First report of pepper chlorosis-associated virus infecting tobacco (Nicotiana tabacum) plants in Sichuan Province in China

Tobacco (Nicotiana tabacum) is an economically important crop in China, and more than 30 viruses have been reported to infect tobacco (Yin et al. 2022). In July 2022, we observed interveinal necrosis on tobacco leaves in fields in Sichuan Province (N 27.9172, E 105.6662) (Fig. 1). Total RNA was isolated from multiple leaves of one plant using an RNAprep Pure Polysaccharide Polyphenol Plant Total RNA Extraction Kit (TIANGEN, Beijing, China). Total RNAs were pooled, and a TruSeq Stranded Total RNA with RiboZero Gold Kit (Illumina, San Diego, CA, USA) was used to eliminate ribosomal RNA. An RNA-Seq library was constructed using VAHTS Universal V6 RNA-seq Library Prep (Nanjing Vazyme, China). High-throughput sequencing was performed on the Illumina DNBseq platform (BGI-ShenZhen, China), which yielded 20,102,087 reads with an average length of 150 nt (total size >6 Gb). Unaligned reads were assembled de novo using SPAdes (Bankevich et al. 2012). Contigs with length ≥200 nt were subjected to local BLASTn and BLASTx analyses against the GenBank nt and nr databases, respectively (Wang et al. 2022). A total of 23 contigs were identified through BLASTx (e-value cut-off = 10 -3), ranging from 631 to 1555 bp long, with 82% to 96% coverage to partial genomic sequences of pepper chlorosis-associated virus (PepCaV-Higashitsuno_2021; Accessions: LC719619 to LC719621) and one contig (6459 bp) with 99% similarity to tobacco mosaic virus (Accession: OP525281) isolate DSMZ PV-0109 from Germany. The complete genome sequence of PepCaV was obtained using primers based on the assembled contigs. The 5'- and 3'-terminal regions of the RNA genome were obtained by 5'- and 3'-rapid amplification of cDNA ends. These amplicons were cloned using the pEASY-Blunt Zero Cloning Kit (TRANSGEN, Nangjing, China) and sequenced by Sanger sequencing. Complete genome sequences of tripartite PepCaV from tobacco samples were 7697, 1808, and 1557 nucleotides long (Accession: OR451987 to OR451989) and showed genome organization typical of the genus Ophiovirus in the family Aspiviridae. The complete sequences of RNA1, RNA2 and RNA3 genome segments shared 92.36%, 90.43%, and 95.24%, nucleotide sequence identities, respectively, with the isolate PepCaV-Higashitsuno_2021 pepper isolate (Accession: LC719619 to LC719621) (Shimomoto et al. 2023), but PepCaV has not been reported to infect N. tabacum. In June 2023, 10 plants collected from each place of Macheng (N 27.9094, E 105.6740), Xiangyang (N 28.0936, E 105.6249) and Moni (N 27.8899, E 105.5936) showing interveinal necrosis symptoms were tested using RT-PCR using PepCaV-MP610-F (5'-TGTTCTCTGCTATGCGGTTG -3') and PepCaV-MP610-R (5'-AGCAATCTCGCACCTGAAGT-3') to product 610bp amplicon. Twenty-five tobacco plants were positive for PepCaV. Single sequence from each location was submitted to GenBank (Accession: PP728631 to PP728633). Sap extracts from the original field leaf samples collected from Sichuan Province were used to mechanically inoculate tobacco plants (10 plants) at the four-leaf stage. After 7 days, leaf samples were tested using RT-PCR assay specific to PepCaV and TMV while samples were positive only for TMV but failed to transmit PepCaV by mechanical inoculation. According to previous literature, ophioviruses may be transmitted though soilborne fungus (Jeong et al. 2014). Further research is needed to understand the transmission, epidemiology, and pathological properties of the PepCaV. To our knowledge, this is the first report documenting natural PepCaV infection of tobacco plants in China, providing a scientific basis for PepCaV infection control in tobacco plantations.

First Report of Pectobacterium polaris Causing Soft Rot on Broccoli in China

In April 2023, soft rot symptoms were observed in broccoli (Brassica oleracea L. var. italica) commercial fields in Songming County, Yunnan province, China (103°12'E, 25°31'N). The disease incidence in these fields (6 ha in size) was high, exceeding 50%, and it caused significant yield loss. The affected plants displayed characteristic symptoms, with the roots and stems of broccoli becoming soft, yellowish-brown, rotten, and emitting a foul odor. To identify the causal agent, soft rot symptomatic stems were surface sterilized by dipping them in 75% ethanol for 30 seconds, followed by three successive rinses with sterile distilled water. Tissue specimens were then plated onto nutrient agar (NA) plates and incubated at 28°C for 24 hours. (Wang et al. 2022). Three representative bacterial isolates HYC22041801-HYC22041803 from broccoli were selected for further analysis. The colonies on NA plates appeared as white, small, round, and translucent with smooth edges. Physiological and biochemical tests were performed, along with 96 phenotypic screenings using the BIOLOG GENIII microplate system (Biolog, Hayward, CA, USA). Three isolates were negative for D-arabitol, maltose, and sorbitol, but were positive for cellobiose, α-D-glucose, sucrose, glycerol and gentiobiose tests, which are consistent with the reported type strain P. polaris NIBIO1006T (Chen et al. 2021). Total genomic DNA was extracted from three bacterial isolates using the QIAamp DNA Mini Kit (QIAGEN, USA). The 16S rRNA region and nine housekeeping genes (gapA, icdA, mdh, mtlD, pel, pgi, pmrA, proA and rpoS) were amplified with universal primers 27F/1492R (Monciardini et al., 2006) and designed specific primers (Xie et al., 2018), respectively. All amplicons were sequenced and deposited in GenBank with accession numbers ON723841-ON723843 and ON723846-ON723872. The BLASTn analysis of the 16S rRNA amplicons confirmed that the isolates HYC22041801-HYC22041803 belonged to the genus Pectobacterium. Phylogenetic trees based on 16S rRNA gene sequences and multilocus sequence analysis of other nine housekeeping genes of the three isolates were constructed and the results revealed that three isolates clustered with P. polaris type strain NIBIO1006T, which was previously isolated from potato (Dees et al., 2017). To confirm the pathogenicity, nine broccoli seedlings were stab inoculated with a bacterial suspension (108 CFU·ml-1), while sterile distilled liquid LB medium was used as a negative control. The seedlings were kept at 80% relative humidity and 28°C in a growth chamber. Three trials were conducted per isolate (HYC22041801-HYC22041803). After 3 days, the inoculated petioles showed soft rot symptoms similar to those observed initially in the field, while control plants remained asymptomatic. All three isolates were re-isolated successfully from symptomatic tissues to complete Koch's postulates. P. polaris has been previously reported as the causative agent of blackleg in potato in several countries, including Norway, Poland, Russia, and China (Handique et al. 2022; Wang et al. 2022). Additionally, it was reported to cause soft rot in Chinese cabbage in China (Chen et al. 2021). However, this is the first report of P. polaris causing soft rot disease in broccoli in China. This discovery is of great importance for vegetable growers because this bacterium is well established on Cruciferous vegetables in the local area, and effective measures are needed to manage this disease.

Puccinia suaveolens Causing Leaf Rust on Cirsium setosum in China

Thistle, Cirsium setosum (Willd.) M. Bieb., is widely distributed in China as a common weed in fields. It is also used as a traditional Chinese medicine for cooling blood, stopping bleeding, dispelling stasis, detoxifying, and resolving carbuncle. In 2023, we found a rust disease on plants of Cirsium setosum in the experimental field of Hebei Agricultural University, Baoding, Hebei Province, China, with incidence of 15% - 25% (Fig. S1 A, B). The diseased leaves turned yellow, and the leaf edges were slightly rolled. The yellow, oil-like pycnia and pycniospores covered the baxial surface of leaves, and brown pustules were produced after 2-3 weeks. On the adaxial surface of the leaves, the brown rust pustules were mainly along the leaf veins. Stems were also be infected later, and dark pustules were scattered. The diseased plants were relatively short and small, and produced relatively small or no flowers compared to healthy plants. A total of 100 plants with typical leaf rust symptoms and signs were collected. To confirm the pathogenicity, healthy plants of thistle were sprayed with 5 ml of urediospores suspension (2.6×105/ml), and plants sprayed with sterile distilled water were treated as control. The sprayed plants were incubated under high moist conditions at 18°C for 24 h, and the inoculated plants were grown at 20°C in a greenhouse. Ten days after inoculation, the plants inoculated with urediniospores showed rust symptoms with uredinia and urediniospores on the leaves (Fig. S1 C), while the control plants were healthy. For morphological characterization, urediospores were picked from the naturally infected plants and placed in a drop of sterile water on a glass slide using a sterile needle, and observed and measured under a microscope. Urediospores were nearly spherical, brown-yellow, and measured 15 - 25 μm in diameter (n=100) (Fig. S1 D). Telia were scattered on the baxial surface of the naturally infected leaves, and teliospores were oval, yellow-brown, double-celled, with very short hyaline pedicels, and measured 15-20 × 15-30 μm (n=100) (Fig. S1 E). For molecular characterization, about 200 μg of urediniospores was collected and placed in a 1.5 ml sterile centrifuge tube, and genomic DNA was extracted using the cetyl-trimethylammonium bromide method (Gawel et al. 1991). The internal transcribed spacer (ITS) region of the rDNA and the D1/D2 domain were amplified using primer pairs ITS1/ITS4 (White et al. 1990) and NL1/NL4 (Borhani et al. 2013) in polymerase chain reaction (PCR), respectively. The PCR products were sequenced, and their sequences were aligned and compared with those deposited in GenBank. The obtained sequences were deposited in GenBank (OR600240 for ITS and OR598614 for D1/D2), which were 100% identical with 100% coverage to the ITS sequence (ON063373.1) and the D1/D2 sequence (ON063379.1) of Puccinia suaveolens (Menzies 1953). Based on the morphological characteristics and DNA sequences, the isolates were identified as P. suaveolens (Fig. S1 and Fig. S2). Thistle rust caused by Puccinia obtegens has been reported in some other parts of China (Zhang 2012). To the best of our knowledge, this is the first report of P. suaveolens causing leaf rust on C. setosum in China. This discovery is helpful for control of leaf rust on thistle grown for Chines medicine and other purposes, and the rust species could be used for biological control of thistle as a weed in crop fields.

