Brief Summary of Minutes of Annual Meeting

SAES-422 Multistate Research Project W-4008:

Integrated Onion Pest and Disease Management

Annual report for 2023

W4008 Committee Officers – 2023:

Chair: Frank Hay, Cornell University.  

Vice-Chair: Gabriel LaHue, Washington State University.

Secretary: Rob Wilson University of California.

Past Chair: David Burrell, National Onion Labs.

Contributors:

 CA: Brenna Aegerter*, Alex Putman, Jaspreet Sidhu*, and Rob Wilson

CO: Jane Davey, Eduardo Gutierrez-Rodriguez, and Mark Uchanski

GA: Bhabesh Dutta, Brian Kvitko, Jake Fountain, and Stormy Sparks

ID: Brenda Schroeder, Mike Thornton, and James Woodhall

NM:  Chris Cramer and Brian Schutte

NY: Brian Nault, Christy Hoepting, Sarah Pethybridge and Frank Hay.

PA: Beth Gugino

OR: Stuart Reitz and Jeremiah Dung

TX: Subas Malla

UT: Claudia Nischwitz

WA: Lindsey duToit, Timothy Waters, and Gabriel la Hue.

 *not current members of W4008

Accomplishments

Objective 1. Evaluate onion germplasm for resistance to pathogens and insects.

California

An onion variety trial was conducted within a commercial field in San Joaquin County to evaluate susceptibility of 12 onion varieties to bacterial bulb rots. Although there was no disease in the field trial, bulbs from each plot were stored and tested in the laboratory using two different potential screening assays. The objective was to continue our evaluation of these post-harvest assays as a possible means to complement field screening trials, which are challenging for numerous reasons. Unfortunately, the postharvest assay results do not correlate well with field trial results based on what we have seen in multiple years at multiple locations across the U.S.

Colorado

Laboratory and field studies were conducted to evaluate differences between 15 onion varieties (yellow, white, and purple/red) in relation to the human pathogen Salmonella.   Despite the documented antimicrobial properties of onion metabolites, no cultivar showed a significant reduction in Salmonella populations in laboratory or field studies.

Idaho

Thirteen commercial onion varieties and pre-commercial onion lines were screened for storability in cooperation with seed companies and McCain Foods.  Ten entries had more than 85% marketable bulbs after 9 months storage at 2 deg C.

 A total of 10 varieties grown in the Onion Variety Trial at the OSU Malheur Experiment Station were evaluated for resistance to Botrytis aclada.  Over 90% of inoculated bulbs showed symptomatic Botrytis neck rot disease progression among all varieties, with an average of 98% incidence. Disease severity among all inoculated bulbs was also much greater than the control group, with an average of 11.8% of halved bulb surface area discolored. Variety ‘Montero’ had the highest average disease severity (17.3% discolored inner surface), while ‘Tucannon’ had the lowest (7.4%). It should be noted that while ‘Montero’ tied for the lowest rate of natural infection, it rated poorly when inoculated.  Twenty varieties in the onion variety trial at OSU were also evaluated for pink root susceptibility.

New Mexico

NMSU breeding lines exhibited a lower Fusarium basal rot (FBR) incidence and severity than an FBR-susceptible and an FBR-resistant cultivar. Our breeding efforts have shown a reduction in FBR disease severity and incidence with selection. When exposed to high onion thrips pressure conditions conducive for Iris yellow spot (IYS) disease development, other NMSU breeding lines exhibited fewer thrips per plant and a lower disease severity early in the growing season, and greater bulb size at harvest than a commercial cultivar grown under the same conditions.

New York

An onion variety trial was conducted in Elba, NY that investigated the relative susceptibility of multiple commonly grown onion varieties to onion thrips and bacterial bulb rot. The trial was arranged as a split-plot design with variety as the main factor and insecticide treatment as the sub-plot factor and treatments replicated four times. Data were also collected on plant characteristics including leaf color, upright plant architecture, vigor, neck diameter, maturity and yield. Data entry, analysis and summary are pending.

