Fusarium spp., Cylindrocarpon spp., and Environmental Stress in the Etiology of a Canker Disease of Cold-Stored Fruit and Nut Tree Seedlings in California

Identification and Pathogenicity of Fusarium Species Associated with Young Vine Decline in California

Grapevine trunk diseases are caused by a broad diversity of fungal taxa that have serious impacts on the worldwide viticulture industry due to significant reductions in vineyards yield and lifespan. Field surveys carried out from 2018 to 2022 in California nurseries and young vineyards revealed a high incidence of Fusarium. Since Fusarium species are important pathogens of other perennial crops, the present study aimed to identify and determine the pathogenicity of the Fusarium species on grapevines. Morphology of the fungal colonies coupled with multilocus phylogenetic analyses using nucleotide sequences of the translation elongation factor 1-alpha (tef1) and the RNA polymerase II second largest subunit (rpb2) genes revealed the occurrence of 10 species clustering in six species complexes, namely F. fujikuroi (FFSC), F. oxysporum (FOSC), F. solani (FSSC), F. sambucinum (FSAMSC), F. incarnatum-equiseti (FIESC), and F. tricinctum (FTSC) species complexes. The species F. annulatum (FFSC) was the most prevalent in samples from both symptomatic young vineyards (73.5% incidence) and nursery propagation material (62.5% incidence). Pathogenicity of the 10 most frequent species was confirmed by fulfilling Koch's postulates on living woody tissue of 1103 Paulsen rootstocks. Our results suggest that Fusarium spp. are involved in the development of young vine decline, probably as opportunistic pathogens when grapevines are under stress conditions.

Characterization and Pathogenicity of Fungal Species Associated with Dieback of Apple Trees in Northern Italy

Severe dieback symptoms were recently observed on apple (Malus × domestica) trees in Northern Italy, representing a growing concern for producers. Surveys were conducted over a 3-year period (2019 to 2021), and five apple orchards, from 5 to 12 years old, were monitored. A total of 33 fungal isolates isolated from symptomatic plants was selected for characterization. The species identification was achieved through multilocus phylogenetic analyses performed on sequences of three genomic loci (ITS, tub2, and tef1). Morphological features were assessed, and the average growth rate at different temperatures was determined. Seven species were identified in association with dieback of apple trees: Botryosphaeria dothidea, Cadophora luteo-olivacea, Diaporthe rudis, Diplodia seriata, Eutypa lata, Kalmusia longispora, and Paraconiothyrium brasiliense. All the species were pathogenic when inoculated on healthy apple plants. B. dothidea resulted in the most aggressive infections. This study provides an insight into the fungal species diversity associated with apple dieback and provides basis for further investigations to assess the phytosanitary status of plant materials to recommend and implement effective management strategies.

Profiling Walnut Fungal Pathobiome Associated with Walnut Dieback Using Community-Targeted DNA Metabarcoding

Walnut dieback can be caused by several fungal pathogenic species, which are associated with symptoms ranging from branch dieback to fruit necrosis and blight, challenging the one pathogen-one disease concept. Therefore, an accurate and extensive description of the walnut fungal pathobiome is crucial. To this end, DNA metabarcoding represents a powerful approach provided that bioinformatic pipelines are evaluated to avoid misinterpretation. In this context, this study aimed to determine (i) the performance of five primer pairs targeting the ITS region in amplifying genera of interest and estimating their relative abundance based on mock communities and (ii) the degree of taxonomic resolution using phylogenetic trees. Furthermore, our pipelines were also applied to DNA sequences from symptomatic walnut husks and twigs. Overall, our results showed that the ITS2 region was a better barcode than ITS1 and ITS, resulting in significantly higher sensitivity and/or similarity of composition values. The ITS3/ITS4_KYO1 primer set allowed to cover a wider range of fungal diversity, compared to the other primer sets also targeting the ITS2 region, namely, GTAA and GTAAm. Adding an extraction step to the ITS2 sequence influenced both positively and negatively the taxonomic resolution at the genus and species level, depending on the primer pair considered. Taken together, these results suggested that Kyo set without ITS2 extraction was the best pipeline to assess the broadest fungal diversity, with a more accurate taxonomic assignment, in walnut organs with dieback symptoms.