Impact of Infection Timing and Autumnal Fungicide Applications on Perennation of Pseudoperonospora humuli and Severity of Hop Downy Mildew

Pseudoperonospora humuli, causal agent of hop downy mildew, is known to survive winter as systemic mycelium in the crown and developing buds of hop (Humulus lupulus). Field studies were conducted over three growing seasons to quantify the association of infection timing to overwintering of P. humuli and development of downy mildew. Cohorts of potted plants were inoculated sequentially from early summer to autumn, overwintered, and then evaluated for symptoms of systemic downy mildew in emerging shoots. Shoots with systemic P. humuli developed after inoculation at any time in the previous year, with the most severe disease typically resulting from inoculation in August. Independent of the timing of inoculation, diseased shoots emerged coincident with the emergence of healthy shoots, beginning as early as late February and continuing through late May to early June. Surface crown buds on inoculated plants exhibited internal necrosis associated with P. humuli at rates ranging from 0.3 to 1.2%, whereas P. humuli was detected by PCR on 7.8 to 17.0% of asymptomatic buds depending on the timing of inoculation and year. Four experiments were conducted to quantify the impact of foliar fungicides applied in autumn on downy mildew the following spring. There was a small reduction of disease in only one study. Together, these studies indicate that infection by P. humuli that leads to overwintering can occur over a broad period of time, but delaying infection until autumn tends to reduce disease levels in the following year. However, in established plantings, postharvest application of foliar fungicides appeared to have little impact on severity of downy mildew in the ensuring year.

Viability of Moniliophthora roreri on Cocoa Beans Under Microfermentation and Long-Term Survival on Carrier Materials

The viability of Moniliophthora roreri inoculum was evaluated during the microfermentation process of diseased and healthy pulp-seed masses and on a range of carrier materials: aluminum, cloth, glass, paper, plastic, raffia, and rubber tire. Fungal survival was assessed before the microfermentation (0 h) and every 24 to 96 h by the growth of colonies in potato-dextrose-agar (PDA) and sporulation in seed shells. Colonies of M. roreri and sporulation on seed shells were observed from seeds not submitted to microfermentation. No growth was recovered from diseased cocoa beans after 48 h under the microfermentation. The viability of M. roreri spores recovered from carrier materials was evaluated at 7, 15, 30, 45, and 100 days after inoculation (DAI) by collecting spores and plating them on Sabouraud dextrose yeast extract agar amended with chloramphenicol (50 mg l1). The viability was determined by counting germinated and ungerminated spores under a light microscope (40×) after incubating in a moist chamber at 26 ± 2°C for 72 h. Spores maintained long-term viability on all tested carrier materials toward the end of the experiment (overall 26%) with significant differences (<0.05) among them. Maximum spore viability occurred at 7 and 15 DAI, with cloth and plastic carrier materials considered at high risk of acting as vehicles for the fungal spread. Mathematical models of spore viability over time were fit to the data using the Bayesian information criterion. Findings confirmed the importance of the fermentation process to hamper M. roreri growth and the potential of carrier materials for fungal dispersal.

First report of milkweed (Euphorbia geniculata) as an alternative host for Colletotrichum truncatum in soybean fields in India

Soybean (Glycine max, L.), a major oilseed crop of India faces anthracnose disease caused by Colletotrichum truncatum (Nataraj et al. 2021). Several weeds serve as alternative hosts for Colletotrichum spp. (Hartman et al. 1986). Around 24.67% of soybean fields in the study area were infested with Euphorbia geniculata (Kutariye et al. 2021). In September 2021, milkweed plants died in the field, showing irregular circular lesions with wavy margins on the stem, change in color of veins and veinlets from brown to black and leaves exhibiting a twisted appearance at ICAR-Indian Institute of Soybean Research, India. Later on plants completely died and acervuli of average size 284 µm were visualized under stereo microscopy. Twenty milkweed samples were collected, rinsed, and surface sterilized with NaOCl (1%). Fungus isolation was done from leaf and stem and transferred to sterilized Petri plates with Potato dextrose agar (PDA). The plates were incubated at 25 ± 2°C for 48 h with dark/light (10h/14h) cycle. The fungi produced circular, raised, black to light grey colonies. Sickle shaped aseptate conidia, measuring 23.14 µm length, 3.18 µm width and hyphal width 5.49 µm were confirmed using a compound microscope with 20X magnification. The fungus was purified via hyphal tip method and pure culture was maintained on PDA at (26 ± 2°C). Milkweed seedlings in clay pots were inoculated with a conidial suspension of the fungus (106 conidia/mL) prepared from ten days old culture using serial dilution technique. Soybean variety JS 95-60 was inoculated by atomizing 20 ml of the same suspension on each plant. The negative controls for both milkweed and soybean were inoculated with sterile distilled water. Veinal necrosis and acervuli formation were observed on both milkweed and soybean, but no signs or symptoms of disease were observed in the controls. The re-isolated fungus from both the diseased hosts resembled original culture as they produced black to light grey colonies, sickle shaped aseptate conidia and ITS sequence (OR124845) exhibiting 100% resemblance to C. truncatum isolate C-17 (MN736513), thus confirming Koch's postulates. The pathogen was classified as Colletotrichum spp. based on morphological and cultural characters and the pathogenicity test (Rajput et al. 2021). To confirm identity of the pathogen infecting milkweed, DNA was extracted from the reisolated fungus using the HiPurA Fungal DNA Purification Kit (HiMedia, India). The internal transcribed spacer (ITS) region, beta-tubulin (TUB2) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes were amplified (Kumar et al. 2021). The GAPDH gene was amplified under similar reaction conditions except for annealing temp 59°C. For species level identification, the ITS, TUB2 and GAPDH gene sequences were submitted to GenBank with accession numbers OR004468, OQ869780 and OQ869781, respectively. The BLAST analysis of TUB2 and GAPDH gene showed sequence homology of 100% and 98.43% respectively with C. truncatum culture-collection CBS:151.35 (GU228156, GU228254). The isolate was identified as C. truncatum on the basis of molecular analysis, corroborating the above morphological identification. This is the first report of C. truncatum infecting milkweed in India, indicating milkweed as an alternative host in soybean fields, potentially raising inoculum levels and carryover between crops.

First Report of Colletotrichum siamense causing Anthracnose on Ficus microcarpa in China