Texas

The Texas A&M (TAM) AgriLife Vegetable Breeding program evaluated 21 advanced TAM onion lines, including 28 commercial checks against Pink Root (Setophoma terrestris) in a randomized complete block design with four replications. Four TAM germplasm (31034, 34066, 50084, and 50089) were resistant with a low severity of 5 (on a scale of 0 to 100). Additionally, 65 early generation TAM lines were evaluated in an augmented trial with ten commercial checks. A total of 39 TAM germplasm showed resistance to pink root with a severity of 5. Germplasm were further evaluated for variety development.

Utah

Six onion varieties were screened in a field trial for susceptibility to slippery skin caused by Burkholderia gladioli. In three varieties quite a few transplants died which significantly reduced the stand.  Stand density was correlated with rot, with the denser the stand, the less rot was observed.  This effect masked any potential differences between varieties in disease susceptibility.

Washington

Bulbs in storage for the 2022 Washington State University (WSU) Onion Cultivar Trial were evaluated in Feb. 2023 after 5 months in storage for quality and bulb rots. Results were summarized and shared with onion stakeholders on the WSU Onion Alerts (>600 subscribers), demonstrating how 53 cultivars fared under Columbia Basin production conditions. The 2023 Washington State University Onion Cultivar Trial was planted in April 2023 near Moses Lake, WA, with three replicate plots of each of 52 cultivars submitted by onion seed companies. The plots were evaluated regularly for diseases and pests. Bulbs were harvested in September to assess yield, and 50 bulbs/plot were placed in storage to evaluate for storage quality and bulb rots in Feb. 2024. The WSU Onion Field Day, at which stakeholders could look at the plots and hear presentations on onion research and other issues, was held on Aug. 31 in-person, and was well attended (>100 attendees).

 Objective 2. Investigate the biology, ecology and management of onion insect pests.

California

A field trial was conducted in Tulelake to evaluate the efficacy of insecticide seed treatments for suppressing seed corn maggot feeding on seedling onions.

Idaho

An experimental insecticide (GWN12030) applied as a foliar or drip application was compared to grower standard practices for impact on onion thrips populations and incidence of Iris Yellow Spot Virus (IYSV). Both application rate and timing were important factors in efficacy.  All treatments that significantly reduced insect populations on disease incidence also improved the yield of > 3 inch diameter onions.

New York

Surveys of maggot species infesting onions across northern production regions of North America revealed that seedcorn maggot is the dominant pest of onion in the Klamath and Columbia Basins, while onion maggot is the dominant pest of onion in the Treasure Valley and Great Lakes regions. This information is valuable for determining which insecticide seed treatments should be considered for different regions.

 Field research studies in 2023 documented onion maggot insensitivity to spinosad in one New York onion field. The LC₅₀ of that population to spinosad increased 165- fold since 2018 and 2.3-fold since 2021. Lumiverd (spinosad) seed treatment failed to protect the crop from onion maggots in that field in 2023, indicating that the population has developed resistance to spinosad.

 Field research studies in 2023 also revealed onion thrips insensitivity to spinetoram in several onion fields in the Elba production region. While we cannot confirm that onion thrips populations have developed resistance to spinetoram, there is considerable concern that this may be the situation.

 An on-farm small-plot trial revealed that a tank mix of Lannate LV + Agri-Mek SC + Warrior II was found to be as effective as the industry standard, Exirel 16 fl oz/A for controlling thrips when pressure was high (4 thrips/leaf at the time of the first spray). This treatment may be used instead of Radiant in the 2023 growing season.

Syngenta’s new insecticide, isocycloseram (PLINAZOLIN^® technology), was evaluated as a seed treatment for maggot control and as a foliar spray for onion thrips control. Isocycloseram alone and especially in combination with either thiamethoxam (Cruiser 5FS) or clothianidin + imidacloprid (Sepresto) provided protection of the crop against onion maggot and seedcorn maggot. Onion maggot control using isocycloseram alone and in combination with other products occurred in the field where the population is resistant to Spinosad. Isocycloseram also was one of the most effective products for protecting onions from onion thrips, including locations where onion thrips populations were not controlled effectively by spinetoram.

