, 4 other Daucus species, 5 herbs, 1 naturalised weed, and 2 Australian native plants. When 7 of these host species were exposed to infection in the field, all became infected systemically. In both glasshouse and field, the types of symptoms that developed in infected plants and their severity varied widely from host to host. Following inoculation with infective sap, the virus was detected in inoculated leaves of 1 additional species in the Apiacaeae, and 2 species of Chenopodiaceae. A field survey did not reveal any alternative hosts likely to be important as CarVY infection reservoirs. When 34 accessions of wild carrot germplasm and 16 of other Daucus spp. were inoculated with infective aphids, symptom severity varied widely among accessions but no source of extreme resistance to CarVY was found. Tests on seedlings grown from seed collected from individual infected plants or field plantings (most with CarVY incidences of >92%) of cultivated carrot (34 135 seeds), wild carrot (20 978 seeds), Anethum graveolens (22 921 seeds), and 3 other host species (3304 seeds) did not detect any seed transmission of CarVY. The implications of these results for control of the virus in carrot crops, minimising the losses it causes, and avoiding its introduction to new locations are discussed.

Occurrence of virus infection in seed stocks and 3-year-old pastures of lucerne (Medicago sativa)

In tests on seed samples from 26 commercial seed stocks of lucerne (Medicago sativa) to be sown in south-western Australia in 2001, infection with Alfalfa mosaic virus (AMV) was found in 21 and Cucumber mosaic virus (CMV) in 3 of them. Bean yellow mosaic virus (BYMV) and Pea seed-borne mosaic virus (PSbMV) were not detected in any. Incidences of infection within individual affected seed samples were 0.1–4% (AMV) and 0.1–0.3% (CMV), and the infected seed stocks were from 3 (CMV) and at least 11 (AMV) different lucerne cultivars. In a survey of 31 three-year-old lucerne pastures in the same region in 2001, in randomly collected samples, AMV was found in 30 and luteovirus infection in 11 pastures. Pastures in high, medium, and low rainfall zones were all infected. Incidences of AMV within individual infected pastures were high, with 50–98% of plants infected in 20 of them and only 3 having <10% infection, but luteovirus incidences were only 1–5%. In addition to various cultivar mixtures, at least 8 (AMV) and 3 (luteoviruses) different individual lucerne cultivars were infected. When the species of luteovirus present were identified, they were Bean leaf roll virus, Beet western yellows virus ( = Turnip yellows virus), or Subterranean clover red leaf virus ( = Soybean dwarf virus). CMV and legume-infecting potyviruses (BYMV, PSbMV, and Clover yellow vein virus) were not detected in any of the lucerne samples. Acyrthosiphon kondoi infestation was common in the samples collected, and A. pisum and Aphis craccivora were also found. Widespread infection in lucerne stands, and their frequent colonisation by aphid vectors, are cause for concern not only because of virus-induced production losses in lucerne itself but also because they provide virus infection reservoirs for spread to nearby grain legume crops and annual legume pastures.

Lettuce big-vein disease: sources, patterns of spread, and losses

Most batches of lettuce seedlings taken over an 18-month period from a vegetable nursery were infested with lettuce big-vein disease (LBVD) with an up to 31% incidence. Using lettuce seedlings in bait tests, contamination was detected at the nursery in potting mix composted for different periods and in dirt from under the benches, and at the bark supplier's site in this ingredient of the potting mix and waste 'bark' from the ground. In a field experiment in which lettuce seedlings from the infested nursery were inoculated with infested roots or left uninoculated before transplanting into subplots on land with no history of lettuce planting, disease progress followed a sigmoid curve with the former but an almost straight line with the latter. However, significant clustering of symptomatic plants was found only in the subplot with the uninoculated plants. Leaf symptoms of LBVD were more severe in lettuces infested later, whereas symptoms in those infested earlier were obvious initially but then became milder. The disease impaired formation of hearts: the proportion of symptomatic plants that lacked hearts was 24–36% when leaf symptoms first appeared 5–7 weeks after transplanting, but 14–16% after 8–9 weeks. When leaf symptoms first appeared at 5–6 weeks, there was a fresh weight loss of 14–15% for heads (all plants) and 39% for hearts (excluding plants without hearts). When leaf symptoms first appeared 7 weeks after transplanting, there was no significant yield loss for heads and only a 14% loss for hearts. At 8–9 weeks, there were no significant yield losses for heads or hearts.