Chinese banyan, Ficus microcarpa Linn. f. (Moraceae), is a native and popular landscape tree with high ornamental  and economic value in South China. In May 2018, a foliar disease occurred on F. Microcarpa in Zhanjiang city (21˚17'51''N, 110˚18'16''E), Guangdong Province, China. The incidence of the disease was 10% and severity was 20% (n = 100 investigated plants). Symptoms first appeared on leaves as small black spots (up to 5 mm diam) with yellow haloes. The spots gradually enlarged in size and coalesced, resulting in necrosis and early shedding of the leaves. Ten symptomatic leaves from 10 plants were collected. The margins of the samples were cut into small pieces (5 × 5 mm), surface disinfected (75% ethanol 30 s, 0.1% HgCl2 45 s, rinsed 3 times with sterile distilled water), and placed on potato dextrose agar (PDA) at 28°C with a photoperiod of 12 h. After 5 days, a total of four isolates with 100% isolation frequency were obtained and three representative strains (T6, T6-1, and T6-2) were used for morphological and molecular characterization. Colonies on PDA were white to gray with cottony mycelia, a few bright orange conidial masses developed near the inoculum point. Conidia were single-celled, smooth-walled, straight, colorless, fusiform with obtuse to slightly rounded ends, and measured 13.2 to 17.5 (avg. 15.3) × 4.3 to 6.0 (avg. 5.2) μm in size (n = 50). Appressoria were oval to irregular in shape, dark brown, and ranged from 5.8 to 9.1 (avg. 7.5) × 4.8 to 6.5 (avg. 5.6) µm (n = 50). Morphological characteristics of the isolates agreed with the description of Colletotrichum siamense (Prihastuti et al. 2009). The ITS (Nos. MK225610, ON979519, ON979520), TUB2 (Nos. MK348533, OQ130408, OQ130409), ACT (Nos. MZ852755, OQ116933, OQ116934) and GAPDH (Nos. MZ852756, OQ130406, OQ130407) sequences were 99.35%, 98.76%, 98.92% and 97.60% identical to the type C. siamense strain MFLU 090230 (accession nos. NR_144784, FJ907438, FJ907423 and FJ972575) through BLAST analysis, respectively. A phylogenetic tree was generated using the concatenated sequences of ITS, ACT, GAPDH and TUB2. The result showed that the 3 isolates were clustered with C. siamense strains including the type MFLU 090230. To perform pathogenicity tests, One leaf (wounded and/or unwounded) per seedling (n = 20, 2-year-old) was inoculated by dropping a 10 ul droplet of the conidial suspension (1 × 105) of isolate T6 or sterile distilled water (as control) at one point, respectively. These plants were wrapped in polyethylene bags for 2 days and incubated in a greenhouse at 28± 2°C with 80-90% relative humidity. Initial symptoms appeared within 5 days on all wounded and unwounded inoculated leaves, while the controls remained symptomless. Colletotrichum siamense was reisolated from the lesions in terms of morphology and molecular characterization, whereas no fungus was isolated from control leaves. The pathogenicity test was repeated three times under the same conditions. The diseases caused by C. siamense have been reported to cause anthracnose in a wide range of hosts (Weir et al. 2012), but not in F. microcarpa. In China, C. gloeosporioides has been reported to cause anthracnose on potted banyan F. microcarpa (Yao et al. 2016), but not C. siamense. This is the first report of C. siamense causing anthracnose on F. microcarpa in China. This study provides valuable information for the identification and control of the anthracnose on F. microcarpa.

Evaluating In Vitro Fitness Parameters of G143A-Containing and Wild-Type Corynespora cassiicola Isolates from Mississippi Soybean

Quinone outside inhibitor (QoI) fungicides have been widely used to manage diseases of soybean including target spot caused by Corynespora cassiicola. However, resistance to QoI fungicides has recently been reported within the C. cassiicola population from Alabama, Arkansas, Mississippi, and Tennessee as a result of isolates in the population containing the G143A amino acid substitution. Therefore, the relative fitness and stability of isolates containing the G143A substitution compared with wild-type C. cassiicola isolates from Mississippi soybean were investigated by analyzing several fitness parameters in vitro. In addition, in vivo virulence assays were conducted in the greenhouse on a target spot-susceptible cultivar. The evaluations of fitness considered the difference between isolates from the wild-type and G143A-containing genotypes by evaluating colony growth parameters following the first and the 10th subcultures on microbiological media. When considered as an average of all G143A-containing isolates, the G143A-containing isolates following the 10th subculture produced 6.2% greater colony diameter growth but produced 2.3% less conidia. Conversely, over the same period, wild-type isolates produced 6.7% less colony growth but produced 10.9% more conidia. Based on our results, the C. cassiicola isolates that contained the G143A substitution appear stable since successive subculturing did not significantly affect the measured fitness parameters. The lack of fitness cost accompanying the genotypic shift to the G143A amino acid substitution which confers fungicide resistance in C. cassiicola indicates that these isolates may have fitness advantages and may remain stable in the population as well as displace wild-type isolates with repeated fungicide applications of QoI-containing products.

First Report of Pestalotiopsis clavispora causing Leaf Spot on Pandanus amaryllifolius in China

Pandanus amaryllifolius, also known as pandan, is a perennial herb, growing in Indonesia, China and the Maluku Islands (Wakte et al. 2009). It is the only plant with aromatic leaves in the Pandanaceae. It is widely used in food, medicine, cosmetics and other industries, and is also known as "Oriental Vanilla." Pandan is planted in Hainan province over 1,300 ha and is the main plant intercropped among the forest trees. From 2020, the leaf spot was surveyed for three years. Diseased leaves occurred on 30 to 80% of the surveyed plants, with an incidence of 70% and yield losses of 40%. The disease occured from mid-November to April and was most severe at low temperatures and humidity. Initial symptoms were pale green spots, that formed dark brown, nearly circular lesions. As the lesions expanded, their centers became greyish white, with yellow halos at the junction of the diseased and healthy tissue. When the humidity was high, there were small black spots scattered in the center of the lesion. Symptomatic leaf samples were collected from four different sites. The leaf surface was disinfested with 75% ethyl alcohol for 30 s and washed with sterile distilled water three times. Samples from the junction of diseased and healthy tissue (0.5 × 0.5 cm) were removed and placed on potato dextrose agar (PDA) medium containing 100 µg/mL of cefotaxime sodium and cultivated in a dark incubator at 28°C. After two days, hyphal tips from the edges of growing colonies were transferred to fresh PDA plates for further purification. Following Koch's postulates, colonies from strains were used as inoculum in pathogenicity tests. Colonies with 5 mm diameter were inoculated upside onto fresh and healthy pandan leaves via wounding method (pinpricked by sterilized needles) and non-wounding method. Sterilized PDA was used as control. All plants were setted three replicates and were incubated at 28℃ for 3 to 5 days. When symptoms on leaves similar to those in the field appeared, the fungus were reisolated The colonies formed on PDA were also consistent with the original isolate (Scandiani et al, 2003). After seven days, the colony covered the whole petri dish with white, petal-shaped growth with a slight concentric, annular bulge in the center, irregular edges, with black acervuli emerging at a later stage of colony growth. Conidia were fusiform, 18.1±1.6 × 6.4±0.3 μm, showing four septations and five cells, the middle three cells were brownish black to olivaceous, and the apical cell colorless with two to three filaments, 21.8±3.5 μm long. The caudate cell was colorless with one stalk 5.9±1.8 μm long (Zhang et al. 2021; Shu et al. 2020). According to the colony and conidia characteristics, the pathogen was initially identified as Pestalotiopsis spp. (Benjamin et al. 1961). To confirm the pathogen identity, we used the universal primers ITS1/ITS4, targeting primers EF1-728F/EF1-986R and Bt2a/Bt2b sequences (Tian et al. 2018). The sequences of the PCR products were deposited in NCBI GenBank with accession numbers OQ165166 (ITS), OQ352149 (TEF1-α) and OQ352150 (TUB2). BLAST results showed that the sequences of the ITS, TEF1-α and TUB2 genes shared 100% homology with the sequences of Pestalotiopsis clavispora. The maximum likelihood method was used in the phylogenetic analysis. The result showed that LSS112 was clustered with Pestalotiopsis clavispora with a support rate of 99%. Based on morphological and molecular characteristics, the pathogen was confirmed as Pestalotiopsis clavispora. To our knowledge, this is the first report of leaf spot of pandan caused by Pestalotiopsis clavispora in China. This research will be immediately helpful for the diagnosis and control the disease on pandan.

Large-Scale Surveys of Blackleg of Oilseed Rape (Leptosphaeria biglobosa) Revealed New Insights into Epidemics of This Disease in China

Blackleg of oilseed rape caused by Leptosphaeria maculans/L. biglobosa is a worldwide important disease. L. maculans is more virulent than L. biglobosa, so it causes a great concern for oilseed rape production. In China, blackleg (L. biglobosa) of oilseed rape was reported in the 2000s, but epidemiological features of blackleg have not been well elucidated. Moreover, whether L. maculans exists in China is still an open question. Therefore, a 5-year survey was done in China to collect blackleg-occurrence data for characterizing the features of blackleg epidemics and to identify the blackleg pathogens for assessing the risk of L. maculans invasion. The results showed that all the 19 surveyed provinces had blackleg on oilseed rape, and the most frequently occurring provinces are Gansu, Qinghai, Shaanxi, and Hubei. Phoma stem canker was the most common symptom, which was associated with stem cracks on winter oilseed rape and with stem-weevil activities on spring oilseed rape. Temperature and rainfall were the main factors for blackleg epidemics on winter oilseed rape, whereas rainfall was the main factor for blackleg epidemics on spring oilseed rape. Brassica campestris and B. juncea oilseed rapes were more susceptible than B. napus to blackleg. Oilseed rapes cultivated under the continuous dry land-cropping pattern were more prone to blackleg than those cultivated under the paddy land/dry land-cropping pattern. All 6,015 fungal isolates from blackleg plant tissues belonged to L. biglobosa. These results are helpful for understanding the blackleg epidemics of oilseed rapes and for management of this disease in China.