 Co-applying methomyl (Lannate LV) with spirotetramat (Movento/Senstar) early in the season significantly reduced populations of onion thrips early in the season as well as the numerical incidence of IYSV later in the season. Bulb yield was numerically greater in fields with lower levels of IYSV, but differences were not statistically significant.

Oregon

Onion thrips and thrips-transmitted IYSV are the key pests of onion in the Treasure Valley of eastern Oregon and southwestern Idaho. At present, insecticides remain the cornerstone for management of these pests. In 2023, we demonstrated that good thrips management reduces the incidence and severity of foliar fungal pathogens (Stemphylium leaf blight) and onion bulb rots. We also demonstrated that yield and size profiles of onions increase with shorter intervals between insecticide applications. In general, tank-mixing insecticides does not improve thrips management.

Texas

The TAM AgriLife Vegetable Breeding program evaluated insecticide programs to manage thrips. The trial was direct seeded at the Uvalde Center on October 15, 2022. The experiment was laid out in split-plot design with four replications. The main plot was variety with three levels (‘Hornet’, ‘Mata Hari’, and ‘Don Victor’), and the sub-plot was insecticide program with three levels (threshold application based on 1 thrips per leaf, weekly application, and control). We selected three products based on different modes of action (abamectin [Agri-Mek SC], spirotetramat [Movento], and spinetoram [Radiant SC]). Insects were counted on 10 tagged plants each week from February 23 to April 10, 2023. Insecticide programs included Agri-Mek SC followed by Movento and Radiant SC. Each insecticide was sprayed two subsequent times. The action threshold and weekly methods were not significantly different in managing thrips, however, both methods were significantly different from the control. The action threshold method needed only three applications of chemicals, whereas the weekly method used six applications of chemicals. In addition, onion germplasm comprising 18 Texas A&M selections and 24 commercial varieties were screened for thrips tolerance in a germplasm evaluation nursery. Two Texas A&M onion germplasm (40052 and 50023) showed tolerance to thrips.

Washington

Insecticide efficacy trials were conducted in 2023 to evaluate control options for onion thrips (Thrips tabaci) management. One trial featured a new insecticide set to be registered for use in 2024, plinazolin insecticide. Plinazolin was as efficacious as Radiant (spinetoram), currently the most effective insecticide available to onion producers in WA. Another trial evaluated organic products currently registered for use on onions. None of the organic insecticides provided consistent efficacy against onion thrips.

 Seedcorn maggot research was conducted in collaboration with stakeholders. Based on all data for both spray application plot locations, Diazinon as a pre application is still effective at reducing seedcorn maggot pressure and damage.  Based on stand counts and harvest data from two trial locations for seed treatments, Lumiverd, Sepresto 75 WS, Lumiverd + Sepresto 75 WS, and Plinazolin+ Cruiser 5FS as a seed treatment, effectively reduced seedcorn maggot pressure for a better plant stand and improved harvest yield. The commercial standards currently are Diazinon pre-plant and seed treatment with Lumiverd or Lumverd + Cruiser. Plinazolin will not be registered as a seed treatment in 2024, but when it becomes available in future years will be an alternative option for growers to manage seedcorn maggot in onions.

Objective 3. Investigate the biology, epidemiology and management of onion plant pathogens.

 California

A field trial was conducted in Kern County to evaluate the efficacy of bactericides against bacterial diseases (foliar symptoms and bulb rot). However, there was insufficient disease to draw good conclusions about efficacy.  Two field trials were conducted to examine efficacy of timing fungicide applications for downy mildew control based on environmental conditions measured using on-site weather stations.  Environmental disease models were used to determine if the measured weather conditions were favorable for disease and warranted fungicide application.  This strategy was compared to the standard approach of applications at regular intervals. The weather models indicated that conditions in 2023 were generally not favorable for disease, and indeed we did not observe disease in any trial location or year.  This result shows that this modeling approach can identify when environmental conditions are unfavorable for onion downy mildew development.