Yield losses caused by virus infection in four combinations of non-persistently aphid-transmitted virus and cool-season crop legume

Field experiments provided quantitative information on the yield losses caused by virus infection within 4 different combinations of non-persistently aphid-transmitted virus and cool-season crop legume: Alfalfa mosaic virus (AMV) in chickpea, faba bean and lentil, and Cucumber mosaic virus (CMV) in lentil. Virus infection foci were introduced into plots and naturally occurring aphids spread infection from these to the other plants. Plants were tagged individually when typical virus symptoms first appeared during the growing period. Paired plant comparisons between symptomatic and asymptomatic plants were made to measure different yield loss parameters. Late infection with AMV in faba bean cv. Fiord diminished shoot dry weight by 41% and seed yield by 45%, but plants infected earlier recovered sufficiently from their initial shock reaction not to produce significant yield losses. In plants of lentil cv. Matilda first showing symptoms at different times, infection with AMV decreased shoot dry weight by 74–76%, seed yield by 81–87% and individual seed weight by 10–21%, while CMV diminished shoot dry weight by 72–81%, seed yield by 80–90% and individual seed yield by 17–25%. Early infection with AMV killed plants of chickpea cv. Tyson while later infection decreased shoot dry weight by 50%, seed yield by 98% and individual seed weight by 90%. The first tentative evidence for seed transmission of AMV in faba bean is reported with a transmission rate of 0.04%.

The complete nucleotide sequence of Subterranean clover mottle virus

The complete nucleotide sequence of Subterranean clover mottle virus (SCMoV) genomic RNA has been determined. The SCMoV genome is 4,258 nucleotides in length. It shares most nucleotide and amino acid sequence identity with the genome of Lucerne transient streak virus (LTSV). SCMoV RNA encodes four overlapping open reading frames and has a genome organisation similar to that of Cocksfoot mottle virus (CfMV). ORF1 and ORF4 are predicted to encode single proteins. ORF2 is predicted to encode two proteins that are derived from a -1 translational frameshift between two overlapping reading frames (ORF2a and ORF2b). A search of amino acid databases did not find a significant match for ORF1 and the function of this protein remains unclear. ORF2a contains a motif typical of chymotrypsin-like serine proteases and ORF2b has motifs characteristically present in positive-stranded RNA-dependent RNA polymerases. ORF4 is likely to be expressed from a subgenomic RNA and encodes the viral coat protein. The ORF2a/ORF2b overlapping gene expression strategy used by SCMoV and CfMV is similar to that of the poleroviruses and differ from that of other published sobemoviruses. These results suggest that the sobemoviruses could now be divided into two distinct subgroups based on those that express the RNA-dependent RNA polymerase from a single, in-frame polyprotein, and those that express it via a -1 translational frameshifting mechanism.

Yield limiting potential of necrotic and non-necrotic strains of Bean yellow mosaic virus in narrow-leafed lupin (Lupinus angustifolius)

The yield losses caused by necrotic and non-necrotic strains of Bean yellow mosaic virus (BYMV) in narrow-leafed lupin (Lupinus angustifolius) were quantified in field experiments. Clover plants infected with either were introduced into plots to provide infection sources, and aphids spread infection to the lupin plants. When the effects of virus infection were examined in individual lupin plants infected with necrotic BYMV, they were killed by early infection so there was no seed production. With late infection, shoot dry wt, seed yield, and seed number were decreased by at least 55%, 80%, and 74%, respectively. With non-necrotic BYMV, shoot dry wt, seed yield, and seed number diminished with increasing duration of plant infection, these decreases ranging over 27–88%, 48–99%, and 35–98% for late to early infection, respectively. In partially infected stands in which both necrotic and non-necrotic BYMV were spreading, an additional incidence of 28% in plots with introduced non-necrotic strain foci over that in plots without introduced foci was sufficient to decrease overall seed yield significantly. However, an additional incidence of 10% was insufficient to do so in plots with introduced necrotic strain foci. In plots into which different numbers of clover plants infected with non-necrotic BYMV were introduced, subsequent incidence of infection depended on the magnitude of the initial virus source, and yield was decreased by 21–24%, 31–43%, and 64–66% with 4, 8, or 16 foci/plot, respectively. With both types of strain, yield loss in infected plants was mainly due to failure to produce any seed or to fewer seeds being produced, but smaller seed size also contributed. These results show that non-necrotic strains of BYMV have considerable yield-limiting potential in narrow-leafed lupin crops despite causing milder symptoms than necrotic strains. No evidence was obtained of seed-transmission of non-necrotic BYMV in narrow-leafed lupin, but a 0.2% seed transmission rate was detected in yellow lupin (Lupinus luteus).

