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POSTERS: Host resistance screening

Comprehensive Disease Survey on USDA-ARS Cucumber Germplasm Collections in Search for Downy Mildew Resistance at Charleston SC.
Mihir Mandal - Claflin University. Jennifer Ikerd- U.S. Vegetable Laboratory, USDA, ARS, Chandrasekar Kousik- U.S. Vegetable Laboratory, USDA ARS, Livy Williams- USDA, Phillip Wadl- USDA-ARS Charleston, Lina Quesada-Ocampo- North Carolina State University

Downy mildew (DM) caused by Pseudoperonospora cubensis can affect a wide variety of cucurbit crops leading to significant yield losses. Cucumber cultivars with resistance to DM were available for over 30 years and widely grown across the U.S.A. After breakdown of resistance in 2004, intensive fungicide programs are being used to prevent DM from completely destroying the crop. We focused on identifying new sources of DM resistance in the USDA-ARS Cucumber Germplasm Collection against local P. cubensis isolates. In summer 2017, 180 plant introductions (PIs) were evaluated for DM resistance in field trials. The DM tolerant ‘Peacemaker’, the old resistant ‘Expedition’ (prior to 2004), and the susceptible ‘SMR-58’ were used as checks. Twenty-three PIs with varying levels of tolerance/resistance were identified and re-screened in fall 2017 and summer 2018. Six PIs were tolerant and two PIs had high levels of resistance compared to ‘Peacemaker’. In summer 2018, an additional 608 PIs were screened and of these 52 were tolerant and 3 were resistant. Severe DM was observed on ‘SMR-58’ and ‘Expedition’ in 2018. Differences in DM severity on ‘Expedition’ and ‘Peacemaker’ were observed indicating potential differences in the pathogen populations in 2017 and 2018. The pathogen population is being characterized using microsatellite markers. The five resistant cucumber PIs will be useful for breeding resistant commercial cultivars