 To evaluate if downy mildew can be detected in air samples and used as an indicator of when to start applying fungicides, we placed air samplers to trap spores of the pathogen at the field trial locations described above.  The samplers collected air continuously, and samples were brought back to the lab one to three times per week.  In the lab, pathogen spores were quantified microscopically, and using quantitative PCR (qPCR).  Few spores of the pathogen were detected during the months-long monitoring period, a result which agrees with observations of no symptoms in the field trials and unfavorable conditions from the weather models.

Colorado

Trials were conducted to examine bacterial bulb rot management with bactericide (in season) and postharvest fogging trials during storage.

Georgia

The effect of neck-clipping length on post-harvest incidence of external and internal bacterial bulb rot in onion was evaluated in a field/storage trial.  Four rows of ‘Century’ onions were transplanted into 6-ft beds (panels) on 15 Dec at the Vidalia Onion and Vegetable Research Center, Lyons, GA. The fertility program was consistent with University of Georgia Extension Service recommendations. Experimental design consisted of a randomized complete block with four replications. Treated plots were 20-ft long and were separated on each side by non-treated border panels. Plots were separated by a 3 ft bare-ground buffer within the row. Thrips and disease management program was followed according to the UGA Cooperative Extension recommendation. Natural infection was relied upon. At harvest maturity, onion bulbs were undercut using a bed ridge frame undercutter (Parma Inc.,) followed by a three-day field curing period. Following curing, dried necks of onion bulbs were clipped manually at four different lengths; 1-in., 2-in., 3-in. and close-to the shoulder of the bulb (0-in.). Roots were also clipped but care was taken not to clip too close to the basal plate. Onion bulbs from each replicate plot (four replicates) were bagged and stored at 4℃ for 35 days. After storage, onion bulbs were individually cut using a sterile knife to determine the incidence of internal rot. Internal bulb rot was evaluated after a month of storage.  Individual onion bulbs were cut open with a sterile knife and assessed visually. The onion neck-clipping length had a significant effect on internal bulb rot incidence but not on the external rot. Significantly higher incidence of internal bulb rot was observed with the neck-clipping length of one inch or when the necks were cut close to the shoulders compared with the two and three inches. Internal rot was associated with mainly Pantoea spp., and Enterobacter spp. based on arbitrarily-collected symptomatic samples. Bulb rot due to post-harvest fungal pathogens (Aspergillus spp.) was not observed. 

 The ability of Bacillus subtilis QST 713 (Serenade ASO) or Bacillus mycoides isolate J (LifeGard WG) applied in rotation or tank-mix with copper (Nordox 75 WG) to manage center rot in onion leaves and bulbs was assessed in a field trial. Alternating the application of BCAs with copper at longer day intervals (10 or 15 d) could achieve a similar level of center rot control as their rotation at shorter day intervals (5 d). Under low disease pressure, two to three applications could be eliminated from the spray program without compromising disease suppression.  B. subtilis QST 713 and B. mycoides isolate J strains from respective products were copper-sensitive at concentrations >250 ppm. Our concurrent study on the tank mix of BCAs with copper showed that the standalone copper treatment had a similar level of disease control as their tank mix application. Frequent applications of copper may control disease relatively better than treatments without copper but the integration of BCA with copper in spray programs may largely depend upon their compatibility and modes of action of the BCA products.