A non-aphid-transmissible isolate of bean yellow mosaic potyvirus has an altered NAG motif in its coat protein

An isolate of Bean yellow mosaic virus (BYMV) not transmitted by aphids (NAT) was compared with the aphid-transmissible isolate (MI) from which it was derived. For each isolate, the sequence of the coat protein and parts of the helper component was determined. A single nucleotide substitution caused a NAG to NAS alteration in the coat protein of the non aphid-transmissible isolate. Loss of aphid transmissibility in isolate BYMV(MI)-NAT was most likely caused by this mutation within the NAG motif. Systemic movement and accumulation of the virus in infected plants were not affected by the mutation.

Quantification of Yield Losses Caused by Barley yellow dwarf virus in Wheat and Oats

Grain yield data obtained from five field experiments in Western Australia from 1992 to 1994, in which insecticide applications suppressed the spread of Barley yellow dwarf virus (BYDV) in wheat and oats, were used to quantify the relationships between incidence of BYDV and yield gaps, 500-seed weight, and percent shriveled grain. Yield gaps ranged from 0 to 2,700 kg/ha, and the relationship between yield gap and incidence of BYDV was always linear. Single point yield loss models revealed that BYDV infection explained most of the variation in yield gaps. There was a significant linear relationship between incidence of BYDV and 500-seed weight for wheat, but not for oats. The percent shriveled grain always increased with an increase in incidence of BYDV in wheat but not in oats. Cost-benefit relationships were determined for the return on investment when deploying imidacloprid-treated seed and/or one or two foliar applications of pyrethroid insecticides to reduce incidence of BYDV and to decrease the yield gaps in wheat and oats due to BYDV.

Incidence of virus infection in experimental plots, commercial crops, and seed stocks of cool season crop legumes

Experimental plots of cool season crop legumes growing at diverse locations in Western Australia were inspected for plants with suspect virus symptoms over 4 growing seasons (1994, 1997, 1998, 1999), and plant samples were tested for infection with alfalfa mosaic (AMV), bean yellow mosaic (BYMV), cucumber mosaic (CMV), and pea seed-borne mosaic (PSbMV) viruses. All 4 viruses were detected in faba bean (Vicia faba); BYMV, CMV, and PSbMV in field pea (Pisum sativum); AMV, CMV, and PSbMV in lentil (Lens culinaris); and AMV and CMV in chickpea (Cicer arietinum). Among minor crop species, AMV, BYMV, and CMV were found in narbon bean (V. narbonensis) and grass pea (Lathyrus sativus); BYMV and CMV in dwarf chickling (L. cicera); BYMV in bitter vetch (V. e r v i l i a ) and L. clymenum; and AMV in fenugreek (Trigonella foenum-graecum). Incidences of individual viruses varied widely from site to site but plot infection sometimes reached 100%. Symptom severity varied widely with virus–crop combination. In large-scale surveys of commercial crops of field pea and faba bean over 2 (1998, 1999) and 3 (1994, 1998, 1999) growing seasons, respectively, randomly collected samples from each crop were tested for presence of AMV, BYMV, CMV, and PSbMV. In 1999 they were also tested for beet western yellows virus (BWYV). All 5 viruses were detected in both species. BWYV was found in 35% of faba bean and 56% of the field pea crops sampled in 1999, with incidences of infection in individual crops up to 40% and 49%, respectively. PSbMV was found in 42% and BYMV in 18% of field pea crops in 1999. In individual crops, highest infection incidences of BYMV and PSbMV detected were 31% for BYMV in faba bean in 1998 and 9% for PSbMV in field pea in 1999. CMV and AMV incidences in both species never exceeded 7% of crops or 4% of plants within individual crops. Infection by 2 different viruses within individual crops was common, even 3 were sometimes found. Cultivars infected with most viruses were Fiesta and Fiord for faba bean, and Dundale, Laura, and Magnet for field pea. BYMV was detected in the crop tested of dwarf chickling. In tests on seed samples from Western Australia of 30 commercial seed stocks of field pea, 11 of faba bean, and 50 of chickpea, PSbMV was detected in 11, 1, and 1, respectively; CMV in 1, 1, and 3; BYMV in 3, 1, and 0; and AMV in 0, 0, and 1. This appears to be the first record of seed transmission of CMV in pea and faba bean. Seed samples from Victoria were also found to contain viruses: PSbMV in pea and AMV in lentil. Widespread infection with viruses in evaluation plots and commercial crops of cool season crop legumes is a cause for concern, especially where individual crop incidences are high and 2 or more viruses are present. Sowing of infected seed stocks leads to introduction of randomly dispersed sources of virus infection within the crop sown, resulting in spread of infection and yield losses. Appropriate control measures are discussed.

