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Evaluation and Identification of Oospores on Cucurbit Downy Mildew Infected Field Samples

Jake Jones: University of Maryland College Park, Lower Eastern Shore Research and Education Center

<div><em>Pseudoperonospora cubensis</em>, which causes cucurbit downy mildew (CDM), is one of the most devastating pathogens of cucurbit crops worldwide. The pathogen is heterothallic, with two mating types (A1 and A2) both recently found in the United States. <em>Pseudoperonospora cubensis</em> largely reproduces asexually via sporangia capable of long distance spread, but sexual reproduction resulting in oospores that serve as survival structures is possible, if both mating types are present. To date, no data of naturally occurring oospores within the field have been recorded, although the survival structure can be induced under laboratory conditions. However, putative <em>P</em>. <em>cubensis</em> oospores were observed and photographed in CDM infected squash leaves from field samples in Maryland and Delaware during 2016-17 growing seasons. Oospore inoculation on detached leaves did not result in any disease symptom development. A custom, pathogen-specific molecular probe, for use in fluorescent <em>in situ</em> hybridization (FISH), is currently being used to verify the oospores are <em>P</em>. <em>cubensis</em> and not another oomycete pathogen. If confirmed, cucurbit growers and plant breeders will need to be aware that the virulence of <em>P</em>. <em>cubensis</em> could shift again, with the introduction of new genetic variation from sexual recombination.</div>