Fusarioid community diversity associated with conifer seedlings in forest nurseries across the contiguous USA

INTRODUCTION

Fusarioid fungi that cause damping-off and root diseases can result in significant losses to conifer crops produced in forest nurseries across the USA. These nurseries are vital to reforestation and forest restoration efforts. Understanding the diversity of Fusarioid fungi associated with damping-off and root diseases of conifer seedlings can provide an approach for targeted management techniques to limit seedling losses and pathogen spread to novel landscapes.

METHODS

This study identifies 26 Fusarium spp. (F. acuminatum, F. annulatum, F. avenaceum, F. brachygibbosum, F. clavus, F. commune, F. cugenangense, F. diversisporum, F. elaeagni, F. elaeidis, F. flocciferum, F. fredkrugeri, F. fujikuroi, F. grosmichelii, F. ipomoeae, F. lactis, F. languescens, F. luffae, F. odoratissimum, F. oxysporum, F. queenslandicum, F. redolens, F. torulosum, F. triseptatum, F. vanleeuwenii, & F. verticillioides), 15 potential species within Fusarium and Neocosmospora species complexes (two from F. fujikuroi species complex, nine from F. oxysporum species complex, three from F. tricinctum species complex, and one from Neocosmospora species complex), and four Neocosmospora spp. (N. falciforme, N. metavorans, N. pisi, & N. solani) and associated host information collected from conifer-producing nurseries across the contiguous USA.

RESULTS

Phylogenetic analyses identified Fusarioid fungi haplotypes that were associated with 1) host specificity, 2) localization to geographic regions, or 3) generalists found on multiple hosts across diverse geographic regions.

DISCUSSION

The haplotypes and novel species identified on conifer seedlings should be considered for further analysis to determine pathogenicity, pathogen spread, and assess management practices.

First Report of Fusarium acuminatum Causing Leaf Blight on Garlic in China

In June 2021, leaf blight symptoms were detected on garlic plants (Allium sativum) in southeastern Jiangsu (Nantong municipality; 120.61° E, 33.25° N) in China. Two-month-old garlic plants exhibited leaf tip die back and light brown lesions in new and old leaves (Figure 1). The symptoms were observed in 40% of the plants in a 60-square-meters commercial field surrounded by rice fields, and were similar to those reported for Botrytis porri, Septoria allii and Stemphylium eturmiunum causing leaf blight on garlic (Dumin et al. 2021; Park et al. 2013; Zhang et al. 2009). Six samples of symptomatic tissue collected in Nantong municipality, approximately 1 cm2 in size, were sterilized in 2% NaOCl for 15 min and washed twice with sterile ddH2O. The pathogen was isolated from all collected samples on PDA medium, containing 50 µg/mL chloramphenicol, at 26°C. Pink colonies with orange pigmentation were observed after 7 days. Internal transcribed spacer (ITS), elongation factor 1-α (EF1-α), RNA polymerase II largest subunit (RPB1) and RNA polymerase II second largest subunit (RPB2) genes were amplified using ITS1/ITS4, EF1-728F/EF1-986R, RPB1-R8/RPB1-F5 and fRPB2-7CF/fRPB2-11aR primers, respectively. A total of 17 isolates were obtained, with nine of the isolates sharing the same sequences (strain NJC21), six of the isolates sharing the same sequences (strain NJC22), and the other two isolates showing different sequences (strains NJC23 and NJC24). The obtained sequences were submitted in GenBank under accession numbers OL655398-OL655401 (ITS), and OL741712-OL741723 (EF1-α, RPB1, RPB2). The obtained ITS sequences shared >99% homology to the ITS gene from F. acuminatum IBE000006 (EF531232), the EF1-α sequences shared 99% homology to the EF1-α gene from F. acuminatum F1514 (LC469785), the RPB1 sequences shared >99% homology to the RPB1 gene from F. acuminatum JW 289003 (MZ921675), and the RPB2 sequences shared 100% homology to the RPB2 gene from F. acuminatum NL19-077002 (MZ921813) or 100% homology to the RPB2 gene from F. acuminatum MD1 (MW164629). A phylogenetic tree was constructed using MEGA7 with related Fusarium strains (Figure 2). Microscope observations after incubation in potato-sucrose-agar (PSA) medium showed the presence of oval microconidia, fusiform macroconidia, septate mycelium and chlamydospores, and agree with the morphology of F. acuminatum (Marek et al. 2013). The pathogenicity was screened with two-week-old wounded and non-wounded garlic plants using a 1 × 106 spores/mL solution (20 µL). Sterile ddH2O was used in the control experiment. The inoculated plants were incubated at 26°C and 60% relative humidity for 3 days, detecting similar lesions compared to those observed in the field. The pathogen was recovered from 5 different lesions, from different plants, and its identity was confirmed by sequence analysis. Recently, F. acuminatum was reported to cause garlic bulb rot in Serbia (Ignjatov et al. 2017). Although F. acuminatum is well known as a causal agent of root rot (Li et al. 2021; Tang et al. 2021), F. acuminatum has also been found causing leaf blight on onion (Parkunan et al. 2013) and muskmelon (Yu et al. 2021). This is the first report of F. acuminatum causing leaf blight on garlic, demonstrating the host and tissue promiscuity of this pathogen. China is the largest producer of garlic in the world with nearly 20 million tons harvested in 2020. This report will help to better understand the pathogens that are affecting garlic production in China.