Cucumber mosaic cucumovirus infection of cool-season crop, annual pasture, and forage legumes: susceptibility, sensitivity, and seed transmission

Seven field experiments were done in 19944—98 to determine the relative susceptibilities and sensitivities of a wide range of alternative crop, annual pasture, and forage legumes to infection with cucumber mosaic virus (CMV). Seed harvested from some species was tested for seed transmission of the virus. Most of the 24 genotypes of Cicer arietinum and 39 of Lens culinaris tested in 2 replicated field experiments were ranked as highly susceptible or susceptible; moderate resistance was recorded in 8Lens culinaris genotypes, the most resistant of which was ILL7163, and in C. arietinum cv. Amethyst Mutant. Sensitivity varied from low to high in different Lens culinaris genotypes, whereas in C. arietinum they were all sensitive or highly sensitive. In 4 other experiments, 12 species (49 genotypes) of other crop legumes were ranked as follows: Vicia narbonensis susceptible to moderately resistant, V. ervilia susceptible, Pisum sativum resistant, and V. faba resistant to potentially highly resistant; Lathyrus cicera,L. clymenum, L. ochrus, L. sativus, L. tingitanus, V. benghalensis, V. monantha, and V. s a t i v a were not infected. V. ervilia andV. faba were very sensitive to infection, but V. narbonensis had intermediate sensitivity and P. s a t i v u m was tolerant. When single genotypes of each of 16 pasture and forage species were tested in 2 replicated field experiments, 1 was highly susceptible, 3 were susceptible, 9 moderately resistant, 2 resistant, and 1 was potentially highly resistant. The 4 most susceptible were the sensitive species Trifolium incarnatum and T. isthmocarpum and the intermediately sensitive species T. michelianum and T. vesiculosum. T. squarrosum (intermediate sensitivity) and T. spumosum (very sensitive) were resistant and Ornithopus sativus was not infected. In sap inoculations, L. ochrus,L. sativus, and P. sativum occasionally became infected. In aphid inoculations,Lens culinaris ILL7163 and V. faba became infected only rarely and V. benghalensis cv. Popany developed a systemic hypersensitive reaction. The following were not infected in the field or glasshouse: L. cicera ATC80521, L. clymenum C7022, O. sativus cv. Cadiz, and V. sativa cv. Languedoc.Seed transmission of CMV was detected for the first time in one crop species, V. narbonensis(0.1mp;mdash;0.8%), and confirmed in C. arietinum (0.2–0.3%) and Lens culinaris (0.3%). It was also detected in T. cherleri (0.05%), T. clypeatum (0.05%), T. dasyurum (0.1%), T. incarnatum (5%), T. purpureum (0.04%), T. spumosum (0.5%), T. squarrosum (0.1%), and T. vesiculosum (1%), but not in 8 other pasture or forage species. The high susceptibility and sensitivity to CMV of some alternative crop, annual pasture, and forage legumes is cause for concern, especially when they are intended for sowing in CMV-prone high rainfall zones. Infection of seed stocks with CMV is also of concern as it leads to inadvertent introductions of the virus.

