Development of a sequential sampling plan for classifying Thrips tabaci (Thysanoptera: Thripidae) populations in onion fields.

Onion thrips, Thrips tabaci Lindeman, is a global pest of onion crops, causing substantial economic damage by diminishing bulb yields and transmitting plant pathogens. Insecticides are used to manage T. tabaci infestations with control decisions traditionally based on action thresholds that require visually counting thrips on a fixed, predetermined number of onion plants per field. However, this approach for treatment decisions is inefficient when thrips populations are well above or below the action threshold. The aim of this research was to develop a sequential sampling plan that would provide a rapid and reliable classification of thrips populations in commercial onion fields above or below prespecified management thresholds. The study was conducted in a total of 24 commercial onion fields in New York in 2021 and 2022. Taylor's power law and Wald's Sequential Probability Ratio Test were used in concert to develop each sampling plan. Simulated and historical field data of thrips populations were used to further validate the efficacy of each sampling plan. Results demonstrated the sequential sampling plan required an average of 78% fewer samples to make a control decision compared with the traditional fixed-sampling approach. Treatment decisions were reached in 72% of cases after inspecting only 10 plants, while only 6% of the cases required examining more than 25 plants. Comparisons with fixed-sample sizes ranging from 23 to 68 plants revealed a 96% agreement in decision-making and a 78% reduction in sampling effort when using the sequential sampling plans.

First Report of Anthracnose on Banana Peppers Caused By Colletotrichum scovillei in New York.

In early August 2023, a disease outbreak on hot banana peppers (Capsicum annuum cv. Golden Dagger) was reported in Cattaraugus County, New York (NY). Disease incidence was at least 60%. Affected developing and mature fruit had at least one tan, soft, sunken lesion with salmon-colored spore masses surrounded by brown, necrotic margins. Microscopic observation of the lesions identified acervuli and setae typical of Colletotrichum spp. Isolations were made from these lesions by spreading conidia from the acervuli on 2% water agar (WA) + 0.02% (w/v) ampicillin. Colonies were hyphal tipped and transferred onto clarified V8 juice agar (CV8) and incubated at 20°C. The isolation frequency was 100% and a total of six isolates were obtained: Coll23Pep001, Coll23Pep003, Coll23Pep005, Coll23Pep007, Coll23Pep008, and Coll23Pep010. After 10 days, colonies were subcultured to potato dextrose agar (PDA) and CV8. On PDA, colonies appeared off-white to dark gray with sparse aerial mycelia. On CV8, the colony was pale gray with acervuli and orange-colored spore masses in the center. Conidia were hyaline, smooth and fusiform to round, and tapered at both ends. Mean conidial dimensions (n = 20) were 20.2 (13.75 to 25) µm long × 4.7 (3 to 6.25) µm wide. To confirm the identity of the isolates, DNA was extracted, and PCR performed to amplify the internal transcriber spacer (ITS) region (primers ITS1/ITS4; White et al. 1990), and actin (ACT) (primers ACT-512F/ACT-783R; Carbone and Kohn 1999) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes (primers GDF1/GDR1; Guerber et al. 2003). Pairwise alignment of the sequences showed all isolates had 100% similarity to C. scovillei ex. holotype CBS 126529 (Damm et al. 2012). Sequences from all isolates were deposited in GenBank with accessions PP556967 to PP556972 (ITS), PP565766 to PP565771 (ACT), and PP565772 to PP565777 (GAPDH). For pathogenicity testing, all isolates were grown on CV8 at 20°C in the dark for 10 days. Conidia were harvested by flooding the plates of each isolate with sterile distilled water and filtering the suspension through a double layer of cheesecloth. The concentration of the conidial suspension was adjusted to 5 × 105 spores per ml. Pathogenicity of the six isolates was tested on banana pepper fruit by using a sterile toothpick to pierce the skin at the two opposite ends. A droplet (10 µl) of the conidial suspension was placed on each wound. The same number of fruit were inoculated without wounding, and non-inoculated control fruit received a droplet of sterile distilled water (either wounded or unwounded). The experiment was repeated twice. All fruit were placed in a humid box at room temperature for 7 days. All wounded and inoculated fruit developed sunken lesions filled with salmon-colored conidial masses. Disease did not occur on the unwounded, inoculated fruit nor the non-inoculated controls. C. scovillei was recovered from all inoculated fruit by reisolating onto CV8 media and isolates had similar morphology and conidial dimensions to the original isolates. To the best of our knowledge, this is the first report of C. scovillei causing anthracnose on pepper in NY. C. scovillei has been reported in South Carolina (Toporek and Keinath 2021), Brazil (Caires et al. 2014), eastern Asia (de Silva et al. 2019), and Kosovo (Xhemali et al. 2023). The pathogen is particularly aggressive on pepper and poses substantial threats to pepper production around the world.

