A rapid glove-based inoculum sampling technique to monitor Erysiphe necator in commercial vineyards

Authors: LOWDER, SARAH - Oregon State University; Neill, Tara; PEETZ, AMY B - Revolution Crop Consultants, Llc; MILES, TIM - Michigan State University; MOYER, MICHELLE - Washington State University; OLIVER, CHARLOTTE - Washington State University; STERGIOPOULOS, I - University Of California, Davis; DING, SHUNGPING - California Polytechnic State University; Mahaffee, Walter - Walt

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/14/2023
Publication Date: 4/20/2023
Citation: Lowder, S.R., Neill, T.M., Peetz, A., Miles, T.M., Moyer, M.M., Oliver, C., Stergiopoulos, I., Ding, S., Mahaffee, W.F. 2023. A rapid glove-based inoculum sampling technique to monitor Erysiphe necator in commercial vineyards. Plant Disease. https://doi.org/10.1094/PDIS-02-23-0216-RE.
DOI: https://doi.org/10.1094/PDIS-02-23-0216-RE

Interpretive Summary: For western US grape producers, grape powdery mildew is a constant threat that can render the crop unmarketable, thus, growers make numerous fungicide applications each season in an attempt to reduce crop loses. Recently, mildew populations resistant to several fungicides has been identified to occur in all western US production regions. This research demonstrates that a novel sampling method is faster and more accurate in assessing presence of mildew than traditional methods and has the added utility of also monitoring for the present of fungicide resistant mildew populations. The sampling method can be used to more accurately assess disease risk and inform growers of potential for fungicide resistance.

Technical Abstract: Information on the presence and severity of grape powdery mildew (GPM), caused by Erysiphe necator, has long been used to guide management decisions. While recent advances in the available molecular diagnostic assays and particle samplers have made monitoring easier, there is still need for more efficient field collection of E. necator. The use of vineyard worker gloves worn during canopy manipulation as a sampler (glove swab) of E. necator was compared with samples identified by visual assessment with subsequent molecular confirmation (leaf swabs) and airborne spore samples collected by rotating-arm impaction traps (impaction traps). Samples from U.S. commercial vineyards in Oregon, Washington, and California were analyzed using two TaqMan qPCR assays targeting the internal transcribed spacer regions or cytochrome b gene of E. necator. Based on qPCR assays, visual disease assessments misidentified GPM up to 59% of the time with a higher frequency of misidentification occurring earlier in the growing season. Comparison of the aggregated leaf swab results for a row (n=915) to the row’s corresponding glove swab had 60% agreement. The latent class analysis (LCA) indicated that glove swabs were more sensitive than leaf swabs in detecting E. necator presence. The impaction trap results had 77% agreement to glove swabs (n=206) taken from the same blocks. The LCAs estimated that the glove swabs and impaction trap samplers varied each year in which was more sensitive for detection. This likely indicates that these methods have similar levels of uncertainty and provide equivalent information. Additionally, all samplers, once E. necator was detected, were similarly sensitive and specific for detection of the A-143 resistance allele. Together, these results suggest that glove swabs are a viable sampling method for monitoring the presence of E. necator and, subsequently, the G143A amino acid substitution associated with resistance to quinone outside inhibitor fungicides in vineyards. Glove swabs could significantly reduce sampling costs due to the lack of need for specialized equipment, and time required for swab collection and processing.