First Report of radish tubers rot caused by Enterobacter asburiae in China

Radish (Raphanus sativus L.) is a widely consumed vegetable in China. However, radish is susceptible to diseases, which limits its yield and development in Harbin, China. In September 2021, rotten white radish tubers were observed in the field. The incidence of this disease reached 70% in October 2021, which led to huge economic losses (i.e., 30%-40%). Water-soaked lesions appeared on the radish tubers and appeared brown-yellow, which looked similar to ginger tuber rot caused by Enterobacter asburiae (Zhang et al. 2020). The interior was rotten with no considerable smell. Over time, the lesions gradually spread into all tubers of radish. Small square pieces of radish (0.5 cm × 0.5 cm) were excised from the junction of diseased and healthy tuber, disinfected with 75% alcohol, and washed three times with distilled water then ground to prepare tissue suspensions for plating. Under 28 ℃ for 16h, single colonies were isolated from the beef extract culture medium. Single colonies appeared oval, white, and smooth, with bright and slightly raised surfaces, and with moist neat edges. Gram-negative bacterial strain CCGL 988 was obtained, with an average size of 1-2 µm × 0.5-1 µm, and 3-4 flagella. Physiological and biological test results showed that strain CCGL 988 produced acid utilizing sucrose, glucose, maltobiose, D-Sorbitol, and mannitol; negative for Voges-Proskauer, methyl red, malanate, ornithine decarboxylase, arginine decarboxylase, and lysine decarboxylase. According to the results, strain CCGL 988 was identified as Enterobacter asburiae (Hoffmann et al. 2005). The 16S rDNA region of the strain was amplified using PCR with 27F/1492R primers (López et al. 2019), and partial gyrB, atpD, rpoB genes were amplified according to Zhang et al. (2020), infB gene was amplified with primers (F:TCAATGCGTGCTCGTGGTGCTC; R: TCGATACAGTGCCACTTCACG). The 16S rDNA, gyrB, atpD, rpoB and infB sequences were deposited in GenBank under accession numbers: ON999069, OP006448, OP006449, OP006450, and OP542231, respectively. These five sequences shared 99.80%, 100%, 100%, 100% and 100% of identity with E. asburiae (GenBank Accession: NO. CP011863). Maximum-likelihood phylogenetic tree clustered CCGL 988 with E. asburiae (MEGA7, bootstrap n = 1,000). Strain CCGL 988 was able to produce pectate lyases, polygalacturonases, cellulases, proteases, and extracellular polysaccharide using the methods described by Hugouviex-Cotte-Pattat et al. (2014), and Condemine et al. (1999). Koch's postulates were conducted by inoculating 20 µl of the bacterial suspension (108 CFU/ml) on the needle wound on the surface of six healthy radish tubers; six radish tubers incubated with sterile water were negative controls. Radish tubers were incubated at 28 ℃ with 80% humidity. The inoculated radish was slightly rotten after 7 days. Water-soaked lesions with light yellow were initially observed; after 12 days, the lesions expanded gradually and appeared deep yellow. No symptoms were found in the control radish. This experiment was carried out three times, each time with three replications. The bacterium was reisolated from infected radish tuber and was confirmed to be E. asburiae by the same molecular and morphological characterization as described above. This study is the first report of E. asburiae causing radish tuber rot in China. It serves as a basis for future studies to develop management strategies for the disease to prevent radish yield loss.

Genome sequence resource of Pectobacterium polaris QK413-1 that causes blackleg on potato in Fujian Province, China

The Pectobacterium pathogens cause soft rot and blackleg diseases on many plants and crops, including potatoes. Here we first report a high-quality genome assembly and announcement of the P. polaris strain QK413-1, which causes blackleg disease in potatoes in China. The QK413-1 genome was sequenced and assembled using the PacBio Sequel II and Illumina sequencing platform. The assembled genome has a total size of 5,005,507bp with a GC content of 51.81%, encoding 4782 open reading frames, including 639 virulence genes, 273 drug resistance genes, and 416 secreted proteins. The QK413-1 genome sequence provides a valuable resource for the control of potato blackleg and research into its mechanism.

Genome sequence resource of Pectobacterium polaris QK413-1 that causes blackleg on potato in Fujian Province, China

The Pectobacterium pathogens cause soft rot and blackleg diseases on many plants and crops, including potatoes. Here we first report a high-quality genome assembly and announcement of the P. polaris strain QK413-1, which causes blackleg disease in potatoes in China. The QK413-1 genome was sequenced and assembled using the PacBio Sequel II and Illumina sequencing platform. The assembled genome has a total size of 5,005,507bp with a GC content of 51.81%, encoding 4782 open reading frames, including 639 virulence genes, 273 drug resistance genes, and 416 secreted proteins. The QK413-1 genome sequence provides a valuable resource for the control of potato blackleg and research into its mechanism.

Field Occurrence and Overwintering of Oospores of Pseudoperonospora cubensis in the Southeastern United States

In the United States, the cucurbit downy mildew pathogen, Pseudoperonospora cubensis, has been shown to form oospores under laboratory conditions, but there are no reports on the formation of oospores in naturally infected cucurbit plants in the field. This study investigated the occurrence of oospores in naturally infected leaves from cucurbit fields in North Carolina and South Carolina from 2018 to 2020. Oospore viability and survival was also determined outdoors during the winter in North Carolina during this study period using soil containing leaves infested with oospores. About 5% of 1,658 naturally infected cucumber and cantaloupe leaves sampled during the study had oospores, with a mean density of 585 oospores per cm² of infected leaf tissue. Absolute oospore viability, as assessed using the plasmolysis method, declined linearly (slope = -0.27; P < 0.0001) over the 6-month exposure period from 67.8% in November to 19.3% in May. Other variables being equal, the decrease in oospore viability was significantly affected by soil temperature (b = -0.03 to -0.05; P < 0.0001) and number of rainy days (b = 21.6 to 40.46; P < 0.05), while the effects of soil moisture on oospore viability were less clear. About 20% of the oospores exposed to outdoor conditions at the end the study period were putatively viable and deemed potentially infective. However, these putatively viable oospores failed to germinate or initiate disease when inoculated onto cucumber or cantaloupe leaves. These results indicate that oospores might require some unrecognized stimuli or physiological factors to initiate germination and infection. Nonetheless, viability of oospores at the end of the winter season suggests that once exposed to the right conditions that stimulate germination, these oospores could potentially serve as a primary inoculum source in the southeastern United States where winter temperatures are cold enough to kill cucurbits plants.

First report of leaf spot on Deutzia crenata caused by Neopestalotiopsis ellipsospora in China

Deutzia crenata Sieb. et Zucc, native to Japan, with white flowers in early summer, is a high quality ornamental shrub widely planted in China. In October 2021, a new leaf spot disease was observed on approximately 70% of the 320 D. crenata trees growing in Nanjing Botanical Garden, Jiangsu Province, China. The disease started as irregular small gray spots on the leaf of D. crenata that coalesced into larger lesions. Infected leaves turned yellow (Figure S1A) and leaves with multiple spots withered. To isolate the pathogen, leaf sections (3 to 4 mm) were excised from the lesion margin, surface sterilized in 75% alcohol for 30 s and then in 1.5% NaClO for 90 s, rinsed three times in sterilized distilled water, plated on potato dextrose agar (PDA) and incubated at 25℃in the darkness. Pure cultures were obtained by monosporic isolation. The colony of a representative isolate (L-1), growing on PDA was circular, white, and cottony, and the surface undulate and pale luteous (Figure S1B). The reverse was similar in color (Figure S1C). The conidial masses were black and appeared over PDA plates after 12 days (Figure S1D). Conidia [18.3 to 28.4×5.4 to 8.5 µm (mean 24.5×6.7 µm)] (n=35) were fusiform to ellipsoid and four-septate (one basal and one apical cell hyaline, and three brown median cells), with two to three apical appendages (Figure S1E). These characteristics were consistent with the description of Neopestalotiopsis sp. (Maharachchikumbura et al. 2014). Three regions of the internal transcribed spacer (ITS), translation elongation factor 1-alpha (TEF1α), and β-tubulin (TUB) genes (GenBank Accession No. OM663738, No. OM687134 and No. OM687133, respectively) were amplified and sequenced with the primers pairs ITS1/ITS4 (Innis et al. 1990), EF1-526F/EF1-1567R (Maharachchikumbura et al. 2014) and Bt2a/Bt2b (Glass and Donaldson 1995), respectively. The obtained sequences were 95.4-99.8% similar to those from Neopestalotiopsis sp. accessions in GenBank. A neighbor-joining phylogenetic tree was generated by combining all sequenced loci in MEGA7. The isolate L-1 clustered in the N. ellipsospora clade with 98% bootstrap support (Figure S2). To test pathogenicity, three detached healthy leaves and three one-year-old D. crenata seedlings were inoculated with 20 μL conidia suspension (1×106 spores/mL) on the left sides of leaves. The right side of each leaf was inoculation with 20 µL of sterile water as the experimental control. All plants were covered with clear polyethylene bags and incubated in a greenhouse (Institute of Botany, Jiangsu Province and Chinese Academy of Sciences) at 25℃, 80% relative humidity, and a 12-h light/dark cycle. The experiment was repeated three times. After 5 days of inoculation, leaf spots typical of those observed in the orchards were observed on the left sides of all inoculated leaves and the right sides did not have any leaf spot symptoms (Figure S1F-G). The same fungus was isolated from the diseased spots of the inoculated leaves to complete Koch,s postulates (Figure S1H). N. ellipsospora is known to cause leaf spots on Camellia sinensis and sweet potato, infects fruits of Ardisia crenata in China (Maharachchikumbura et al. 2014; Maharachchikumbura et al. 2016; Wang et al. 2019), and causes stem spots on Acanthopanax divaricatus in Korea (Yun et al. 2015). This is the first report of N. ellipsospora causing leaf spot on D. crenata in the world. The occurrence of this disease needs to be monitored, because it can reduce the ornamental value of D. crenata. This finding provides the foundation to further investigate the biology and epidemiology of this disease so that effective strategies can be developed to manage this disease.