Nighttime Applications of Germicidal UV Light to Suppress Cercospora Leaf Spot in Table Beet.

Cercospora leaf spot (CLS), caused by the hemibiotrophic fungus Cercospora beticola, is a destructive disease affecting table beet. Multiple applications of fungicides are needed to reduce epidemic progress to maintain foliar health and enable mechanized harvest. The sustainability of CLS control is threatened by the rapid development of fungicide resistance, the need to grow commercially acceptable yet CLS-susceptible cultivars, and the inability to manipulate agronomic conditions to mitigate disease risk. Nighttime applications of germicidal UV light (UV-C) have recently been used to suppress several plant diseases, notably those caused by ectoparasitic biotrophs such as powdery mildews. We evaluated the efficacy of nighttime applications of UV-C for suppression of CLS in table beet. In vitro lethality of UV-C to germinating conidia increased with increasing dose, with complete suppression at 1,000 J/m2. Greenhouse-grown table beet tolerated relatively high doses of UV-C without lethal effects despite some bronzing on the leaf blade. A UV-C dose >1,500 J/m2 resulted in phytotoxicity severities greater than 50%. UV-C exposure to ≤750 J/m2 resulted in negligible phytotoxicity. Older (6-week-old) greenhouse-grown plants were more susceptible to UV-C damage than younger (2- and 4-week-old) plants. Suppression of CLS by UV-C was greater when applied within 6 days of C. beticola inoculation than if delayed until 13 days after infection in greenhouse-grown plants. In field trials, there were significant linear relationships between UV-C dose and CLS control and phytotoxicity severity, and a significant negative linear relationship between phytotoxicity and CLS severity at the final assessment. Significant differences between UV-C doses on the severity of CLS and phytotoxicity indicated an efficacious dose near 800 J/m2. Collectively, these findings illustrate significant and substantial suppression by nighttime applications of UV-C for CLS control on table beet, with potential for incorporation in both conventional and organic table beet broadacre production systems.

Identifying Onion Fields at Risk of Iris Yellow Spot Virus in New York.

Iris yellow spot virus (IYSV) poses a significant threat to dry bulb onion, Allium cepa L., production and can lead to substantial yield reductions. IYSV is transmitted by onion thrips, Thrips tabaci (Lindeman), but not via seed. Transplanted onion fields have been major early season sources of IYSV epidemics. As onion thrips tend to disperse short distances, seeded onion fields bordering transplanted onion fields may be at greater risk of IYSV infection than seeded fields isolated from transplanted ones. Additionally, seeded onion fields planted early may be at greater risk of IYSV infection than those seeded later. In a 2-year study in New York, we compared IYSV incidence and onion thrips populations in seeded onion fields relative to their proximity to transplanted onion fields. In a second study, we compared IYSV incidence in onion fields with either small or large plants during midseason. Results showed similar IYSV incidence and onion thrips populations in seeded onion fields regardless of their proximity to transplanted onion fields, while IYSV incidence was over four times greater in large onion plants than in small ones during midseason. These findings suggest a greater risk of onion thrips-mediated IYSV infection in onion fields with large plants compared with small ones during midseason and that proximity of seeded fields to transplanted ones is a poor indicator of IYSV risk. Our findings on IYSV spread dynamics provided valuable insights for developing integrated pest and disease management strategies for New York onion growers.

Spatiotemporal Patterns of Iris Yellow Spot Virus and Its Onion Thrips Vector, Thrips tabaci, in Transplanted and Seeded Onion Fields in New York.

Onion thrips, Thrips tabaci (Lindeman), transmits iris yellow spot virus (IYSV) and is one of the most important pests of Allium crops. IYSV is a member of the species Tospovirus iridimaculaflavi in the genus Orthotospovirus of the family Tospoviridae. This virus typically reduces overall onion bulb quality and weight but can also prematurely kill onion plants. IYSV is neither seed nor mechanically transmitted. Onion fields are typically established via seeds and transplants. A decade ago, onion thrips tended to colonize transplanted fields before seeded fields because plants in transplanted fields were larger and more attractive to thrips than smaller onions in seeded fields. Therefore, we hypothesized that the incidence of IYSV in transplanted fields would be detected early in the season and be spatially aggregated, whereas IYSV would be absent from seeded fields early in the season and initial epidemic patterns would be spatially random. In 2021 and 2022, IYSV incidence and onion thrips populations were quantified in 12 onion fields (four transplanted fields and eight seeded fields) in New York. Fields were scouted four times throughout the growing season (n = 96 samples), and a geospatial and temporal analysis of aggregation and incidence was conducted to determine spatiotemporal patterns in each field type. Results indicated that spatial patterns of IYSV incidence and onion thrips populations were similar early in the season, indicating that transplanted onion fields are no longer the dominant early-season source of IYSV in New York. These findings suggest the need to identify other important early-season sources of IYSV that impact New York onion fields.

