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Poster: Biology & Disease Mgmt: Oomycetes


Breadth of resistance of Phytophthora fruit rot resistant watermelon germplasm to Phytophthora capsici isolates from across United States of America
C. KOUSIK (1), J. Ikerd (1), M. Mandal (2) (1) U.S. Department of Agriculture, Agricultural Research Service, U.S. Vegetable Laboratory, U.S.A.; (2) ORISE Participant, U.S. Vegetable Laboratory, USDA, ARS., U.S.A.

Phytophthora fruit rot of watermelon caused by Phytophthora capsici is particularly severe in southeastern U.S where optimal conditions for disease development are prevalent. We have developed Phytophthora fruit rot resistant (PFR) germplasm lines (USVL020-PFR, USVL203-PFR, USVL489-PFR and USVL782-PFR) for use in breeding programs. The breadth of resistance of these lines to 20 P. capsici isolates collected from various states and different crops was evaluated. Mature fruit of PFR lines or susceptible checks ‘Mickylee’ and ‘Sugar Baby’ were collected from plants grown in a field. Fruit were inoculated by placing a 7-mm agar plug from a 4-day-old colony of a given P. capsici isolate. Inoculated fruit were kept in a walk-in-humid chamber (>95% RH, temperature 26±2 ºC) for 5 days. Ratings on lesion diameter and visible pathogen growth were then recorded. Variability in aggressiveness among the 20 isolates on checks and some PFR lines was observed. Check cultivars were susceptible to all isolates whereas PFR lines were significantly more resistant. However, PFR lines were not immune to all isolates. Significant differences in lesion size on PFR lines for some isolates was observed. In most cases very small lesions (0.8mm) formed on PFR lines under the agar plug and on the fruit surface. 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.