Soil Disinfestation: From Soil Treatment to Soil and Plant Health

This feature article tracks 100 years of soil disinfestation, from the goal of eradicating soilborne pathogens and pests to much milder approaches, aimed at establishing a healthier soil, by favoring or enhancing the beneficial soil microflora and introducing biological control agents. Restrictions on the use of many chemical fumigants is favoring the adoption of nonchemical strategies, from soilless cultivation to the use of physical or biological control measures, with more focus on maintaining soil microbial diversity, thus enhancing soil and plant health. Such approaches are described and discussed, with special focus on their integrated use.

Spatiotemporal Dynamics of Stemphylium Leaf Blight and Potential Inoculum Sources in New York Onion Fields

Stemphylium leaf blight (SLB) caused by Stemphylium vesicarium is the dominant foliar disease affecting large-scale onion production in New York. The disease is managed by fungicides, but control failures are prevalent and are attributed to fungicide resistance. Little is known of the relative role of inoculum sources in initiation and spread of SLB epidemics. Plate testing of 28 commercially available organic onion seedlots from 2016 and 2017 did not detect S. vesicarium. This finding suggests that although S. vesicarium has been reported as seed-transmitted, this is unlikely to be a significant inoculum source in commercially available organic seed lots and even less so in fungicide-treated seed used to establish conventional fields. The spatial and spatiotemporal dynamics of SLB epidemics in six onion fields were evaluated along linear transects in 2017 and 2018. Average SLB incidence increased from 0 to 100% throughout the cropping seasons with an average final lesion length of 28.3 cm. Disease progress was typical of a polycyclic epidemic and the logistic model provided the best fit to 83.3% of the datasets. Spatial patterns were better described by the beta-binomial than binomial distribution in half of the datasets (50%) and random patterns were more frequently observed by the index of dispersion (59%). Geostatistical analyses also found a low frequency of datasets with aggregation (60%). Spatiotemporal analysis of epidemics detected that the aggregation was influenced by disease incidence. However, diseased units were not frequently associated with the previous time period according to the spatiotemporal association function of spatial analyses by distance indices. Variable spatial patterns suggested mixed inoculum sources dependent upon location, and likely an external inoculum source at the sampling scale used in this study. A small-plot replicated trial was also conducted in each of 2 years to quantify the effect of S. vesicarium-infested onion residue on SLB epidemics in a field isolated from other onion fields. SLB incidence was significantly reduced in plots without residue compared with those in which residue remained on the soil surface. Burial of infested residue also significantly reduced epidemic progress in 1 year. The effect of infested onion residue on SLB epidemics in the subsequent onion crop suggests rotation or residue management may have a substantial effect on epidemics. However, the presence of an inoculum source external to fields in onion production regions, as indicated by a lack of spatial aggregation, may reduce the efficacy of in-field management techniques.

Potential Susceptibility of Six Aquatic Plant Species to Infection by Five Species of Phytophthora

Investigations of the susceptibility of aquatic plants to species of Phytophthora are limited. Therefore, the objective of this study was to assess the potential susceptibility of six aquatic plant species, frequently used in constructed wetlands or vegetated channels, to infection by five species of Phytophthora commonly found at nurseries in the southeastern United States. In a greenhouse experiment, roots of each plant species (Agrostis alba, Carex stricta, Iris ensata 'Rising Sun', Panicum virgatum, Pontederia cordata, and Typha latifolia) growing in aqueous solutions were exposed to zoospores of each of the species of Phytophthora (Phytophthora cinnamomi, Phytophthora citrophthora, Phytophthora cryptogea, Phytophthora nicotianae, and Phytophthora palmivora). Zoospore presence and activity in solution were monitored with a standard baiting bioassay with rhododendron leaf discs as baits. Experiments were initiated in 2016 and repeated in 2017 and 2018. During the 2016 trials, Phytophthora spp. were not isolated from the roots of any of the plants, but some roots of C. stricta, P. virgatum, and T. latifolia were infected with multiple species of Phytophthora during trials in 2017 and 2018. Presence of plant roots reduced the percentage of rhododendron leaf discs infected by zoospores of four of the species of Phytophthora but not those infected by P. cinnamomi, which suggested that roots of these plants negatively affected the presence or activity of zoospores of these four species of Phytophthora in the aqueous growing solution. Results from this study demonstrated that certain aquatic plant species may be sources of inoculum at ornamental plant nurseries if these plants are present naturally in waterways or used in constructed wetlands treating water flowing off production areas, which could be of concern to plant producers who recycle irrigation water.

Phytophthora Crown Rot of Florida Strawberry: Inoculum Sources and Thermotherapy of Transplants for Disease Management

Phytophthora crown rot, caused mainly by Phytophthora cactorum but also by P. nicotianae, reported in 2018, is an important disease in the Florida strawberry annual production system. Mefenoxam is the most effective and widely used fungicide to manage this disease. However, because of pathogen resistance, alternatives to chemical control are needed. Phytophthora spp. were rarely recovered during the summer from soil of commercial farms where the disease was observed during the season. In a more detailed survey on research plots, neither of the two species was recovered 1 month after the crop was terminated and water was shut off. Therefore, Phytophthora spp. does not seem to survive in the soil over summer in Florida. In a field trial, asymptomatic nursery transplants harboring quiescent infections were confirmed as the major source of inoculum for these pathogens in Florida. Heat treatment of P. cactorum zoospores at 44°C for as little as 5 min was effective in inhibiting germination and colony formation; however, oospore germination was not inhibited by any of the tested temperatures in vitro. In the field, thermotherapy treatment of inoculated plants was shown to have great potential to serve as a nonchemical approach for managing Phytophthora crown rot in production fields and reducing mefenoxam-resistant populations in nursery transplants.

Survey of Trunk Pathogens in South African Olive Nurseries

Several fungal trunk pathogens are associated with olive trunk diseases in South Africa. Little is known regarding the inoculum sources of these pathogens in the olive industry, and no specific management strategies are in place. The aim of this study was to investigate the status of olive nurseries in South Africa, with regard to the presence of trunk pathogens in olive plant material, to determine whether nursery material can be considered inoculum sources contributing to long-distance dispersal of these pathogens. Isolations were made from asymptomatic cuttings from mother blocks (stage 1), asymptomatic and symptomatic rooted cuttings (stage 2), and 1- to 2-year-old trees (stage 3) of eight cultivars in two nurseries. Known olive trunk pathogens of Nectriaceae, Diaporthaceae, Botrysphaeriaceae, Togniniaceae, Phaeomoniellaceae, and Pleurostomataceae were recovered. Neofusicoccum australe was detected in a single stage 1 cutting. Stage 3 material showed the highest incidence of fungi from these families, with Pleurostoma richardsiae having the highest incidence in both nurseries (82.2 and 36.7% of the 1- to 2-year-old trees). Phaeoacremonium parasiticum was present in 28.9% of the trees from one nursery (stage 3). The remaining pathogens occurred in ≤13.3% of the material. These results indicate that nursery propagation material from mother blocks harbors low levels of trunk pathogens and that additional infections occur during the nursery process. Management strategies should focus on the prevention and elimination of infections in mother blocks as well as during the propagation process to ensure that pathogen-free material is delivered to producers.