 Center rot complex in onions is attributed to various Pantoea species, including P. ananatis, P. agglomerans, P. allii, and P. stewartii ssp. indologenes. These bacterial pathogens utilize the HiVir and alt gene clusters to induce necrosis and colonize onion tissues. However, not all Pantoea spp. possess these gene clusters. Research on virulence factors like motility, quorum sensing, and EPS production have been extensively studied in P. ananatis but not in P. agglomerans and P. stewartii ssp. indologenes, leaving their gene functions unexplored. In this study, we investigated the impact of quorum sensing (luxIR), EPS production (epsG), and flagellar motility (motB) genes on the pathogenicity of P. ananatis PNA 97-1, P. agglomerans AR1a, and P. stewartii subsp. indologenes PNA 03-3. Deletion of luxIR reduced acyl-homoserine lactones (AHLs) production, motB gene removal eliminated swimming motility, and epsG deletion decreased EPS production in all three Pantoea species. However, quorum sensing and flagellar motility gene deletions did not affect the pathogenicity of these Pantoea spp. on onions, contradicting previous findings. Further, we identified epsG as a novel gene contributing to exopolysaccharide production but was not involved in onion necrosis symptoms in three Pantoea spp. Our study used strains that are HiVir and alt positive, unlike previous strains that were HiVir positive only and lacked the alt gene cluster. Further studies would focus on investigating the roles of these three genes in three Pantoea spp. without the the alt gene cluster background.

Idaho

Curing duration and storage assays were completed by inoculating onion bulbs (cv. Vaquero) with Rahnella spp. and incubation of bulbs at 25°C (77°F), 30°C (86°F) or 35°C (95°F) for 2 days or 2 weeks and storage at 5°C for 4 and 6 months.  The 2 day curing assays determined that bulbs cured for 25°C (77°F) and stored for four or six months exhibited 42% or 58% bulb rot, respectively.  Bulbs cured for 2 weeks at 25°C (77°F) exhibited similar amounts of bulb rot.  Bulbs cured at 35°C (95°F) for 2 days or 2 weeks and stored for four or six months exhibited around 40% and 50% bulb rot.  Interestingly bulbs cured at 30°C (86°F) for 2 days or 2 weeks and stored for four or six months exhibited around 30% bulb rot.    Field trials were conducted investigating the impact of irrigation and planting date on decay.  A LAMPL assay for P. agglomerans was developed and demonstrated at the Parma IPM field day.

New York

 Three on-farm small-plot trials were conducted to evaluate fungicides for control of Stemphylium leaf blight (SLB) and Botrytis leaf diseases in muck-grown onions. Objectives included screening novel active ingredients for efficacy on these diseases and evaluating 2-3 product tank mixes of mediocre products with medium to low risk of fungicide resistance in an attempt to find a treatment that may be rotated with most effective double product-FRAC 3 treatment. We also staged some of the treatments to help us understand their effect on the development of fungicide resistance. For these treatments, SLB isolates were taken from leaf samples that were collected after the last fungicide sprays, and screened for fungicide sensitivity to propiconazole and tebuconazole. Results of fungicide sensitivity assays are still being analyzed, but a new FRAC 3 active ingredient was identified that had excellent control of SLB. 

Fungicide resistance monitoring

Testing of S. vesicarium against the FRAC 3 active ingredients difenoconazole (Quadris Top, Inspire Super), propiconazole (Tilt), and tebuconazole (Viathon, Luna Experience) has been undertaken to detect shifts in sensitivity of Stemphylium vesicarium, cause of Stemphylium Leaf Blight of onion. 

Between 2021 and 2022, S. vesicarium isolates from Elba and Wayne exhibited a similar profile of sensitivity to difenoconazole, suggesting no change in sensitivity.  However, S. vesicarium isolates from Oswego showed a continuing trend towards increased insensitivity to difenoconazole between 2020, 2021 and 2022. 

In Elba, Wayne and Oswego there appeared to be an increase in S. vesicarium isolates sensitive to tebuconazole (EC₅₀ > 10 µg/ml) remained similar between 2021 and 2022 in Elba and Wayne, and there was a general trend towards increased highly insensitive isolates in Oswego.   

