Development of Phytophthora fruit rot caused by Phytophthora capsici on resistant and susceptible watermelon fruit of different ages

Authors: Kousik, Chandrasekar - Shaker; Ikerd, Jennifer; Turechek, William

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2017
Publication Date: 9/20/2017
Citation: Kousik, C.S., Ikerd, J.L., Turechek, W. 2017. Development of Phytophthora fruit rot caused by Phytophthora capsici on resistant and susceptible watermelon fruit of different ages. Plant Disease. 102(2)370-374. https://doi.org/10.1094/PDIS-06-17-0898-RE.
DOI: https://doi.org/10.1094/PDIS-06-17-0898-RE

Interpretive Summary: Watermelon is an important crop grown in 44 states in the U.S. Many different pests and diseases attack watermelon plants causing extensive damage. In recent years, an old, but re-emerging disease called Phytophthora fruit rot has been causing serious problems in watermelon production in many states in the U.S. including: FL, GA, SC, NC, DE, MI, MD, etc. The National Watermelon Association, a group made up of watermelon growers and shippers from across the U.S., considers Phytophthora fruit rot an important problem for which management solutions are needed. Weather conditions for fruit rot development are always prevalent in southeastern states and hence the pathogen can infect the fruits at any stage. USDA, ARS scientists in Charleston, SC, identified, developed and released Phytophthora fruit rot resistant germplasm lines for use by seed companies and University plant breeders. The present study was conducted to determine if the USVL developed fruit rot resistant germplasm lines were resistant at all fruit ages. Watermelon fruit from susceptible commercial varieties were highly susceptible at all fruit ages while fruit from USVL developed resistant germplasm lines were indeed resistant at all fruit ages. Disease management actions may need to begin as early as fruit set for commercial cultivars that are currently being grown and do not have resistance to fruit rot. The USVL fruit rot resistant watermelon accessions and the information about them will be useful to public and private plant breeders working to incorporate fruit rot resistance in watermelon cultivars.

Technical Abstract: Watermelon is an important crop grown in 44 states in the United States. Phytophthora fruit rot caused by Phytophthora capsici is a serious disease in the southeastern U.S., where over 50% of the watermelons are produced. The disease has resulted in severe losses to watermelon growers, especially in Georgia, South Carolina, and North Carolina during the past few years. Several fruit rot resistant watermelon germplasm lines have been developed for use in breeding programs. To evaluate the development of Phytophthora fruit rot on fruit of different ages, plants of both fruit rot resistant and susceptible lines were planted at weekly intervals for five consecutive weeks in experiments conducted over 3 years (2011 to 2013). Flowers were routinely inspected and hand pollinated to assure having fruit of different ages. For each year of the experiment, different aged fruit were harvested on the same day and inoculated with a 5-mm agar plug from an actively growing colony of P. capsici. Inoculated fruit were maintained in a room set to conditions conducive for disease development (>95% RH, 26±2 ºC). After 5 days lesion size and intensity of sporulation was recorded for each fruit. Lesion diameter and sporulation intensity were significantly greater on fruit of the susceptible lines compared to the resistant lines. The age of the fruit did not have an effect on either measurement on either the susceptible lines (Sugar Baby and PI 536464) or the resistant lines (PI 560020 and PI 595203). Our results showed that resistance to Phytophthora fruit rot in watermelon was not correlated with fruit age. Disease management actions may need to begin as early a fruit set for susceptible cultivars.