Survival of Sclerotinia sclerotiorum Sclerotia in Central New York.

White mold caused by Sclerotinia sclerotiorum is a serious disease affecting many field and specialty crops in New York (NY). The primary inoculum for white mold is sclerotia that are hardened masses of mycelia that survive adverse environmental conditions and periods of non-hosts. However, NY crop guidelines lack rotation and residue management recommendations based on local knowledge of sclerotial survival. A field trial was established in October 2020 by deploying S. sclerotiorum sclerotia in mesh bags on the soil surface or shallowly buried (placed at 3 cm depth in the soil) at Geneva, NY. Bags were periodically collected from 67 to 769 days. At each time, sclerotial retrieval (number of sclerotia) was assessed by counted and viability evaluated through myceliogenic germination. Sclerotial retrieval was significantly affected by soil depth and was higher in those on the surface than buried. Time also affected the retrieval of sclerotia which was significantly reduced after 250 days. The interaction between burial and time had a significant effect on sclerotial viability. Approximately 15% of sclerotia placed on the surface were still viable after 769 days. After 433 days, viability of buried sclerotia was also significantly reduced compared to those on the surface. After 670 days, none of the buried sclerotia were viable. These findings suggest a rotation of at least two years between susceptible crops is required to reduce primary inoculum. However, given that low inoculum densities are sufficient to initiate a white mold outbreak, a longer rotation may be beneficial. In a cultivated system, timely tillage of crop residue to bury sclerotia after harvest to promote degradation is encouraged.

Phylogenetic relatedness can influence cover crop-based weed suppression.

Cover crops are plants grown to provide regulating, supporting, and cultural ecosystem services in managed environments. In agricultural systems, weed suppression services from cover crops can be an important tool to promote sustainability as reliance on herbicides and tillage for weed management has caused pollution, biodiversity loss, and human health issues. However, to effectively use weed suppression services from cover crops, farmers must carefully select species that fit within their rotations and suppress their problematic weeds. Understanding how the relatedness between cover crops and weeds affects their interactions will help farmers select cover crops for targeted weed management. The phylogenetic distance between species reflects their relatedness and was studied through a series of field experiments that compared weed suppression in winter and summer cover crops with tilled controls. This study demonstrates that cover crops can reduce up to 99% of weed biomass and alter weed community structure by suppressing phylogenetically related weed species. Results also suggest that cover crop planting season can influence weed community structure since only overwintering treatments affected the phylogenetic distance of weed communities. In an applied context, these results help develop cover crop-based weed management systems, demonstrating that problematic weeds can be managed by selecting phylogenetically related cover crop species. More broadly, this study provides a framework for evaluating weed communities through a phylogenetic perspective, which provides new insight into plant interactions in agriculture.

First Report of Cucurbit Yellow Vine Disease Caused by Serratia marcescens on Cucurbits in New York.