Between 2021 and 2022, S. vesicarium isolates from Elba and Oswego exhibited a similar profile of sensitivity to propiconazole, suggesting no changes.  In Wayne, highly insensitive isolates (EC₅₀ > 10 µg/ml) were detected in 2021, but not in 2022.

Identification of inoculum sources of Stemphylium

In April/May 2023, prior to or shortly after direct seeding onion, the incidence of S. vesicarium in volunteer onion ranged from 20-80% in each of 10 fields in Elba, NY.  An initial study utilizing SSR’s indicated that genotypes of S. vesicarium present in volunteers were present in the succeeding crop at the end of the season indicating that S. vesicarium overwintering in volunteer onion could be a major contributor to SLB in the succeeding onion crop.   Approximately 25% of onion in NY is grown from bare-root transplants produced and imported from other States.  Samples of 11 batches of bare-root onion were obtained at delivery to farms in NY.  S. vesicarium was detected in all 11 batches, ranging in incidence from 74%-100%.   However, an initial study utilizing SSR’s indicated a poor relationship between genotypes of S. vesicarium present in transplants and those in the crop at the end of the season suggesting that S. vesicarium in transplant onion was a minor contributor to SLB in the succeeding onion crop compared to local sources of inoculum.  One hypothesis is that S. vesicarium isolates on transplants may exhibit less fungicide resistance in comparison to local sources in NY and become outcompeted by local sources when fungicides are applied.   The bulb rotting pathogen Botrytis aclada was also detected in leaves from 8 of the 11 batches of transplant onion ranging in incidence from 1.1-21.5% with identity confirmed by sequencing.  A species of Botrytis previously unrecorded in onion, but is pathogenic to onion, was also detected and is currently being characterized.

Assessment of nematode issues in NY onion.

A survey of fields in Elba, Oswego, Wayne, Yates and Orange was undertaken in June and July 2023 to determine the prevalence of nematode species.  Root Knot Nematode (RKN; Meloidogyne hapla) and root lesion nematode (Pratylenchus spp.) were detected at low population density, unlikely to cause adverse effects on onion yield. Stubby Root Nematode (SRN; Paratrichodorus allius) was prevalent and occurred at population densities which have been associated with yield losses in other onion growing regions in USA.

Monitoring of nematode populations in a grower’s field of non treated areas, and areas treated with Vydate or Velum Prime at planting demonstrated that both nematicides were effective at reducing SRN numbers early in the season.  Velum Prime and Vydate treatment increased the weight of standard sized bulbs in a 30-bulb sample by an average of 86.4 and 71.8% respectively compared to non treated. Velum Prime and Vydate treatment decreased the weight of boiler sized onion in a 30-bulb sample by an average of 76.8 and 62.5% respectively compared to non treated. Regression analysis indicated that for every 10 stubby root nematode/200 ml soil in June there was a loss in mean bulb weight in a 30-bulb sample of 0.128 kg, or 9.3%, mean weight of a bulb of 0.398 kg or 9.5%, mean weight of standard sized bulbs of 0.443 kg or 11.7%, and an increase in boiler sized of 0.111 kg or 58.1%.   SRN was widespread in onion fields in NY and occurred at population densities that would indicate yield loss. 

Oregon

White rot is a devastating soilborne fungal disease that limits the yield, marketability, and production of onion, garlic and other Allium crops. Growth chamber and field trials demonstrated that both encapsulated diallyl disulfide (DADS) and garlic oil inclusion complexes were equally or more effective at reducing populations of white rot sclerotia in soils when compared to liquid DADS or garlic powder.

Pennsylvania

Pathogenicity testing and data analysis was completed for the three-year onion bacterial disease survey conducted in both Pennsylvania and New York. Of the 43 bacterial genera tested using the red scale necrosis assay, Burkholderia and Pantoea were the predominant pathogenic genera, with 74.5% of Burkholderia and 44.1% of Pantoea being pathogenic. A subset of the non-pathogenic isolates were further tested in both whole bulb and foliar pathogenicity assays. Among those 37% of Pantoea (n=27) and 94.1% of Burkholderia (n=17) were found to be pathogen in either foliar, bulb or both assays.

