Detection of Airborne Sporangia of and . in Michigan Using Burkard Spore Traps Coupled to Quantitative PCR.

Cucurbit downy mildew (CDM), caused by the oomycete pathogen , is a devastating foliar disease on cucumber resulting in reduced yields. In 2004, the pathogen re-emerged in the United States, infecting historically resistant cucumber cultivars and requiring the adoption of an intensive fungicide program. The pathogen cannot overwinter in Michigan fields but because of an influx of airborne sporangia CDM occurs annually. In Michigan, spore traps are used to monitor the presence of airborne . sporangia in cucumber growing regions to guide the initiation of a fungicide program. However, sporangia, the causal agent of downy mildew on hop, are morphologically indistinguishable from . sporangia. This morphological similarity reduces the ability to accurately detect . from spore trap samples when examined with the aid of light microscopy. To improve . detection, we adapted a qPCR-based assay to allow the differentiation between . and . on Burkard spore trap samples collected in the field. Specifically, we evaluated the specificity and sensitivity of . detection on Burkard spore trap tapes using a morphological-based and quantitative-PCR (qPCR)-based identification assay and determined whether sporangia of . and . on Burkard samples could be distinguished using qPCR. We found that the qPCR assay was able to detect a single sporangium of each species on spore trap samples collected in the field with values <35.5. The qPCR assay also allowed the detection of . and . in samples containing sporangia from both species. However, the number of sporangia quantified using light microscopy explained only 54 and 10% of the variation in the values of . and . , respectively, suggesting a limited capacity of the qPCR assay for the absolute quantification of sporangia in field samples. After 2 years of monitoring using Burkard spore traps coupled with the qPCR in cucumber fields, . sporangia were detected more frequently than . early in the growing season (May and June). . sporangia were detected ∼5 to 10 days before CDM symptoms were first observed in cucumber fields during both years. This research describes an improved sporangial detection system that is key for the monitoring and management of . in Michigan.