First Report of Bacterial Wilt Caused by Ralstonia solanacearum on Plectranthus amboinicus in Martinique

Plectranthus amboinicus, commonly known as Gwo ten in the French West Indies (Martinique), is a semi-succulent perennial plant of the Lamiaceae family. This aromatic plant wich is widespread naturally throughout the tropics is of economic importance because of the therapeutic and nutritional properties attributed to its natural phytochemical compounds wich are highly valued in the pharmaceutical industry. In March 2019, wilted P. amboinicus plants intercropped with tomato plants (cv. Heatmaster) in order to reduce the insect-pest damages on tomato, were observed in a field located at the CIRAD experimental station in Lamentin, Martinique (14.663194 N, -60.999167 W). Average disease incidence of 65.74% was recorded on P. amboinicus, in 3 plots with an area of 22.04 m2. The initial symptoms observed were irregular, black, necrotic lesions on leaves. After 10 days, plants wilted and black stripes were observed on stems. Within 4 weeks, more than 50% of plants were fully wilted. Longitudinal stem sections of the wilted plants showed brown vascular discoloration. The cut stems of the wilted plants released a whitish bacterial ooze in water. In all, 108 stem sections were collected, surface disinfected with 70% ethanol and each was crushed in 2 mL of Tris-buffer, then processed for bacterial isolation by plating on modified Semi-Selective Medium from South Africa SMSA (Engelbrecht 1994). Typical Ralstonia solanacearum colonies grew on SMSA medium for 100 of the 108 samples after incubation for 48h at 28°C and were identified as Ralstonia solanacearum using diagnostic PCR with 759/760 primers (Opina et al. 1997). A phylotype-specific multiplex PCR (Fegan and Prior 2005) classified all the strains in R. solanacearum Phylotype IIA. A subset of 11 strains was selected for sequevar identification. All the strains were identified as sequevar I-39 (100% nucleotide identity with strain ANT92 - Genbank accession EF371828), by partial egl sequencing (Fegan and Prior 2005) (GenBank Accession Nos. MT314067 to MT314077). This sequevar has been reported to be widespread in the Caribbean and tropical America on vegetable crops (particularly on tomato), but not on P. amboinicus (Deberdt et al. 2014; Ramsubhag et al. 2012; Wicker et al. 2007). To fulfil Koch's postulates, a reference strain, isolated from diseased P. amboinicus (CFBP 8733, Phylotype IIA/sequevar 39), was inoculated on 30 healthy P. amboinicus plants. A common tomato cultivar grown in Martinique (cv. Heatmaster) was also inoculated on 30 plants with the same bacterial suspension. Three-weeks-old plants of both crops grown in sterilized field soil were inoculated by soil drenching with 20 ml of a calibrated suspension (108 CFU/mL). P. amboinicus and tomato plants drenched with sterile water served as a negative controls. Plants were grown in a fully controlled environment at day/night temperatures of 30-26°C ± 2°C under high relative humidity (80%). The P. amboinicus plants started wilting 9 days after inoculation, and within four weeks 60% of the P. amboinicus plants had wilted. The tomato plants started wilting 5 days after inoculation with 62% of wilted plants within four weeks. R. solanacearum was recovered from all symptomatic plants on modified SMSA medium. No symptoms were observed and no R. solanacearum strains were isolated from negative controls plants. To our knowledge, this is the first report of R. solanacearum causing bacterial wilt on Gwo ten (P. amboinicus) in Martinique. The importance of this discovery lies in the reporting of an additional host for R. solanacearum, which can be associated with other crops as tomato crop in order to reduce the abundance of insect-pests. Further studies need to be conducted to assess the precise distribution of bacterial wilt disease on P. amboinicus in Martinique and to develop a plan of action avoiding its association with R. solanacearum host crops as tomato for reducing epidemic risk.

Physical, Cultural, and Chemical Alternatives for Integrated Management of Charcoal Rot of Strawberry

Macrophomina phaseolina, the causal agent of charcoal rot in strawberry, induces plant wilting and collapse. The pathogen survives through the production of microsclerotia in the soil and in strawberry debris. However, its management is difficult, and the disease has become an increasing problem for the strawberry industry. Physical, cultural, and chemical alternatives for integrated management of charcoal rot were evaluated in laboratory and field trials during the 2017-18 and 2018-19 strawberry seasons. In a laboratory trial, M. phaseolina microsclerotia were subjected to heat treatment and germination was inhibited at 52, 56, 80, and 95°C after 30, 10, 1, and 0.5 min of exposure, respectively. In infected strawberry crowns, microsclerotial viability was reduced after 5 min, regardless of temperature, whereas in the field, reduction was observed after 1 min. In field trials, charcoal rot incidence of inoculated strawberry plants transplanted into white-striped plastic-mulched beds was reduced to 20.8%, compared with 60.8% for plants grown in the black plastic mulch. On commercial farms, crop residue removal from infested areas reduced the M. phaseolina population in the soil but did not decrease charcoal rot incidence. Moreover, M. phaseolina propagule densities in the soil and in strawberry debris was reduced by fumigant application at crop termination but surviving propagules allowed the population to increase over the summer. Furthermore, preplant fumigation with metam potassium reduced soil population and charcoal rot incidence. Overall, the adoption of integrated approaches such as physical, chemical, and/or cultural methods played a significant role in reducing M. phaseolina inoculum and contributed to control of the disease in areas with high disease pressure.

First Report of Phytophthora pini Causing Foliage Blight and Shoot Dieback of Rhododendron pulchrum in China

During a 2019-2020 survey for plant pathogenic oomycetes in Nanjing, China, severe foliage blight and dieback were observed on approximately 20 Rhododendron pulchrum plants at three public parks and gardens. Approximately 25% of leaves and shoots were affected. Symptoms included brown to black lesions on leaves and stems, dieback of shoot tips, and wilting. Diseased tissues were collected from a five-year-old shrub with typical disease symptoms at Xuanwuhu Park. They were cut into 10×10 mm2 squares, immersed in 70% ethanol for 30 sec, and placed onto fresh clarified V8 juice agar (cV8A) containing pimaricin, ampicillin, rifampicin, and pentachloronitrobenzene. Phytophthora-like hypae were transferred to new cV8A plates daily. A total of five isolates were obtained after five days of incubation at 25°C. After approximately 20 days, all isolates were identical in morphological traits including semi-papillate sporangia and gametangia (homothallic). Thirty sporangia of a representative isolate Ppi were randomly selected and examined. They were mostly ovoid and sometimes obpyriform, averaging 41.0 ± 3.9 × 24.8 ± 3.2 µm. Antheridia of 30 randomly selected gametangia were paragynous, averaging 16.7 ± 0.7 × 12.4 ± 1.5 µm. Average diameters of oogonia and plerotic oospores were 29.2 ± 0.3 µm and 26.4 ± 1.6 µm, respectively. Chlamydospores were not observed. The above morphological traits suggested the causal agent belonging to the "P. citricola-complex". Isolate Ppi was subjected to sequencing of the rDNA internal transcribed spacer (ITS) region and the ras-related GTP-binding protein 1 (Ypt1) gene. ITS sequence of Ppi (GenBank ACN. MT672594) has 100% identity to that of P. pini (MG865565). It has a 3-nt difference from the ITS sequences of P. acerina (MG518642) and P. citricola (MG865475) and a 4-nt difference from that of P. plurivora (FJ665225). Ypt1 sequence of Ppi (MT680000) has 100% identity to that of P. pini (MK058416). Pathogenicity of Ppi on R. pulchrum was tested using both detached-leaf and whole-plant assays. In the former assay, each of six asymptomatic leaves was symmetrically wounded at both sides using a sterile inoculation needle. A 5×5 mm2 Ppi-colonized cV8A plug was placed on each wound of five leaves. Sterile agar plugs were used for a control leaf. All six leaves were placed on a wet filter paper in a closed container at 25°C. This assay was repeated twice. On the fifth day, all inoculated leaves had necrotic tissues around the wounds, while the control leaves remained asymptomatic. In the whole-plant assay, 20-inch-tall plants were used. Five attached leaves and the twig base of each plant were wounded. A control plant was inoculated in the same manner above, while sterile agar plugs were used. Each plant was covered with a plastic bag and maintained at 25°C. Wet cotton balls were placed in the bags to maintain humidity. After two days, the bag containing cotton balls was removed. This assay was repeated three times. After two weeks, all three inoculated plants in the three replicated trials had severe foliage blight and dieback, whereas control plants remained healthy. Phytophthora isolates recovered from artificially inoculated tissues were identical to isolate Ppi in morphological characters. Rhododendron diseases caused by P. pini were reported in the USA and Finland . This is the first report of P. pini causing foliage blight and dieback on R. pulchrum, an important nursery and landscape plant in China. Additional surveys are ongoing to determine the distribution of this pathogen in Nanjing. Management programs are under development to contain the spread of P. pini and treat diseased plants.

LONG-TERM SURVIVAL OF PSEUDOGYMNOASCUS DESTRUCTANS AT ELEVATED TEMPERATURES

White-nose syndrome is an emerging fungal disease that has devastated hibernating bat populations across eastern North America. The causal pathogen, Pseudogymnoascus destructans (PD), is a psychrophilic fungus with a known maximal growth temperature of 20 C. Although it is widely speculated that PD is primarily spread between hibernacula by the movement of bats, experimental evidence is lacking to demonstrate that PD can endure temperatures experienced by active bats for periods of time that would facilitate dispersal of viable fungus. We used an in vitro culture-based approach to study the survival of PD conidia on three artificial growth media and bat fur. The fungus was incubated at three temperatures it might realistically be exposed to on nonhibernating bats or in the environment outside of caves and mines (24 C, 30 C, and 37 C). When incubated on artificial media, we found that PD conidia were able to survive for a maximum of 150 d when exposed to temperatures of 24 C, 60 d at 30 C, and 15 d at 37 C. At all temperatures, maximal survival duration was recorded when conidia were incubated on brain-heart infusion agar with 10% volume of sheep (Ovis aries) blood. When incubated on bat fur, viable PD was recovered at 180 d, 60 d, and 5 d when exposed to temperatures of 24 C, 30 C, and 37 C, respectively. Our results suggest that viable PD conidia may be able to survive on or within the bodies of bats, which may facilitate long-distance dispersal. The long-term viability of the fungus on various fomites may differ, and therefore must be assessed for each potential substrate.