Genetic Diversity and Potential Inoculum Sources of Fusarium Species Causing Cankers in Bareroot-Propagated Almond Trees in California Nurseries

Previous research determined that Fusarium acuminatum and F. avenaceum are important causal agents of a canker disease in bareroot-propagated fruit and nut trees in California that emerges during cold storage or after transplanting. The disease largely disappeared after 2001, but it reemerged in 2011 in almond trees in at least one nursery. This motivated further study of the etiology and epidemiology of the disease by undertaking studies to determine distribution of the pathogens throughout almond nursery propagation systems and trace possible sources of inoculum. Research initiated in 2013 detected pathogenic Fusarium spp. throughout the almond propagation system, including in healthy trees, in soils, on wheat rotation crops, on equipment, and in the cold-storage facility air. In addition to the two Fusarium spp. implicated previously, F. brachygibbosum and a new Fusarium species, F. californicum, were found to be pathogenic on almond trees. Multilocus sequence typing and somatic compatibility testing confirmed that isolates within a species collected from different materials in the nursery were all highly genetically similar and likely of one clonal lineage. These findings affirm that equipment surfaces, wheat rotation crops, soil, cold-storage facility air, and asymptomatic almond tree materials (i.e., rootstock cuttings, budwood, and scions) can potentially contribute inoculum to increase disease prevalence and severity.

Molecular and Pathogenic Characterization of Cylindrocarpon-like Anamorphs Causing Root and Basal Rot of Almonds

Three almond nurseries were prospected in the South of Spain (Sevilla) to evaluate the sanitary status of the nursery plant material. Samples consisted of main roots, secondary roots and six-month-old basal stems ‘GxN-15’, ‘Nemaguard’, ‘Cadaman’, ‘Rootpac-40’ and ‘Rootpac-20’ rootstocks planted in the soil, and twigs of mother plants from ‘Lauranne’, ‘Guara’, ‘Marcona’, ‘Marta’ and ‘Ferragnes’ almond cultivars. Endophytic and potential pathogenic fungi were identified in mother plants and 70 Cylindrocarpon-like anamorph isolates were detected in the root system and basal stems of analyzed rootstocks. Based on partial sequencing of the his3 gene and multilocus phylogenetic analysis of the concatenated ITS, tub2, his3 and tef1-α partial sequences, seven Cylindrocarpon-like anamorph species were identified as Dactylonectria torresensis, D. novozelandica, D. macrodidyma, Ilyonectria liriodendri, Neonectria sp. 1, N. quercicola and Cylindrocladiella variabilis. Pathogenicity was assessed on young healthy detached twigs of ‘Guara’ almond cultivar and one-year-old ‘Lauranne’ potted almonds grafted onto ‘GxN-15’ rootstocks. Among the seven Cylindrocarpon-like anamorph species, I. liriodendri, Neonectria sp. 1 and N. quercicola were the most aggressive. Inoculated detached shoots developed necrotic lesions 15 days after inoculation. Inoculated trees showed sectorized necrosis in the main and secondary roots and the basal stem of the rootstock 5 months after inoculation. The most aggressive species were able to cause necrosis also in the grafted cultivar, and I. liriodendri, and N. quercicola also reduced the root biomass. This is the first report of Cylindrocarpon-like anamorph species causing root and basal rot of almonds.

