ARS | Publication request: Resistance to Phytophthora fruit rot of watermelon caused by Phytophthora capsici in U.S. Plant Introductions (PI)

Submitted to: Journal American Society Hortscience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2012
Publication Date: N/A

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. It has been particularly severe in parts of Georgia, where over 25,000 acres of watermelon are grown. The National Watermelon Association, a group made up of watermelon growers and shippers from across the U.S., considers Phytophthora fruit rot and important problem for which management solutions are needed. USDA, ARS maintains a large collection of over 1,800 accessions of watermelon that were collected from different regions of the world. In this study, we tested a representative core collection of over 200 wild watermelon accessions for their reaction to Phytophthora fruit rot in the greenhouse and field. After repeated evaluations over three years, we identified and developed several watermelon accessions with moderate to high levels of resistance to the pathogen that causes fruit rot. These watermelons accessions will be useful as a source for public and private plant breeders for incorporating fruit rot resistance in watermelon cultivars.

Technical Abstract: Fruit rot, caused by Phytophthora capsici is an emerging disease in most watermelon producing regions of Southeast U.S., and is considered an important problem by the National Watermelon Association. A non-wound inoculation technique was developed and used to evaluate detached mature watermelon fruit belonging to U.S. watermelon plant introductions (PI, www.ars-grin.gov) for fruit rot resistance. Mature fruit were harvested and placed on wire shelves in a walk-in humid chamber (>95% RH, temperature 26±2 ºC) and inoculated with a 7-mm plug from an actively growing colony of P. capsici. Twenty-four plant introductions that exhibited resistance in a preliminary evaluation of 205 PI belonging to the watermelon core collection in 2009 were evaluated in the field and/or greenhouse experiments in 2010 and 2011. Fruit rot development was rapid on fruit of susceptible controls Black Diamond, Sugar Baby, and PI 536464. Several accessions including PI 560020, PI 306782, PI 186489, and PI 595203 (all C. lanatus var. lanatus) were highly resistant to fruit rot. One C. colocynthis (PI 388770) and a C. lanatus var. citroides PI (PI 189225) also showed fruit rot resistance. Fruit from PI that were resistant also had significantly lower amounts of P. capsici DNA/g of fruit tissue compared to the susceptible commercial cultivars Sugar Baby and Black Diamond. This is the first report on identification of sources of resistance in watermelon against fruit rot caused by P. capsici.