Cucurbits are one of the most significant commodities in New York, with a value of $92.3 million in 2021 (NASS-USDA 2021). In August 2021, several acorn squash (Cucurbita pepo) cultivar Turbinate plants at Cornell AgriTech research farm in Geneva, NY, had chlorotic, wilting leaves, and older leaves appeared scorched. The phloem of stems, bisected at the crown, had a honey-brown discoloration. The incidence of symptomatic plants was 22% in a one-acre planting field. Most of the symptomatic plants rapidly declined and died. The following year, similar symptoms were observed on muskmelon (Cucumis melo), acorn squash, and winter squash (C. pepo) cultivar Bush Delicata at the same location. These symptoms were typical of Cucurbit Yellow Vine Disease (CYVD) caused by the Gram-negative bacterium Serratia marcescens (Bruton et al. 1998, 2003). Moreover, a high incidence of squash bugs (vector of CYVD) was observed. To identify the causal agent, 45 stems from the symptomatic Bush delicata plants were collected. Each stem was cut into small pieces (2 to 3 mm), surface sterilized with 70% ethanol for 60 sec, 10% bleach for 60 sec, and rinsed with sterile water. The tissue was macerated in sterile water, and the resultant suspension was streaked on King's B (KB) medium (King et al. 1954). Plates were incubated at 28°C for 24 h, and 11 developed white, round bacterial colonies that were smooth and creamy in appearance. Single colonies were transferred to new KB plates and incubated for 24 h. The genomic DNA of two isolates (22212 and 22213) was extracted with the Wizard® Genomic DNA Purification Kit Protocol (Promega, Madison, WI). PCR was carried out using YV1 and YV4 primers specific to the 16S rDNA region of S. marcescens and 79F/R primers specific for S. marcescens causing CYVD (Zhang et al. 2005). The DNA sequence of each PCR product was obtained using Sanger sequencing and submitted to GenBank. Accessions OQ584799 and OQ584800 for YV1/YV4 (isolates 22212 and 22213, respectively) exhibited 100% identity to S. marcescens (384/384 bp, nearest accession identity: CP083754). Accession numbers OQ693911 and OQ693912 for 79F/R showed 99% identity to S. marcescens isolates (309/313 bp, nearest accession identity: CP033623). To fulfill Koch's postulates, Bush Delicata squash plants were grown for two weeks in a greenhouse, and three plants per isolate were inoculated using S. marcescens 22212 and 22213, three plants with Escherichia coli DH5a as a non-pathogenic control, distilled water as a mock-inoculated control, and a noninoculated control. Inoculation was performed by taking a single bacterial colony with a small pin and puncturing the plant's lower stem four to five times (Bruton et al. 2003). Twenty-eight days after inoculation, three of the six plants inoculated with the two S. marcescens isolates (two from 22212 and one from 22213) developed CYVD symptoms as observed in the field. Isolations were made from the stems of symptomatic plants and the mock-inoculated controls. PCR was conducted using YV1/YV4 primers and 79F/R primers (Zhang et al. 2005). Only isolations from symptomatic plants amplified with these primers and PCR products were sequenced. These sequences were identical to the original isolates. To our knowledge, this is the first report of CYVD and phytopathogenic S. marcescens in New York. The impact of CYVD can be substantial, with losses up to 100% (Zhang et al. 2005). Therefore, more knowledge on S. marcescens is needed to determine its biology and prevalence in New York.

Enabling Population Biology Studies of Stemphylium vesicarium from Onion with Microsatellites.

Stemphylium leaf blight (SLB), caused by the fungus Stemphylium vesicarium, is dominant within the foliar disease complex affecting onion production in New York (NY). The disease causes premature defoliation and significant reductions in bulb weight and quality. Foliar diseases of onion are usually managed by an intensive fungicide program, but SLB management is complicated by resistance to multiple single-site modes of action. The design of integrated disease management strategies is limited by incomplete knowledge surrounding the dominant sources of S. vesicarium inoculum. To facilitate genomic-based studies of S. vesicarium populations, nine microsatellite markers were developed. The markers were multiplexed into two PCR assays containing four and five fluorescently labeled microsatellite markers. Initial testing of the S. vesicarium isolates found the markers were highly polymorphic and reproducible with an average of 8.2 alleles per locus. The markers were used to characterize 54 S. vesicarium isolates from major NY onion production regions in 2016 (n = 27) and 2018 (n = 27). Fifty-two multilocus genotypes (MLGs) were identified between these populations. Genotypic and allelic diversities were high in both the 2016 and 2018 populations. A greater degree of genetic variation was observed within populations than between years. No distinct pattern of MLGs according to population was identified and some MLGs were closely related between 2016 and 2018. The lack of evidence for linkage among loci also was strongly suggestive of clonal populations with only minor differences between the two populations. These microsatellite markers will be a foundational resource for the testing of hypotheses surrounding the population biology of S. vesicarium and therefore informing disease management.

Distinct Red Blotch Disease Epidemiological Dynamics in Two Nearby Vineyards.

