Submitted to: International Phytophthora Capsici Conference
Publication Type: Proceedings
Publication Acceptance Date: November 8, 2011
Publication Date: November 30, 2011
Citation: Kousik, C.S., Ikerd, J.L. 2011. Host resistance to phytophthora fruit rot in U.S. watermelon plant introductions. International Phytophthora Capsici Conference. p. 26-27. Technical Abstract: Phytophthora capsici, distributed worldwide, is an aggressive pathogen with a broad host range, infecting solanaceous, leguminaceous, and cucurbitaceous crops. Phytophthora fruit rot of watermelon (Citrullus lanatus) caused by P. capsici was first reported in the U.S. in 1940. Since then, the disease has occurred on a regular basis in the U.S. During the past several years, Phytophthora fruit rot has resulted in severe losses to watermelon growers in southeastern states especially GA, SC, and NC. Hence it is considered an important problem and a top research priority by the National Watermelon Association (NWA). There are many effective fungicides available for managing Phytophthora fruit rot of watermelon. However, with chemical fungicides, there is always the risk of the pathogen (P. capsici) developing resistance, as was the case with mefenoxam. Furthermore, the regular use of chemicals increases the chances of human exposure and is not environmentally sound. Host resistance is the cornerstone of any integrated pest management (IPM) program and watermelon cultivars with resistance to P. capsici would be extremely useful in managing Phytophthora fruit rot. Resistance to P. capsici has been identified in some cucurbits, particularly to crown rot. In cucumbers, varying levels of resistance to fruit rot have been identified. However, resistance to Phytophthora fruit rot of watermelon has not yet been identified. In 2009, plants belonging to the core collection of watermelon plant introductions (PI, www.ars-grin.gov) were grown in a field on raised plastic beds in Charleston SC. Five mature fruits from each PI were harvested when the tendrils next to the fruit were dry. The fruit were inoculated with a 7-mm plug of an aggressive SC isolate of P. capsici actively growing on V8 juice agar. The inoculated fruit were kept on wire shelves in an enclosed room where high relative humidity (>95% RH) was maintained. Four days after inoculation, data on diameter of disease lesion and intensity of sporulation were recorded for each fruit. Of the 205 PI evaluated, the majority were highly susceptible and extensive sporulation was observed on most fruit. We identified 25 PI (12%) as potential sources of resistance. Twenty-two (12%) of the 159 Citrullus lanatus var. lanatus PI we evaluated from the core collection, one C. colocynthis (PI 388770) and two C. lanatus var. citroides PI (PI 189225) showed some level of resistance to fruit rot. Variability in the resistant reaction to fruit rot within these PI was also observed. The lack of homogeneity for resistance to fruit rot within a PI indicated a need for careful screening and selection for several generations to develop homozygous resistant germplasm sources. Plants from the most resistant PI were self pollinated and the S1 generation was evaluated in 2010 and 2011. Fruit from PI that were resistant had significantly (P=0.05) lower amounts of P. capsici DNA/g of fruit tissue compared to susceptible commercial cultivars Sugar Baby and Black Diamond. Resistance to Phytophthora fruit rot was not correlated with the age or developmental stage of the fruit. Fruit of two susceptible checks (Sugar Baby and PI 536464) were susceptible and two resistant selections (USVL-903 and 904) were resistant at all fruit developmental stages. We are currently developing Phytophthora fruit rot resistant germplasm from these PI for use in our breeding program.