Detection of Root Physiological Parameters and Potassium and Calcium Currents in the Rhizoplane of the Apple Rootstock Superior Line 12-2 With Improved Apple Replant Disease Resistance

The cultivation of resistant rootstocks is one of the more effective ways to mitigate apple replant disease (ARD). We performed an ion current test, a pot experiment, and a pathogen infection test on the apple rootstocks 12-2 (self-named), T337, and M26. The ion current test showed that exposure to ARD soil extract for 30 min had a significant effect on K+ ion currents at the meristem, elongation, and mature zones of the M26 rhizoplane and on Ca2+ currents in the meristem and elongation zones. ARD also had a significant effect on Ca2+ currents in the meristem, elongation, and mature zones of the T337 rhizoplane. Exposure to ARD soil extract for 5 min had a significant effect on K+ currents in the meristem, elongation, and mature zones of 12-2 and on the Ca2+ currents in the elongation and mature zones. Compared to a 5-min exposure, a 30-min exposure to ARD extract had a less pronounced effect on K+ and Ca2+ currents in the 12-2 rhizoplane. The pot experiment showed that ARD soil had no significant effect on any root architectural or physiological parameters of 12-2. By contrast, ARD soil significantly reduced some root growth indices and the dry and fresh weights of T337 and M26 compared with controls on sterilized soil. ARD also had a significant effect on root metabolic activity, root antioxidant enzyme activity (except superoxide dismutase for T337), and malondialdehyde content of T337 and M26. Pathogen infection tests showed that Fusarium proliferatum MR5 significantly affected the root structure and reduced the root metabolic activity of T337 and M26. It also reduced their root antioxidant enzyme activities (except catalase for T337) and significantly increased the root malondialdehyde content, reactive oxygen levels, and proline and soluble sugar contents. By contrast, MR5 had no such effects on 12-2. Based on these results, 12-2 has the potential to serve as an important ARD-resistant rootstock.

Pest categorisation of Fusarium brachygibbosum

The EFSA Plant Health Panel performed a pest categorisation of Fusarium brachygibbosum Padwick. F. brachygibbosum is a well-characterised fungal plant pathogen with opportunistic behaviour, mostly isolated along with other fungal pathogens in symptomatic hosts. It has been reported from Africa, America, Asia and Oceania where it is has been associated with a wide range of symptoms on approximately 25 cultivated and non-cultivated plant species. The pathogen has been reported in Italy in soil/marine sediments and in quinoa (Chenopodium quinoa) and durum wheat (Triticum turgidum subsp. durum) seeds. The pathogen is not included in the EU Commission Implementing Regulation 2019/2072. This pest categorisation focused on a selected range of host plant species on which F. brachygibbosum fulfilled Koch's postulates and was formally identified by multilocus gene sequencing analysis. Host plants for planting, seed of host plants and soil and other substrates originating in infested third countries are main pathways for the entry of the pathogen into the EU. There are no reports of interceptions of F. brachygibbosum in the EU. Host availability and climate suitability factors occurring in the EU are favourable for the establishment of the pathogen in Member States (MSs). Phytosanitary measures are available to prevent the introduction of the pathogen into the EU. Additional measures are available to mitigate the risk of entry and spread of the pathogen in the EU. Despite the low aggressiveness observed in some reported hosts, it has been shown that, in the areas of its present distribution, the pathogen has a direct impact on certain hosts (e.g. almond, onion, soybean, tobacco) that are also relevant for the EU. The Panel concludes that F. brachygibbosum satisfies all the criteria to be regarded as a potential Union quarantine pest. However, high uncertainty remains regarding the distribution of the pathogen in the EU and some uncertainty exists about its potential impact in the EU. Specific surveys and re-evaluation of Fusarium isolates in culture collections could reduce these uncertainties.

