Broad resistance to fruit rot in USVL watermelon germplasm lines to isolates of Phytophthora capsici from across USA

Authors: Kousik, Chandrasekar - Shaker; Ikerd, Jennifer; Wechter, William - Pat; BARNHAM, SANDRA - Clemson University; Turechek, William

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2021
Publication Date: 2/7/2022
Citation: Kousik, C.S., Ikerd, J.L., Wechter, W.P., Barnham, S., Turechek, W. 2022. Broad resistance to fruit rot in USVL watermelon germplasm lines to isolates of Phytophthora capsici from across USA. Plant Disease. 106:711-719. https://doi.org/10.1094/PDIS-11-20-2480-RE.
DOI: https://doi.org/10.1094/PDIS-11-20-2480-RE

Interpretive Summary: Watermelon is an important crop grown in forty-four states in the USA. 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 is always prevalent in southeastern United and hence the pathogen can infect the fruits at any stage. USDA, ARS 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 five U.S. Vegetable Laboratory (USVL), USDA ARS developed fruit rot resistant germplasm lines were resistant to isolates of the plant pathogen that causes fruit rot from different states and crops in the U.S. Overall, USVL developed resistant germplasm lines were resistant to 20 isolates from nine different states. Three of the USVL developed lines were very resistant to the broad range of isolates. These resistant germplasm lines can be used to develop resistant cultivated type watermelon for managing the disease. Because of the broad resistance in the USVL germplasm lines, cultivated watermelon lines developed using these resistance sources may also have resistance to a broad range of pathogen isolates from different states. These fruit rot resistant watermelons accessions and the information will be useful for public and private plant breeders for incorporating fruit rot resistance in watermelon cultivars.

Technical Abstract: Watermelon is an important cucurbit vegetable crop grown in most states in the United States of America (USA). Phytophthora fruit rot of watermelon caused by Phytophthora capsici has been a major factor, limiting production for the past 15 years in southeastern USA. USDA-ARS released five Phytophthora fruit rot resistant germplasm lines for use in breeding programs. These lines were developed by phenotyping using a local isolate of P. capsici from South Carolina. The present study was undertaken to determine if these resistant lines had broad resistance to diverse P. capsici isolates collected from different states and crops. Five resistant germplasm lines (USVL020-PFR, USVL203-PFR, USVL782-PFR, USVL489-PFR and USVL531-MDR) and two susceptible cultivars Sugar Baby and Mickey Lee used as controls were grown in a field in 2014 and 2015 to produce fruit for evaluation. Mature fruit were harvested and placed in a walk-in growth chamber and inoculated with 20 different P. capsici isolates. The chamber was maintained at 26±2°C and high relative humidity (>95%) using a humidifier. All five resistant germplasm lines were significantly more resistant than the two susceptible controls to all 20 P. capsici isolates. Among the five resistant germplasm lines USVL020-PFR, USVL782-PFR and USVL531-MDR had broad resistance. The potential for the existence of P. capsici races based on watermelon genotypes was observed, especially based on the response of USVL489-PFR. Genetic and virulence variability among the 20 P. capsici isolates was also observed. The broad resistance observed in the five watermelon germplasm lines will be useful for developing commercial watermelon cultivars with broad resistance to P. capsici.