A copper tolerance plate assay was used to evaluate the tolerance of Burkholderia isolates collected from PA and NY. Preliminary analysis has indicated that the majority of isolates (87%, n=38) tested exhibited a reduced copper sensitivity phenotype. Speciation of the Burkholderia isolates is also underway with the majority of isolates speciating as Burkholderia cenocepacia and B. cepacia.

Washington:

Genetic diversity of Iris yellow spot virus.

Barcoding of Thrips tabaci (onion thrips) from different onion producing regions was carried out. Onion thrips is a key pest of onion and acts as a vector of several viruses, including Iris yellow spot virus (IYSV), a serious viral pathogen of onion. There is limited information available on the genetic variation within and among T. tabaci populations in the United States, and key evolutionary parameters. In our study, 83 T. tabaci specimens were collected from A. cepa from 15 locations comprising four states in the United States. A total of 92 mtCOI gene sequences of T. tabaci was analyzed to understand the genetic diversity and structure of T. tabaci from onion. Seven distinct haplotypes of T. tabaci were identified from the collection, while nine T. tabaci sequences retrieved from GenBank comprised 5 haplotypes. Overall, 15 haplotypes of T. tabaci infesting A. cepa were identified in the world, including 10 haplotypes in the United States. In the phylogenetic analysis, all populations collected during the study clustered with the thelytokous lineage, while T. tabaci sequences retrieved from GenBank corresponded with the leek-associated arrhenotokous lineage. The greatest genetic variation was found in Elba, NY and Malheur, OR populations with 3 haplotypes in each. Results suggest that haplotypes 1 and 7 are more frequently prevailing haplotypes in the northwestern United States, with haplotype 1 being predominant all over the country. The eastern United States appears to have a more diverse group of haplotypes. Ongoing research included collection and typing of onion samples for the presence of IYSV. More than 20 isolates were identified, and selected regions of the IYSV RNA genome were amplified, with the nucleotide sequences determined. Sequence and bioinformatics analyses are being carried out. Information from this effort will provide insights into genome diversity, population structure, and evolutionary patterns of the virus, which could help refine IYSV detection.

Onion bacterial diseases.

As part of the ‘Stop the Rot’ USDA NIFA SCRI Project No. 2019-51181-30013, ongoing identification and characterization of bacterial strains was conducted from a survey over 3 seasons in 11 states across the USA to determine the diversity and prevalence of bacteria associated with onion diseases.  Hundreds of bacterial isolates from WA and CA were tested for pathogenicity on onion using scale, foliar, and bulb assays; and sequencing for genus and species determination. Results from five field trials in 2022-23 were finalized after rating bulbs in storage in Feb. 2023. Rolling onion tops at onset of ‘tops down’ did not affect bulb rot; undercutting early increased yield and reduced bulb rot compared to undercutting at 100% tops down or not undercutting; early topping of bulbs (while green) reduced yield by 54% and increased bulb rot by 32-35%. In 2023, we repeated field trials to evaluate timing of topping onion bulbs, effect of chemigated vs. spray boom applications of pesticides on bacterial diseases, and postharvest application of disinfectants on control of bacterial bulb rots in storage. The bulbs were harvested in Aug.-Sep. 2023 and placed in storage for bulb rot evaluations in Feb. 2024. Bulb rot ratings in storage have been completed and data analyses are in progress. Results will be published in Plant Disease Management Reports.

A PNW bacterial disease risk model was developed using 4 risk categories: current/cumulative risk, field variables, environmental variables, and production variables. Weekly scores were plotted over the season. The model was demonstrated at farms in WA and tested on ~12 bulb crops in 2023. Storage results from the farms are pending for validation. Data from trials in previous years also are being used to test the model. This was led by Project Manager, MacKay, who resigned in 2023 for medical reasons. du Toit is seeking expertise to continue developing the model and develop an app for stakeholders.