Occurrence of Canker and Wood Rot Pathogens on Stone Fruit Propagation Material and Nursery Trees in the Western Cape of South Africa

Dieback and canker of young stone fruit trees can cause suboptimal growth and even death under severe conditions. One source of inoculum of canker pathogens could be through nursery trees harboring latent infections that would not be visible to inspections done according to the deciduous fruit scheme. The objectives of this study were to identify the canker and wood rot fungal pathogens present in nursery stone fruit trees as well as in propagation material and to evaluate their pathogenicity. Isolations were made from scion and rootstock propagation material and from certified nursery stone fruit trees. The plant material sampled did not have any external symptoms. The certified nursery trees when cross-sectioned displayed brown discoloration from the pruning wound, the bud union, and often the crown. Fungal species isolated were identified by sequencing of the relevant barcoding genes and phylogenetic analyses thereof. Canker- and wood rot-associated fungi were identified. Buds used for budding had low levels of infection, with 1.2% of dormant buds infected and 0.4% of green buds infected. The dormant rootstock shoots had a canker pathogen incidence of 6.2% before they were planted in the nursery fields and increased inasmuch as the ungrafted, rooted rootstock plants had 11.1% infection with canker and wood rot pathogens. Out of 1,080 nursery trees, the canker- and wood rot-associated fungi infected 21.8% of trees. The canker-causing pathogens that were isolated the most were Cadophora luteo-olivacea and Diplodia seriata. A low incidence of wood rot fungi was found, with only 1.5% of nursery trees infected. In total, 26 new reports of fungal species on stone fruit in South Africa were made. Of these, 22 have not been found on stone fruit worldwide. The pathogenicity trials' results confirmed the pathogenic status of these newly reported species. All of the isolates tested formed lesions significantly longer than the control, 4 months after wound inoculation of 2-year-old shoots of two plum orchards. Lasiodiplodia theobromae was the most virulent species on both plum cultivars. The results of this research showed that nursery stone fruit trees and propagation material can harbor latent infections. Different management practices need to be evaluated to prevent these infections to ensure healthier stone fruit nursery trees.

Fungi and Oomycetes in the Irrigation Water of Forest Nurseries

The aim of the present study was to assess fungal and oomycete communities in the irrigation water of forest nurseries, focusing on plant pathogens in the hope of getting a better understanding of potential pathogenic microorganisms and spreading routes in forest nurseries. The study sites were at Anykščiai, Dubrava, Kretinga and Trakai state forest nurseries in Lithuania. For the collection of microbial samples, at each nursery five 100-L water samples were collected from the irrigation ponds and filtered. Following DNA isolation from the irrigation water filtrate samples, these were individually amplified using ITS rDNA as a marker and subjected to PacBio high-throughput sequencing. Clustering in the SCATA pipeline and the taxonomic classification of 24,006 high-quality reads showed the presence of 1286 non-singleton taxa. Among those, 895 were representing fungi and oomycetes. The detected fungi were 57.3% Ascomycota, 38.1% Basidiomycota, 3.1% Chytridiomycota, 0.8% Mucoromycota and 0.7% Oomycota. The most common fungi were Malassezia restricta E. Guého, J. Guillot & Midgley (20.1% of all high-quality fungal sequences), Pezizella discreta (P. Karst.) Dennis (10.8%) and Epicoccum nigrum Link (4.9%). The most common oomycetes were Phytopythium cf. citrinum (B. Paul) Abad, de Cock, Bala, Robideau, Lodhi & Lévesque (0.4%), Phytophthora gallica T. Jung & J. Nechwatal (0.05%) and Peronospora sp. 4248_322 (0.05%). The results demonstrated that the irrigation water used by forest nurseries was inhabited by a species-rich but largely site-specific communities of fungi. Plant pathogens were relatively rare, but, under suitable conditions, these can develop rapidly, spread efficiently through the irrigation system and be a threat to the production of high-quality tree seedlings.