Grapevine red blotch virus (GRBV) causes red blotch disease and is transmitted by the three-cornered alfalfa hopper, Spissistilus festinus. GRBV isolates belong to a minor phylogenetic clade 1 and a predominant clade 2. Spatiotemporal disease dynamics were monitored in a 1-hectare 'Merlot' vineyard planted in California in 2015. Annual surveys first revealed disease onset in 2018 and a 1.6% disease incidence in 2022. Ordinary runs and phylogenetic analyses documented significant aggregation of vines infected with GRBV clade 1 isolates in one corner of the vineyard (Z = -4.99), despite being surrounded by clade 2 isolates. This aggregation of vines harboring isolates from a non-prevalent clade is likely due to infected rootstock material at planting. GRBV clade 1 isolates were predominant in 2018-2019 but displaced by clade 2 isolates in 2021-2022, suggesting an influx of the latter isolates from outside sources. This study is the first report of red blotch disease progress immediately after vineyard establishment. A nearby 1.5-hectare 'Cabernet Sauvignon' vineyard planted in 2008 with clone 4 (CS4) and 169 (CS169) vines was also surveyed. Most CS4 vines that exhibited disease symptoms one-year post-planting, likely due to infected scion material, were aggregated (Z = -1.73). GRBV isolates of both clades were found in the CS4 vines. Disease incidence was only 1.4% in non-infected CS169 vines in 2022 with sporadic infections of isolates from both clades occurring via secondary spread. Through disentangling GRBV infections due to the planting material and S. festinus-mediated transmission, this study illustrated how the primary virus source influences epidemiological dynamics of red blotch disease.

Sampling, a New iOS Application for Assessment of Damage by Diseases and Insect Pests Using Sequential Sampling Plans.

Regular scouting for plant diseases and insect pests by growers, crop consultants, extension educators, and researchers (herein defined as stakeholders) is the cornerstone of integrated pest management practices. Sequential sampling plans have the potential to save time and labor in field scouting and reduce the frequency of errors surrounding decision-making. The incorporation of the algorithms behind sequential sampling plans into mobile devices can make scouting for diseases and insect pests more straightforward, practical, and enjoyable. Here, we introduce an iOS application called Sampling. The application was designed for stakeholders to use on a mobile device for assessing disease and insect pest incidence in the field using sequential sampling plans. The application allows users to select a disease or insect pest from a prepopulated list and specify the objective of sampling: Estimation or classification. Conducting sequential sampling depends upon different precision levels and action thresholds within each objective. Detailed instructions for each sequential sampling plan are available as a guide. When sampling begins, users enter the number of diseased individuals at each sampling unit. The specific algorithm developed for the disease or insect pest will inform the user when to stop sampling for the desired goal and return the final incidence and precision or threshold achieved. Results are automatically saved in the application, and the user can inspect and share results by exporting them to a range of compatible programs. The initial version of Sampling (1.1) was released with the sequential sampling plans for Cercospora leaf spot of table beet. Sequential sampling plans for additional diseases or pests will be added to Sampling in subsequent versions. Sampling is available as a free download from the Apple Store (https://apple.co/3pUiYKy) and is compatible with iOS 14.0 or greater on the iPhone or iPad.

First Report of Halo Blight on Hop Caused by Diaporthe humulicola in New York.

In late July and August 2015, foliar disease was observed in three hop (Humulus lupulus; unknown cultivars) yards in Ontario, Otsego, and Putnam counties, New York (NY). Disease incidence ranged between 70 and 90% of plants, and up to 25% of the leaves per plant were affected. Leaf symptoms were large, necrotic patches with a chlorotic halo (2 to 10 cm diam.). Leaves and dry, easily shattered cones were placed at high humidity for 10 days. Pycnidia were abundant in leaf lesions which extruded conidia. Pycnidia were also observed on cone bracts and bracteoles. Fifteen isolations were made from each yard by placing a pycnidium onto 2% water agar + 0.02% (w/v) ampicillin. Colonies were hyphal tipped and transferred to potato dextrose agar (PDA) before incubation at 20°C with a 12-h photoperiod. Colonies on PDA had flat mycelia and were white to cream in color. The isolation frequency was 100%. To induce sporulation, five isolates were grown on PDA with autoclaved alfalfa stems for 7 to 10 days. Alpha conidia were hyaline, and oval with obtuse ends. Mean alpha conidial dimensions were (n = 20): 9.1 m × 3.4 µm (BE1; Ontario Co.); 11.8 × 3.8 µm (BE34; Ontario Co.); 9.6 × 4.1 µm (BE10; Ontario Co.); 10.2 × 3.7 µm (BE52; Otsego Co.); and 10.3 × 3.6 µm (BE69; Putnam Co.). Beta conidia were not observed. DNA was extracted and PCR performed to amplify the internal transcribed spacer (ITS) region (primers ITS1/ITS4; White et al. 1990), translation elongation factor 1-α (TEF; EF1-728F/EF1-986R; Carbone and Kohn 1999), a partial region of ß-tubulin (TUB; Bt2a/Bt2b; Glass and Donaldson 1995), a partial region of histone 3 (H3) (H3; CYLH3F/H3-1b Crous et al. 2004), and calmodulin (CAL; CAL-228F/CAL2Rd; Groenewald et al. 2013) genes. For all NY isolates, sequence similarity was >99% to D. humulicola CT2018-3 for the ITS region, and TEF, HIS, and CAL genes. Sequence similarity to CT2018-3 for the TUB region ranged from 86.96% (BE-1) to 96.15% (BE-10). . Analyses with the ITS, TEF, CAL, and HIS sequences supported our identification of the NY isolates as D. humulicola. Sequences were deposited in GenBank (OM370960 to OM370984). For pathogenicity testing, BE-34 and BE-69 were grown on PDA + autoclaved alfalfa stems at room temperature and a 12-h photoperiod for 10 days. Conidia were harvested by flooding the plate with sterile water. Conidial concentration was quantified, and the inoculum suspension diluted to ~5  105 (+ 0.01% polysorbate-20)/ml. Five cv. Cascade plants were sprayed with inoculum until run-off and covered with a plastic bag for 72 h. Non-inoculated control plants were sprayed with 0.01% polysorbate-20 and bagged. Plants were placed in a misting chamber and exposed to alternating 25°C light/18°C dark with a 16 h photoperiod. Mist was applied for 1 h daily. Necrotic lesions like the field specimens were observed on all inoculated plants after 28 days with no symptoms on control plants. Diseased leaves were detached and placed in a humid chamber for 2 days, and pycnidia observed in lesions. The reisolation frequency of D. humulicola was 100%. Conidia from the isolates had similar morphology to the original isolates. This is the first report of halo blight caused by D. humulicola on hop in NY. Halo blight has been reported on hop and associated with significant yield loss through cone shattering in MI (Higgins et al. 2021), CT (Allan-Perkins et al. 2020), and Quebec, Canada (Hatlen et al. 2021). Research is needed to determine if management is warranted.

