Monday, June 18, 2012

Phytophthora Crown Rot of Squash

Last week we received a yellow squash sample from a commercial field operation in the foothills of North Carolina. As soon as we ripped open the package and the smell hit our noses, we knew we would find something rotten inside! Expecting the worst, we dug in to diagnose the sample. Luckily, the symptoms were classic for Phytophthora crown rot and easily diagnosed by plating out infected tissue on semi-selective media.

Crown Rot (Photo: E. Lookabaugh)
Phytophthora crown rot, also known as Phytophthora blight, is one the most destructive diseases of vegetables in North Carolina. The crown rot pathogen, Phytophthora capsici, attacks peppers, tomatoes, eggplant, squash, pumpkin, cucumbers, watermelons, and muskmelons. Symptoms vary depending on the crop and the part of the plant affected. On squash and pumpkins, a watery crown and fruit rot are common symptoms. Wilting and death soon follow. Fruit that comes in contact with contaminated soil develop dark lesions with concentric rings.

Fruit Symptoms (Photo: NCSU Plant Pathology)
Phytophthora is a soilborne water mold that produces swimming spores, called zoospores. Because the zoospores swim through soil water or are splashed onto susceptible tissue, Phytophthora crown rot is favored by heavy rainfall or irrigation. Excess water accumulates in the crown of squash and pumpkins, making these two hosts particularly vulnerable to crown infection.

Crown Rot (Photo: E. Lookabaugh)
Controlling crown rot can be difficult because the pathogen survives in soil for many years. Alternative crops for infested fields include tobacco, potatoes, and sweetpotatoes, which are not susceptible to Phytophthora capsici. Rotating with corn and small grains for 2 to 4 years is highly recommended. Crucifers like cabbage or broccoli also are good rotation crops. Avoid planting in poorly drained or low areas, and avoid excess irrigation. Some peppers cultivars have good to moderate crown rot resistance. Farms with a history of Phytophthora blight may benefit from the use of chemical sprays.

For more information, see our fact sheet.