Seasonal Variation in Host Susceptibility to Fusarium Canker in Young Almond Trees

Loss of water that reduces the relative water content (RWC) of bark can occur during processing, cold storage, and planting of bare-root stone fruit trees. In California nurseries and newly planted orchards, this stress can predispose young almond trees (Prunus dulcis) to a canker disease caused primarily by Fusarium species. While reduced bark RWC contributes to disease development, anecdotal observations suggest a seasonal effect on host physiology may also influence disease severity. We evaluated the effect of season and the impact of drying and reduced RWC on susceptibility of almond branch segments excised from orchard trees (cv. Nonpareil) to Fusarium acuminatum, Fusarium avenaceum, Fusarium brachygibbosum, and Fusarium californicum sp. nov. With lesion size as the criterion, excised inoculated branch segments were most susceptible in spring, of intermediate susceptibility during winter dormancy, and least susceptible during summer and fall. Consistent with an earlier study, branches with RWC between 80 and 85% yielded lesions that were significantly larger than lesions from branches with bark that was above or below that range. However, the effect of reduced bark moisture on lesion size was only apparent in the spring. These results affirm the importance of avoiding conditions that diminish moisture status in bare-root almond trees in Fusarium canker disease management, especially during transport and planting operations in the spring, a period of high physiological vulnerability. California nurseries apply fungicides to bare-root trees prior to cold storage to reduce "mold" growth. Of eight fungicides currently registered for use on almond trees, fludioxonil (Scholar), fluopyram/trifloxystrobin (Luna Sensation), and fluxapyroxad/pyraclostrobin (Merivon) were most inhibitory to in vitro mycelial growth of F. acuminatum, F. avenaceum, and F. brachygibbosum. However, our almond branch disease assay did not demonstrate preventive or curative fungicide action against infections by F. acuminatum or F. avenaceum.

Canker and Wood Rot Pathogens Present in Young Apple Trees and Propagation Material in the Western Cape of South Africa

Canker and wood rot pathogens cause dieback and, in severe cases, the death of young apple trees. Recently, a higher occurrence of cankers was observed on 1-year-old apple trees in the Western Cape Province of South Africa. This study aimed to assess the phytosanitary status of nursery trees and propagation material as possible inoculum sources for canker pathogens. Thirteen 1-year-old apple orchards showing canker or dieback symptoms were sampled. Certified nursery apple trees were collected from four nurseries as well as scion and rootstock mother plant material. Isolations were made from the discoloration observed in the vascular tissue of the plant parts and from asymptomatic material. Possible canker and wood rot species were identified with PCR and sequence comparisons of the relevant gene regions and phylogenetic analyses. Similar canker and wood rot species were isolated from 1-year-old diseased apple trees, nursery apple trees, and the propagation material. Forty-five fungal species associated with canker or wood rot symptoms were identified. The top five most abundant fungal species found causing disease on commercial 1-year-old trees were also found in high numbers causing latent infection in certified apple nursery trees. These species were Didymosphaeria rubi-ulmifolii sensu lato, Diplodia seriata, Schizophyllum commune, Didymella pomorum, and Coniochaeta fasciculata, with D. rubi-ulmifolii sensu lato being the dominant species in both sampling materials. In all, 65% of certified nursery apple trees, 5% of scion shoots used for budding, and 21% of rooted rootstock cuttings from layer blocks had latent infections of canker and wood rot pathogens. Pathogenicity trials were conducted with isolates of 39 species, inoculated onto 2-year-old branches of 14-year-old Golden Delicious trees. All species caused lesions that were significantly longer than the control. This study confirmed the presence of canker and wood rot pathogens in apple propagation material as well as certified nursery apple trees, which will aid the improvement of management practices in nurseries.

Taxonomy and multi-locus phylogeny of cylindrocarpon-like species associated with diseased roots of grapevine and other fruit and nut crops in California