Control of Phoma Leaf Spot and Root Decay of Table Beet in New York.

Disease caused by Neocamarosporium betae (syn. Phoma betae, Pleospora betae) results in reductions in plant populations, foliar disease (Phoma leaf spot [PLS]), and root disease and decay in table beet. Disease caused by N. betae has reemerged as prevalent in organic table beet production in New York. The disease can also cause substantial issues in conventional table beet production. To evaluate in-field control options for conventional and organic table beet production, small-plot, replicated trials were conducted in each of two years (2019 and 2021). The fungicides, propiconazole and difenoconazole, and premixtures, pydiflumetofen + fludioxonil or pydiflumetofen + difenoconazole, provided excellent PLS and root decay control. Azoxystrobin provided excellent (69.9%) control of PLS in 2019 and lesser (40%) control in 2021. Field trial results complemented in vitro sensitivity testing of 30 New York N. betae isolates that were all highly sensitive to azoxystrobin (mean effective concentration to reduce mycelial growth by 50%, EC50 = 0.0205 µg/ml) and propiconazole (mean EC50 = 0.0638 µg/ml). Copper octanoate and microbial biopesticides containing either Bacillus amyloliquefaciens D747 or B. mycoides strain J provided moderate (68.5 to 74.6%) PLS control as reflected in epidemic progress. The Gompertz model provided the best fit to PLS epidemics reflecting a polycyclic epidemic. Reductions in PLS severity were associated with significant decreases in Phoma root decay and increases in canopy health and the time-to-death of leaves compared with nontreated control plots. Prolonging leaf survival is critical for mechanical harvest of roots. These findings underpin the design of programs for foliar disease control in conventional and organic table beet production. Assessment of PLS severity in the field will better inform postharvest management decisions.

Comparing the Fungicide Sensitivity of Sclerotinia sclerotiorum Using Mycelial Growth and Ascospore Germination Assays.

The infection of the floral tissues of snap bean and other crops by Sclerotinia sclerotiorum, the causative agent of white mold, is by ascospores. Irrespective of the fungicide mode of action being evaluated, in vitro fungicide sensitivity tests are conducted almost exclusively using mycelial growth assays. This is likely because of difficulties and time involved in sclerotial conditioning required to produce apothecia and ascospores. The objective of this research was to compare estimates of fungicide sensitivity between mycelial growth and ascospore germination assays for S. sclerotiorum. Sensitivity assays were conducted using serial doses of three fungicides commonly used to control white mold: boscalid, fluazinam, and thiophanate-methyl. A total of 27 isolates were evaluated in replicated trials conducted for each fungicide and assay type. The effective concentration to reduce mycelial growth or ascospore germination by 50% (EC50) was estimated for each isolate, fungicide, and assay type. The median EC50 values obtained from ascospore germination assays were 52.7, 10.0, and 2.7 times higher than those estimated from the mycelial growth for boscalid, fluazinam, and thiophanate-methyl, respectively. No significant correlation was found between EC50 values estimated by the two methods. These findings highlight differences that may be important in evaluating the sensitivity of S. sclerotiorum given the fungicide mode of action and how they will be used in the field.