Alfalfa mosaic and pea seed-borne mosaic viruses in cool season crop, annual pasture, and forage legumes: susceptibility, sensitivity, and seed transmission

Field experiments determined the susceptibilities and sensitivities of a wide range of crop, annual pasture, and forage legumes to infection with alfalfa mosaic (AMV) and pea seed-borne mosaic (PSbMV) viruses. Seed harvested from most of the species was tested for virus seed transmission. With AMV, all 23 Cicer arietinum genotypes tested were ranked as highly susceptible, and 9 out of 19 Lens culinaris genotypes as highly susceptible, 8 susceptible, 1 moderately resistant, and 1 resistant. Genotypes of Vicia narbonensis (5), Lathyrus cicera (5), L. sativus (5), L. ochrus(2), V. sativa (1), and V. benghalensis (1) were highly susceptible, susceptible, or moderately resistant. Genotypes of Pisum sativum (5) and V. faba(3) were susceptible, moderately resistant, or resistant but 1 genotype of V. faba was not found infected. Sensitivities ranged from low in L. ochrus to high in some genotypes of most species tested exceptV. benghalensis. The 20 genotypes (19 species) of pasture and forage legumes ranged from ‘not found infected’ in Hedysarum coronarium to ‘highly susceptible’ in Ornithopus sativus and Trifolium resupinatum. Sensitivity varied from low in T. michelianum to very high in Biserrula pelecinusand Ornithopus sativus. With PSbMV, the genotypes ofP. s a t i v u m (17), V. narbonensis (5), and L. cicera(3) were ranked as highly susceptible, susceptible, or moderately resistant, while those of L. ochrus(3), V. faba(6), V. sativa (3), V. benghalensis (2) and V. ervilia(1) were either moderately resistant or resistant. The genotypes of C. arietinum (6) and Lens culinaris (6) were all resistant. With L. sativus, 2 genotypes were resistant and 1 was not found infected. Sensitivities ranged from low in some P. sativum genotypes to high in some ofL. ciceraand V. narbonensis. The seed coats of 9 crop legume species developed necrotic ring markings, a serious quality defect due to PSbMV infection. Of the 19 genotypes (1/species) of pasture and forage legumes, 4 were resistant with only symptomless infection developing and the remainder not found infected. In glasshouse inoculations to genotypes not found infected in the field, AMV infected V. faba cv. Ascot systemically butH. coronarium cv. Grimaldi (with AMV) and L. sativus BIO L254 (with PSbMV) only became infected in inoculated leaves, H. coronarium developing a localised hypersensitive reaction. Seed transmission of AMV was detected in L. cicera(2%), L. sativus (0.9–4%), V. benghalensis(0.9%), V. narbonensis (0.1%), and V. sativa (0.7%). It was also found in 15 pasture and forage legume species, ranging from 0.05% in T. michelianum to 7% in Trigonella balansae. Seed transmission of PSbMV was detected in L. cicera(0.4%), L. clymenum (5%), L. ochrus (0.7%), L. sativus (1%), P sativum(1–18%), V. benghalensis (0.1%), V. faba (2%), and V. sativa (0.3%). The implications of these findings and their importance to the management of these and other virus diseases are discussed.

Bean yellow mosaic potyvirus infection of alternative annual pasture, forage, and cool season crop legumes: susceptibility, sensitivity, and seed transmission