Black foot disease is a common and destructive root disease of grapevine caused by a multitude of cylindrocarpon-like fungi in many viticultural areas of the world. This study identified 12 cylindrocarpon-like fungal species across five genera associated with black foot disease of grapevine and other diverse root diseases of fruit and nut crops in the Central Valley Region of California. Morphological observations paired with multi-locus sequence typing of four loci, internal transcribed spacer region of nuclear rDNA ITS1-5.8S-ITS2 (ITS), beta-tubulin (TUB2), translation elongation factor 1-alpha (TEF1), and histone (HIS), revealed 10 previously described species; Campylocarpon fasciculare, Dactylonectria alcacerensis, D. ecuadoriensis, D. macrodidyma, D. novozelandica, D. torresensis, D. valentina, Ilyonectria capensis, I. liriodendri, I. robusta, and two new species, Neonectria californica sp. nov., and Thelonectria aurea sp. nov. Phylogenetic analyses of the ITS+TUB2+TEF1 combined dataset, a commonly employed dataset used to identify filamentous ascomycete fungi, was unable to assign some species, with significant support, in the genus Dactylonectria, while all other species in other genera were confidently identified. The HIS marker was essential either singly or in conjunction with the aforementioned genes for accurate identification of most Dactylonectria species. Results from isolations of diseased plant tissues revealed potential new host associations for almost all fungi recovered in this study. This work is the basis for future studies on the epidemiology and biology of these important and destructive plant pathogens.

Characterization of Fusarium and Neocosmospora Species Associated With Crown Rot and Stem Canker of Pistachio Rootstocks in California

California produces 99.1% of pistachios grown in the United States, and diseases affecting pistachio rootstocks represent a constant challenge to the industry. Field surveys of fungi associated with pistachio rootstocks with symptoms of crown rot and stem canker in three central California counties followed by phylogenetic analyses of translation elongation factor 1-α and second largest subunit of RNA polymerase II gene fragments identified three Fusarium species (Fusarium equiseti, Fusarium oxysporum, and Fusarium proliferatum) and two Neocosmospora species (Neocosmospora falciformis and Neocosmospora solani). F. oxysporum and N. falciformis were the fungal species most frequently recovered from symptomatic pistachio trees. Inoculations of detached twigs of cultivar Kerman pistachio Pioneer Gold I and clonal University of California, Berkeley I (UCBI) rootstocks showed that all five species could colonize pistachio wood and cause vascular discolorations. Pathogenicity tests in potted pistachio trees completed Koch's postulates and confirmed that F. oxysporum, F. proliferatum, N. falciformis, and N. solani were capable of producing rot and discoloration in stems of clonal UCBI rootstocks, the most widely planted pistachio rootstock in California. To our knowledge, this study is the first to present insights into the biodiversity and biology of Fusarium and Neocosmospora species associated with pistachio trees in California.

Predisposition in plant disease: exploiting the nexus in abiotic and biotic stress perception and response

Predisposition results from abiotic stresses occurring prior to infection that affect susceptibility of plants to disease. The environment is seldom optimal for plant growth, and even mild, episodic stresses can predispose plants to inoculum levels they would otherwise resist. Plant responses that are adaptive in the short term may conflict with those for resisting pathogens. Abiotic and biotic stress responses are coordinated by complex signaling networks involving phytohormones and reactive oxygen species (ROS). Abscisic acid (ABA) is a global regulator in stress response networks and an important phytohormone in plant-microbe interactions with systemic effects on resistance and susceptibility. However, extensive cross talk occurs among all the phytohormones during stress events, and the challenge is discerning those interactions that most influence disease. Identifying convergent points in the stress response circuitry is critically important in terms of understanding the fundamental biology that underscores the disease phenotype as well as translating research to improve stress tolerance and disease management in production systems.

Infection of Sweetpotato by Fusarium solani and Macrophomina phaseolina Prior to Harvest

The end rot disease complex, caused mainly by Fusarium solani and Macrophomina phaseolina, can be an important postharvest problem in sweetpotato. The disease develops a few weeks after storage roots are harvested and stored. Isolations attempted after harvest showed that the pathogens can be present inside the storage roots before symptoms appear. To determine how and when end rot pathogens enter sweetpotato storage roots, two greenhouse experiments were designed using tissue culture-derived plants free of F. solani and M. phaseolina. In one experiment, plants were grown in autoclaved soil, and 1 month after transplanting, plants were inoculated at the soil line with either noninfested toothpicks or toothpicks infested with each fungus alone or combined. In the other experiment, plants were grown in noninfested soil or in soil infested with each fungus alone or combined. F. solani and M. phaseolina were isolated from roots, storage roots, and plant stems below the soil line, at the soil line, and 5 cm above the soil line in both experiments. This suggests these fungi are capable of invading sweetpotato plants and storage roots from infested soil, and can systemically colonize the plant from infected plant propagation material.