Three field trials were carried out in 2023 to investigate the impacts of irrigation and nitrogen management on onion bacterial diseases: (1) an irrigation frequency trial in Pasco, WA, (2) an N application timing trial in Othello, WA, and (3) an N application rate trial in Othello, WA. Total and marketable bulb yield, bulb size class distribution, and the incidence and severity of bacterial bulb rot were assessed at harvest and after 5 to 6 months of storage. Preliminary results suggest total yield increased with more frequent irrigation. However, marketable yield was comparable among plots subjected to irrigation frequency treatments, due to more frequent irrigation leading to an increase in bulbs culled due to bacterial rot. Similarly, in the N rate trial, total yield at harvest increased with increasing N rates for the cv. Calibra, but so did the weight of bulbs culled due to bacterial rot, such that statistically separable differences in marketable yield were not detected. No statistically separable differences were observed among N timing treatments.

Objective 4. Investigate the biology, epidemiology and management of weedy plant species that impact onion production.

California

A study was conducted in Tulelake to evaluate the efficacy of herbicides applied post-plant to control annual broadleaf weeds in direct seeded processing onions.

New Mexico

A post planting, delayed preemergence application of pendimethalin resulted in similar or better control of annual weeds than current weed control methods using Bensuilde and DCPA herbicides or pendimethalin applied at the 2-leaf stage for autumn-sown and winter-sown onions in New Mexico. This same application of pendimethalin did not impact onion stand and bulb yield while also not leaving any detectable residues on onion bulbs after harvest.

New York

Three small-plot onion herbicide trials were conducted in commercial muck onion fields: 1) To optimize weed control and crop safety with pre-emergent herbicides; 2) To improve control of volunteer potatoes with post-emergent herbicides with emphasis on incorporating new herbicide Optogen into the program; and 3) To extend weed control season long with late applications of pre-emergent herbicides. Data analysis is underway.  However, none of the treatments evaluated in this trial were more effective than repeated applications of Goal 2XL for controlling volunteer potato. Keeping volunteer potatoes as small as possible, ideally 4-6” by applying low doses of Goal 2XL 0.5-2 fl oz in young onions (flag+ to 1.25 leaf) when volunteer potatoes are 2-4” is key to achieving high rates of control (90% or more) when high rates of Goal 2XL may be applied to 2.5 leaf onion. This study demonstrated that adding Buctril 2E and Optogen to Goal 2XL 4 fl oz did not increase crop injury beyond that of Goal 2XL 4 fl oz alone. Thus, these tank mixes may be incorporated into POST herbicide spray program without increased risk of crop injury or negative impact on volunteer potato control. Although weed pressure was very low in this trial, our preliminary results suggest that pre-emergent herbicide applications made to 6-leaf (Jun 28, 77 d PHI) and 9-leaf (Jul 20, 55 d PHI) onion significantly reduced weed pressure at harvest. Generally, the 9-leaf applications, Chateau +/- Prowl H2O and Zidua demonstrated the most effective season-long residual weed control. Optogen 3.42 fl oz/A @ 6-leaf numerically reduced weed density but was not significantly different than the nontreated.

Washington:

Two onion newsletter articles were written and distributed to >600 subscribers, mostly in the Pacific Northwest USA, via the WSU Onion Alerts. The WSU Onion Field Day at Skone and Connors Farms, Moses Lake, WA on August 31 was attended by ~100 attendees, who inspected 3 replicate plots of 52 cultivars. du Toit, Waters, and Wohleb assisted onion stakeholders with disease, entomology, and production questions through the 2023 season. The USDA NIFA SCRI ‘Stop the Rot’ annual team meeting was held on 5-7 March 2024 at the WSU Mount Vernon NWREC, with 25 attendees, immediately following the W-4008 annual meeting at the same location the morning of 5 March 2024. The team shared updates on all objectives of the project, including to those who had been attending the W-4008 meeting.