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Title: Fungicide rotation schemes for managing Phytophthora fruit rot of watermelon across Southeastern United States (NC, SC, and GA)

item Kousik, Chandrasekar - Shaker

Submitted to: Proceedings of the International Oomycete Conference
Publication Type: Abstract Only
Publication Acceptance Date: 12/1/2015
Publication Date: 12/8/2015
Citation: Kousik, C.S., Pingheng, J., Pina Quesada, O. 2015. Fungicide rotation schemes for managing Phytophthora fruit rot of watermelon across Southeastern United States (NC, SC, and GA). Proceedings of the International Oomycete Conference. pp 23.

Interpretive Summary: N/A

Technical Abstract: Phytophthora capsici has been documented as a pathogen on a wide variety of vegetable crops in the family Solanaceae, Cucurbitaceae, Fabaceae, and plants belonging to 23 other families. Phytophthora fruit rot of watermelons caused by P. capsici is particularly severe in southeastern U.S where optimal conditions for disease development prevalent and about 50% of the U.S. watermelons are grown (FL, GA, AL, SC, NC, and VA). Between 2003 and 2008 and again from 2013-2015 many watermelon growers in Georgia, South Carolina and North Carolina did not harvest their crop due to severe pre-harvest fruit rot. In some instances fruits rotted after shipping, resulting in rejection of entire loads and loss of revenue. Hence the National Watermelon Association (NWA), considers Phytophthora fruit rot as one of their most important research priorities. In previous research conducted at the U.S. Vegetable Laboratory (USVL), USDA, ARS in Charleston SC, we identified several sources of resistance to Phytophthora fruit rot in watermelon plant introductions (PI) and developed resistant germplasm lines (USVL020-PFR, USVL203-PFR, USVL489-PFR and USVL782-PFR) by single plant selections. In this study we evaluated the breadth of resistance of these germplasm lines to 20 isolates of P. capsici collected from various states (FL, GA, SC, NC, AZ, NM, MI and NY) across the U.S. and from different crops (pepper, watermelon, pumpkin, zucchini, and lima bean). Mature fruit of each of the Phytophthora fruit rot resistant (PFR) line or susceptible check cultivars Mickey Lee and Sugar Baby were collected from plants grown in a field at the USVL farm. Fruit were placed in plastic autoclave bins and inoculated by placing a 7-mm agar plug from an actively growing colony of a given P. capsici isolate on V8 Juice agar. Only one isolate was used for a single fruit. Each isolate had four replications for each germplasm line or check. After inoculations, the fruit were kept in a walk-in-humid chamber (>95% RH, temperature 26±2 ºC) to maintain conditions favorable for fruit rot development. Five days after inoculations, ratings on lesion diameter, visible pathogen growth and sporulation intensity was recorded. Variability in aggressiveness among the 20 isolates on the two checks and PFR lines was observed. Overall the susceptible checks Mickey Lee and Sugar Baby were susceptible to all isolates. In general, Mickey Lee appeared to be less susceptible to many isolates compared to Sugar Baby in 2014. The Phytophthora fruit rot resistant (PFR) germplasm lines were significantly resistant compared to the two susceptible checks for all isolates. However, the PFR lines did not appear to be immune to all the isolates tested. For some isolates there were significant differences among the PFR lines with respect to lesion dimeter and visible pathogen growth on the fruit surface. In most cases the lesions formed on the PFR lines were close to the inoculation site (agar plug) on the surface and appeared like a hypersensitive reaction. The PFR lines were also highly resistant in field trials in Charleston SC in 2014 and 2015 compared to the susceptible checks. Our results suggest that watermelon cultivars developed using these PFR lines as resistant sources will be able to provide resistance against Phytophthora fruit rot across many states in the U.S. However, it will be essential to test such newly developed cultivars using these resistance sources in field trials across diverse regions where the disease occurs prior release for commercial use.