Postharvest Incidence of Stem End Rot in 'Hayward' Kiwifruit Is Related to Preharvest Botrytis cinerea Colonization of Floral Parts and Latent Infection

Stem end rot (SER) caused by Botrytis cinerea is the primary postharvest disease in the Chilean kiwifruit industry. Relationships between the postharvest occurrence of SER in 'Hayward' kiwifruit and the temporal dynamics of earlier B. cinerea colonization of the floral parts (petals, sepals, receptacles, styles) was studied in five orchards over two consecutive seasons in Chile. Weather conditions in the first season favored B. cinerea infection with roughly constant colonization of floral parts up to about 120 days after full bloom, but colonization then increased up until harvest. In the second season, colonization was roughly constant throughout. Latent infections of the fruit occurred in both seasons but were high in the first season and low in the second. Incidence of latent infections at harvest were the best predictors (r > 0.8) of postharvest SER. The number of preharvest infection periods calculated using temperature, leaf wetness, and relative humidity satisfactorily predicted SER incidence by an exponential model, R² = 0.90, P < 0.001. Results indicated environmental variables play key roles in the temporal dynamics of B. cinerea colonization. Quantification of latent B. cinerea infections in asymptomatic fruit close to harvest, is a practicable way to predict later incidence of SER during storage.

Prevalence of Acidovorax citrulli in Commercial Cucurbit Seedlots During 2010-2018 in China

Acidovorax citrulli is the causal agent of bacterial fruit blotch (BFB), a serious threat to cucurbit fruit and seed production worldwide. In recent years, the BFB has spread to many areas of China, mainly via the inadvertent distribution of contaminated commercial seeds. To assess the prevalence of seedborne A. citrulli in commercial watermelon and other cucurbitaceous seedlots in China, a 9-year survey was conducted between 2010 and 2018. A total of 4,839 seedlots of watermelon and other cucurbitaceous species were collected from 13 major seed production areas of China and tested by a semiselective media-based colony PCR technique for A. citrulli. Overall, A. citrulli was detected in 18.00% (871/4,839) of all cucurbitaceous seedlots. The bacterium was detected in 21.59% (38/176), 19.19% (33/172), 23.44% (214/913), 40.76% (247/606), 13.28% (85/640), 15.40% (95/617), 13.25% (73/551), 8.03% (48/598), and 6.71% (38/566) of all commercial seedlots tested from the 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, and 2018 growing seasons, respectively. Additionally, the prevalence of A. citrulli in cucurbit seedlots was determined for different seed production areas. The prevalence of A. citrulli in cucurbitaceous seedlots produced in Xinjiang, Gansu, Ningxia, Inner Mongolia, and 9 other provinces was 18.76% (582/3103), 26.34% (103/391), 21.47% (82/382), 11.11% (14/126), and 10.75% (90/837), respectively. This is the first survey for A. citrulli in commercial cucurbit seeds in China, and the relatively high prevalence suggests that commercial seeds represent a substantial source of primary inoculum that can threaten cucurbit seed and fruit production in China.

Cryptic Infection and Systemic Colonization of Leguminous Crops by Verticillium dahliae, the Cause of Verticillium Wilt

Verticillium dahliae, the cause of Verticillium wilt, is a widespread pathogen that affects many crops in California and throughout the world. Cover cropping with leguminous species is often integrated into a rotation scheme for its contribution to soil nitrogen, and can contribute to management of Verticillium wilt provided the chosen crop does not support development of V. dahliae. Seven cool season legumes (faba bean, bell bean, field pea, hairy vetch, common vetch, purple vetch, and woollypod vetch), and three warm season legumes (sesbania, sunn hemp, and black-eyed pea) were evaluated as hosts for reproductive growth of V. dahliae. All 10 legumes were colonized by V. dahliae, while remaining symptomless, when subjected to a root-dip inoculation. Similar results were obtained when plants were grown in infested potting soil, albeit with a lower frequency of infection than in root-dip assays. All tested legumes were also infected in field trials, with the exception of bell bean. Overall, warm season legumes sustained higher rates of infection than cool season legumes. Common vetch was the most extensively colonized of the cool season legumes. Based on the results of this study, legumes may not be an appropriate rotation crop in fields where Verticillium wilt is a problem.

Pathogenicity and a TaqMan Real-Time PCR for Specific Detection of Pantoea allii, a Bacterial Pathogen of Onions

Bacterial diseases of onion are reported to cause significant economic losses. Pantoea allii Brady, one of the pathogens causing the center rot on onions, has not yet been reported in Canada. We report the pathogenicity of P. allii on commercially available Canadian green onions (scallions). All P. allii-inoculated plants, irrespective of the inoculum concentration, exhibited typical leaf chlorotic discoloration on green onion leaves, which can reduce their marketability. Reisolation of P. allii from infected scallion tissues and reidentification by sequencing and phylogenetic analyses of the leuS gene suggest that the pathogen can survive in infected tissues 21 days after inoculation. This is the first report of P. allii as a potential pathogen of green onions. This study also reports the development and validation of a TaqMan real-time PCR assay targeting the leuS gene for reliable detection of P. allii in pure cultures and in planta. A 642-bp leuS gene fragment was targeted because it showed high nucleotide diversity and positively correlated with genome-based average nucleotide identity with respect to percent similarity index and identity of Pantoea species. The assay specificity was validated using 61 bacterial and fungal strains. Under optimal conditions, the selected primers and FAM-labeled TaqMan probe were specific for the detection of nine reference P. allii strains by real-time PCR. The 52 strains of other Pantoea spp. (n = 25), non-Pantoea spp. (n = 20), and fungi/oomycetes (n = 7) tested negative (no detectable fluorescence). Onion tissues spiked with P. allii, naturally infested onion bulbs, greenhouse infected green onion leaf samples, as well as an interlaboratory blind test were used to validate the assay specificity. The sensitivities of a 1-pg DNA concentration and 30 CFU are comparable to previously reported real-time PCR assays of other bacterial pathogens. The TaqMan real-time PCR assay developed in this study will facilitate reliable detection of P. allii and could be a useful tool for screening onion imports or exports for the presence of this pathogen.

Sources of Inoculum and Survival of Macrophomina phaseolina in Florida Strawberry Fields

Macrophomina phaseolina, the causal agent of charcoal rot, affects strawberry crowns, inducing plant collapse. The fungus survives in the soil through the production of microsclerotia and is usually controlled by preplant fumigation of soil. However, in the 2016 to 2017 Florida strawberry season, even after soil fumigation, about 30% plant mortality still occurred in plastic-covered beds that were used for a second season and where crop residue (mainly old strawberry crowns) was disposed of between beds. Therefore, this study was conducted to determine if M. phaseolina can survive on strawberry debris over summer in Florida and if so, verify whether strawberry debris might act as a source of inoculum for new transplants. Crowns from the previous season were collected from commercial farms where charcoal rot had been reported, and M. phaseolina was recovered from all samples. In a research field, infected crowns were buried in the soil at different depths and retrieved every 2 weeks during the summer. After 8 weeks, M. phaseolina could be recovered at all depths. Moreover, inoculation of strawberry plants by drenching the soil, dipping roots, or spraying leaves with a M. phaseolina microsclerotial suspension from pure cultures or infected crowns produced symptoms with differences in incubation periods depending on cultivar susceptibility. Furthermore, infected crowns disposed of in the aisles between beds or buried next to new transplants of cultivars Strawberry Festival, Florida Beauty, and Winterstar induced charcoal rot, with the level of aggressiveness depending on the cultivar susceptibility and inoculum placement in the field.

Determining the Sources of Primary and Secondary Inoculum and Seasonal Inoculum Dynamics of Fungal Pathogens Causing Fruit Rot of Deciduous Holly

Fruit rot of deciduous holly, caused by species of the genera Alternaria, Colletotrichum, Diaporthe, and Epicoccum, is affecting plant production in Midwestern and Eastern U.S. nurseries. To determine the sources of inoculum, dormant twigs and mummified fruit were collected, and leaf spot development was monitored throughout the season from three Ohio nurseries over two consecutive years. Mummified fruit was the main source of primary inoculum for species of Alternaria and Epicoccum, whereas mummified fruit and bark were equally important for species of Colletotrichum and Diaporthe. Brown, irregular leaf spots developed in the summer, and disease incidence and severity increased along with leaf and fruit development. Coalesced leaf spots eventually resulted in early plant defoliation. When tested for their pathogenicity on fruit, leaf spot isolates were able to infect wounded mature fruit and induce rot symptoms, which indicated that leaf spots could serve as a source of secondary inoculum for fruit infections. In addition, spore traps were used to monitor seasonal inoculum abundance in the nurseries. Fruit rot pathogens were captured by the spore traps throughout the season, with peak dissemination occurring during flowering. In this study, we also attempted to understand the role of environmental factors on leaf spot development. Although leaf spot incidence and severity were negatively correlated to mean maximum, minimum and average temperature, a decrease in temperature also coincided with leaf senescence. The role of temperature on leaf spot development should be further studied to fully interpret these results.