Seven field and 5 glasshouse experiments were done during 1994–98 to determine the relative susceptibilities and sensitivities of a wide range of alternative annual pasture, forage, and crop legumes to infection with isolate MI of bean yellow mosaic virus (BYMV). Seed harvested from some species was also tested for seed transmission of the virus. Seven of 18 genotypes belonging to 17 species of annual pasture and forage legumes evaluated in 2 replicated field experiments were ranked as highly susceptible to BYMV, 7 as susceptible, 2 as moderately resistant, 1 as resistant, and 1 as highly resistant. The most susceptible and sensitive were Biserrula pelecinus, Trifolium cherleri, T. incarnatum, and T. spumosum. Ornithopus sativus was resistant but sensitive, whereas Hedysarum coronarium was highly resistant. H. coronarium was not infected when manually inoculated repeatedly with 3 different BYMV isolates. Seventy-three of the 94 genotypes of 7 crop legume species tested in the same replicated field experiments were ranked as highly susceptible, including 58/68 of Lens culinaris. Of the remaining genotypes, 6 were susceptible, 5 moderately resistant, 9 resistant, and 1 highly resistant. Five other crop legumes were included in other field experiments in which these species were ranked as highly susceptible (1) or resistant (4). Overall, the most susceptible and sensitive crop legume species were Lens culinaris (most genotypes), Lathyrus cicera, L. ochrus, and Vicia narbonensis. Lathyrus sativus (3 genotypes only), V. sativa (4 genotypes), Cicer arietinum, Pisum sativum, and V. faba were resistant to isolate MI, and Lens culinaris ILL7163 was highly resistant. When infected, C. arietinum was ranked as highly sensitive but symptoms within the other resistant crop species varied in sensitivity between genotypes. Extreme resistance was confirmed in Lens culinaris ILL7163 when it was manually and aphid-inoculated repeatedly with 3 different BYMV isolates. When testing seedlings for seed transmission of BYMV, germination on moist paper towels before testing usually proved more effective than growing in soil in the glasshouse. Low rates of seed transmission of BYMV (0.03–1%) were detected in 9 alternative pasture or forage and 3 alternative crop legume species. This is the first report of seed transmission of BYMV in these species. The pasture or forage species with the highest seed transmission rates were T. clypeatum and T. spumosum (both 1%). The crop legume species in which seed transmission was found were L. cicera (0.1%), L. sativus (0.2%), and V. sativa (0.5%). The high susceptibility and sensitivity to BYMV in some alternative annual pasture, forage, and crop legumes is a cause for concern, especially when they are intended for sowing in BYMV-prone high rainfall zones. Seed transmission of BYMV also leads to inadvertent introduction of the virus to new sites.

Viruses infecting canola (Bassica napus) in south-west Australia: incidence, distribution, spread, and infection reservoir in wild radish (Raphanus raphinistrum)

Over 2 growing seasons, the incidences of infection with beet western yellows (BWYV), cauliflower mosaic (CaMV), and turnip mosaic (TuMV) viruses were determined in canola (Brassica napus) crops growing in the agricultural area of south-west Australia. Tissue blot immunoassay was used to detect BWYV and enzyme-linked immunosorbent assay to detect CaMV and TuMV. In 1998, BWYV was detected in 59% of 159 crops surveyed, whereas in 1999 it was found in 66% of 56 crops. Incidences within individual infected crops were 1–65% (1998) and 1–61% (1999). Infection occurred widely in high and medium rainfall zones, but was also readily detected in the low rainfall zone. In addition, BWYV was found in canola samples from 5 sites in New South Wales. Most cultivars tested (9 of 10) in the canola crop survey were infected with BWYV. No clear relationship was found between BWYV infection and any particular type of disease symptom. Overall, the incidence of BWYV at the crop edge was marginally greater than that inside the crop. CaMV was detected in 27% of 143 crops in 1998 but in only 2 of 47 in 1999. Incidences within individual infected crops were 1–17% in 1998 but only 1% in 1999. CaMV infected 6 of 10 cultivars and was present in high, medium, and low rainfall zones. Obvious chlorotic ringspot symptoms were associated with CaMV infection. TuMV was detected in 5% of 139 crops in 1998 but in only 1 of 47 from 1999. Incidences within the individual infected crops were 1–5% in 1998 and 1% in 1999; 3 of 10 cultivars were infected and it was found in high and medium rainfall zones. BWYV, CaMV, and TuMV were all found infecting wild radish (Raphanus raphinistrum). In general, incidences of BWYV were greater in wild radish than in canola. In 1998, BWYV was detected in wild radish at 9 of 12 sites sampled in 5 of 6 districts, with infection incidences up to 48%. In 1999, it was detected at all 10 sites sampled in 7 districts, with incidences up to 96%. Infected samples came from all rainfall zones, and from several different types of sites, some of which were distant from canola crops. Despite the presence of possible viral symptoms in wild radish, none was clearly associated with BWYV infection. In contrast, TuMV caused obvious mottle and ‘oak leaf’ patterns in wild radish plants. The finding of widespread virus infection in canola crops and a substantial virus reservoir in wild radish weeds is cause for concern to the canola industry.