Seedborne Cercospora beticola Can Initiate Cercospora Leaf Spot from Sugar Beet (Beta vulgaris) Fruit Tissue.

Cercospora leaf spot (CLS) is a globally important disease of sugar beet (Beta vulgaris) caused by the fungus Cercospora beticola. Long-distance movement of C. beticola has been indirectly evidenced in recent population genetic studies, suggesting potential dispersal via seed. Commercial sugar beet "seed" consists of the reproductive fruit (true seed surrounded by maternal pericarp tissue) coated in artificial pellet material. In this study, we confirmed the presence of viable C. beticola in sugar beet fruit for 10 of 37 tested seed lots. All isolates harbored the G143A mutation associated with quinone outside inhibitor resistance, and 32 of 38 isolates had reduced demethylation inhibitor sensitivity (EC50 > 1 µg/ml). Planting of commercial sugar beet seed demonstrated the ability of seedborne inoculum to initiate CLS in sugar beet. C. beticola DNA was detected in DNA isolated from xylem sap, suggesting the vascular system is used to systemically colonize the host. We established nuclear ribosomal internal transcribed spacer region amplicon sequencing using the MinION platform to detect fungi in sugar beet fruit. Fungal sequences from 19 different genera were identified from 11 different sugar beet seed lots, but Fusarium, Alternaria, and Cercospora were consistently the three most dominant taxa, comprising an average of 93% relative read abundance over 11 seed lots. We also present evidence that C. beticola resides in the pericarp of sugar beet fruit rather than the true seed. The presence of seedborne inoculum should be considered when implementing integrated disease management strategies for CLS of sugar beet in the future.

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.

Stemphylium Leaf Blight: A Re-Emerging Threat to Onion Production in Eastern North America.

Stemphylium leaf blight (SLB), caused by Stemphylium vesicarium, is a foliar disease of onion worldwide, and has recently become an important disease in the northeastern United States and Ontario, Canada. The symptoms begin as small, tan to brown lesions on the leaves that can progress to defoliate plants. Crop loss occurs through reduced photosynthetic area, resulting in smaller, lower-quality bulbs. Leaf necrosis caused by SLB also can compromise bulb storage, as green leaves are required for the uptake of sprout inhibitors applied prior to harvest. The pathogen can overwinter on infested onion residue and infected volunteer plants. Asymptomatic weedy hosts near onion fields may also be a source of inoculum. Production of ascospores of the teleomorph (Pleospora allii) peaks in early spring in northeastern North America, often before the crop is planted, and declines rapidly as daily mean air temperatures rise. Conidia are usually present throughout the growing season. Application of fungicides is a standard practice for management of the complex of fungi that can cause foliar diseases of onion in this region. Recent assessments have shown that populations of S. vesicarium in New York and Ontario are resistant to at least three single-site mode-of-action fungicides. Three disease prediction systems have been developed and evaluated that may enable growers to reduce the frequency and/or number of fungicide applications, but the loss of efficacious fungicides due to resistance development within S. vesicarium populations threatens sustainability. The lack of commercially acceptable onion cultivars with sufficient resistance to reduce the number of fungicides for SLB also limits the ability to manage SLB effectively. Integrated disease management strategies for SLB are essential to maintain profitable, sustainable onion production across eastern North America.

Genome-Wide Association and Selective Sweep Studies Reveal the Complex Genetic Architecture of DMI Fungicide Resistance in Cercospora beticola.

The rapid and widespread evolution of fungicide resistance remains a challenge for crop disease management. The demethylation inhibitor (DMI) class of fungicides is a widely used chemistry for managing disease, but there has been a gradual decline in efficacy in many crop pathosystems. Reliance on DMI fungicides has increased resistance in populations of the plant pathogenic fungus Cercospora beticola worldwide. To better understand the genetic and evolutionary basis for DMI resistance in C. beticola, a genome-wide association study (GWAS) and selective sweep analysis were conducted for the first time in this species. We performed whole-genome resequencing of 190 C. beticola isolates infecting sugar beet (Beta vulgaris ssp. vulgaris). All isolates were phenotyped for sensitivity to the DMI tetraconazole. Intragenic markers on chromosomes 1, 4, and 9 were significantly associated with DMI fungicide resistance, including a polyketide synthase gene and the gene encoding the DMI target CbCYP51. Haplotype analysis of CbCYP51 identified a synonymous mutation (E170) and nonsynonymous mutations (L144F, I387M, and Y464S) associated with DMI resistance. Genome-wide scans of selection showed that several of the GWAS mutations for fungicide resistance resided in regions that have recently undergone a selective sweep. Using radial plate growth on selected media as a fitness proxy, we did not find a trade-off associated with DMI fungicide resistance. Taken together, we show that population genomic data from a crop pathogen can allow the identification of mutations conferring fungicide resistance and inform about their origins in the pathogen population.