Frequent Implication of Multistress-Tolerant Campylobacter jejuni in Human Infections

Campylobacter jejuni, a major cause of bacterial foodborne illnesses, is considered highly susceptible to environmental stresses. In this study, we extensively investigated the stress tolerance of 121 clinical strains of C. jejuni against 5 stress conditions (aerobic stress, disinfectant exposure, freeze-thaw, heat treatment, and osmotic stress) that this pathogenic bacterium might encounter during foodborne transmission to humans. In contrast to our current perception about high stress sensitivity of C. jejuni, a number of clinical strains of C. jejuni were highly tolerant to multiple stresses. We performed population genetics analysis by using comparative genomic fingerprinting and showed that multistress-tolerant strains of C. jejuni constituted distinct clades. The comparative genomic fingerprinting subtypes belonging to multistress-tolerant clades were more frequently implicated in human infections than those in stress-sensitive clades. We identified unique stress-tolerant C. jejuni clones and showed the role of stress tolerance in human campylobacteriosis.

Food-Associated Stress Primes Foodborne Pathogens for the Gastrointestinal Phase of Infection

The incidence of foodborne outbreaks and product recalls is on the rise. The ability of the pathogen to adapt and survive under stressful environments of food processing and the host gastrointestinal tract may contribute to increasing foodborne illnesses. In the host, multiple factors such as bacteriolytic enzymes, acidic pH, bile, resident microflora, antimicrobial peptides, and innate and adaptive immune responses are essential in eliminating pathogens. Likewise, food processing and preservation techniques are employed to eliminate or reduce human pathogens load in food. However, sub-lethal processing or preservation treatments may evoke bacterial coping mechanisms that alter gene expression, specifically and broadly, resulting in resistance to the bactericidal insults. Furthermore, environmentally cued changes in gene expression can lead to changes in bacterial adhesion, colonization, invasion, and toxin production that contribute to pathogen virulence. The shared microenvironment between the food preservation techniques and the host gastrointestinal tract drives microbes to adapt to the stressful environment, resulting in enhanced virulence and infectivity during a foodborne illness episode.

Survival of Escherichia coli in common garden mulches spiked with synthetic greywater

Reuse of domestic wastewater is increasingly practiced as a means to address global demands on fresh water. Greywater is primarily reused via subsurface irrigation of gardens, where the soil environment is seen to be an integral part of the treatment process. The fate of biological contaminants (i.e. pathogens) in the soil is reasonably well understood, but their persistence and survival in soil cover layers is largely unexplored. This study investigated the ability of Escherichia coli to survive in common soil cover layers. Three garden mulches were investigated: pea straw mulch, a bark-based mulch and a coconut husk mulch. Each mulch was treated with an E. coli solution, a synthetic greywater with E. coli, or a freshwater control. Escherichia coli was applied at 1 × 10⁴  most probable number (MPN) per g dry weight mulch. Subsamples were temporally analysed for E. coli. The bark and coconut husk mulches showed a steady decline in E. coli numbers, while E. coli increased in the pea straw mulch for the duration of the 50 days experiment, peaking at 1·8 × 10⁸  MPN per g dry weight mulch. This study highlighted the importance of selection of a suitable material for covering areas that are subsurface irrigated with greywater.

SIGNIFICANCE AND IMPACT OF THE STUDY

Potential for microbial contamination is one of the limiting factors for domestic greywater reuse. Although subsurface irrigation is considered to be one of the lowest risk applications, there is still a possibility of microbes reaching the soil surface if the environmental conditions are not favourable or if soil movement inadvertently exposes the irrigation line. In these circumstances, the soil cover layer may be contaminated by greywater microbes. This study assesses the survival rates of the pathogen indicator organism Escherichia coli in three soil cover materials commonly used worldwide and makes clear recommendations to facilitate the safe reuse of domestic greywater.

Survival and Persistence of Alternaria dauci in Carrot Cropping Systems

Alternaria dauci was recovered in California from carrot crop residue and from volunteer carrot plants in fallow carrot fields. The fungus was not recovered from common weeds surrounding fallow fields. To evaluate further the survival of A. dauci on carrot crop residue, infected carrot leaf tissue was placed in fields or in soil in greenhouse pots, and recovered over time. In California, A. dauci was recovered from infected leaf tissue in both fallow and irrigated fields for as long as 1 year. In Florida, A. dauci was recovered from infected leaf tissue in fallow fields for up to 30 weeks. In greenhouse experiments, A. dauci was recovered from infected leaf tissue for as long as 1 year in dry soil, but only up to 30 weeks in soil that was watered weekly. To determine the infectivity of A. dauci borne on carrot crop residue, infected carrot crops were incorporated into organic and mineral field soils, and soil samples were collected over time. Carrot seed were planted in collected soil, and seedling infection by A. dauci was recorded. Seedling infection was detected up to 13 and 14 weeks after crop incorporation in organic and mineral soil, respectively. Seedling infection was detected for up to 5 weeks in soil that remained dry compared with 3 weeks in flooded soil.

Growth, Injury, and Survival Potential of Yersinia enterocolitica . Listeria monocytogenes , and Staphylococcus aureus in Brine Chiller Conditions †

A model brine system was used to evaluate growth, injury, and survival potential of Yersinia enterocolitica . Listeria monocytogenes , and Staphylococcus aureus . Each strain was incubated for up to 30 days at -12 to 28°C in brain heart infusion broth containing 0.5 to 20% NaCl. Samples were enumerated on a dual agar plating system to assess growth and injury. Y. enterocolitica grew at -2°C in 0.5% brine and at 5°C in 5% NaCl. L. monocytogenes grew at 5°C in 5% NaCl and at 12°C in 9% NaCl. S. aureus grew at 12°C in 5% NaCl. Significant injury was observed for two of the pathogens, but not for L. monocytogenes . Bacteriostatic or lethal conditions were maintained for the three organisms at -2°C and 9% NaCl. While lethal NaCl and temperature combinations were defined for Y. enterocolitica and S. aureus . L. monocytogenes survived for 30 days at -12°C in 20% NaCl. This study provides safety criteria and recommendations for use in the operation of recycle brine systems for cooling processed foods.

Bioluminescence: A Rapid Indicator of Escherichia Coli O157:H7 in Selected Yogurt and Cheese Varieties

Outbreaks of enterohemorrhagic Escherichia coli O157:H7 have been commonly associated with products derived from ground beef, but recently the organism has been implicated as the causative agent in outbreaks involving yogurt and cheese. This finding has raised concern about the potential for its growth and survival in fermented dairy products. A bioluminescent strain of E. coli O157:H7 was used to determine postprocessing survival in yogurt with live cultures at pH 4.17, 4.39, and 4.47 stored at 4 and 10°C. In addition, survival of E. coli O157:H7 was monitored during the manufacture of Cottage, Colby, Romano, and Feta cheeses. Results indicated survival for 8 and 5 days at 4 and 10°C respectively in yogurt at pH 4.17, 17 and 15 days at 4 and 10°C respectively in yogurt at pH 4.39, and 17days at both 4 and 10°C in yogurt at pH 4.47. E. coli O157:H7 did not survive cooking procedures at 56°C in Cottage cheese. However, the pathogen survived for 27, 30, and 27 days in Colby, Romano, and Feta cheeses respectively. A high correlation of r² > 0.89 was obtained between counts of bioluminescenct colonies and standard plate count for all yogurt and cheese varieties, indicating that bioluminescence was a sensitive and rapid indicator of cellular viability for E. coli O157:H7. Survival of the pathogen, as indicated by this method, is possible in highly acidic environments even at refrigeration temperatures. This poses a potential hazard should postprocessing contamination occur.

Use of Luminescent Strains of Salmonella enteritidis To Monitor Contamination and Survival in Eggs

Fresh and retail eggs were exposed to luminescent S. enteritidis cultures containing from 10⁴ to 10⁹ CFU/ml at either room temperature (approximately 21°C) for 3 days or 40°C for 16 h. The entry of S. enteritidis through egg shell was evidenced by luminescence in the eggs which was visualized using an Image Quantifier. The rate of contamination of the eggs increased with increasing inoculum size. Scanning electron microscopy was used to confirm the position of S. enteritidis cells in the eggs. The survival rate of the Salmonella cells in liquid eggs and whole shell eggs during storage at 4°C was investigated. Although S. enteritidis did not grow in eggs during storage at 4°C for up to 8 weeks, cells were able to survive. Under these storage conditions, the count was reduced by 1.7 to 2.5 log cycles per g in liquid egg and 0.8 to 1.4 log cycle per g in whole shell eggs. Similar trends were observed using both plate count and luminescence to monitor survival.