Development of a Sequential Sampling Plan using Spatial Attributes of Cercospora Leaf Spot Epidemics of Table Beet in New York.

Sampling strategies that effectively assess disease intensity in the field are important to underpin management decisions. To develop a sequential sampling plan for the incidence of Cercospora leaf spot (CLS), caused by Cercospora beticola, 31 table beet fields were assessed in the state of New York. Assessments of CLS incidence were performed in six leaves arbitrarily selected in 51 sampling locations along each of three to six linear transects per field. Spatial pattern analyses were performed, and results were used to develop sequential sampling estimation and classification models. CLS incidence (p) ranged from 0.13 to 0.92 with a median of 0.31, and beta-binomial distribution, which is reflective of aggregation, best described the spatial patterns observed. Aggregation was commonly detected (>95%) by methods using the point-process approach, runs analyses, and autocorrelation up to the fourth spatial lag. For Spatial Analysis by Distance Indices, or SADIE, 45% of the datasets were classified as a random pattern. In the sequential sampling estimation and classification models, disease units are sampled until a prespecified target is achieved. For estimation, the goal was sampling CLS incidence with a preselected coefficient of variation (C). Achieving the C = 0.1 was challenging with <51 sampling units, and only observed on datasets with incidence >0.3. Reducing the level of precision, i.e., increasing C to 0.2, allowed the preselected C to be achieved with a lower number of sampling units and with an estimated incidence ([Formula: see text]) close to the true value of p. For classification, the goal was to classify the datasets above or below prespecified thresholds (pt) used for CLS management. The average sample number, or ASN, was determined by Monte Carlo simulations, and was between 20 and 45 at disease incidence values close to pt, and approximately 11 when far from pt. Correct decisions occurred in >76% of the validation datasets. Results indicated these sequential sampling plans can be used to effectively assess CLS incidence in table beet fields.

Cercospora beticola: The intoxicating lifestyle of the leaf spot pathogen of sugar beet.

Cercospora leaf spot, caused by the fungal pathogen Cercospora beticola, is the most destructive foliar disease of sugar beet worldwide. This review discusses C. beticola genetics, genomics, and biology and summarizes our current understanding of the molecular interactions that occur between C. beticola and its sugar beet host. We highlight the known virulence arsenal of C. beticola as well as its ability to overcome currently used disease management strategies. Finally, we discuss future prospects for the study and management of C. beticola infections in the context of newly employed molecular tools to uncover additional information regarding the biology of this pathogen. TAXONOMY: Cercospora beticola Sacc.; Kingdom Fungi, Phylum Ascomycota, Class Dothideomycetes, Order Capnodiales, Family Mycosphaerellaceae, Genus Cercospora. HOST RANGE: Well-known pathogen of sugar beet (Beta vulgaris subsp. vulgaris) and most species of the Beta genus. Reported as pathogenic on other members of the Chenopodiaceae (e.g., lamb's quarters, spinach) as well as members of the Acanthaceae (e.g., bear's breeches), Apiaceae (e.g., Apium), Asteraceae (e.g., chrysanthemum, lettuce, safflower), Brassicaceae (e.g., wild mustard), Malvaceae (e.g., Malva), Plumbaginaceae (e.g., Limonium), and Polygonaceae (e.g., broad-leaved dock) families. DISEASE SYMPTOMS: Leaves infected with C. beticola exhibit circular lesions that are coloured tan to grey in the centre and are often delimited by tan-brown to reddish-purple rings. As disease progresses, spots can coalesce to form larger necrotic areas, causing severely infected leaves to wither and die. At the centre of these spots are black spore-bearing structures (pseudostromata). Older leaves often show symptoms first and younger leaves become infected as the disease progresses. MANAGEMENT: Application of a mixture of fungicides with different modes of action is currently performed although elevated resistance has been documented in most employed fungicide classes. Breeding for high-yielding cultivars with improved host resistance is an ongoing effort and prudent cultural practices, such as crop rotation, weed host management, and cultivation to reduce infested residue levels, are widely used to manage disease. USEFUL WEBSITE: https://www.ncbi.nlm.nih.gov/genome/11237?genome_assembly_id=352037.