Return to Diseases

Phytophthora blight (Phytophthora capsici) affects all cucurbits, although different plant parts are affected on a given host. Symptoms on cucumber, muskmelon, and watermelon are normally found on leaves and fruit. The disease affects all parts of pumpkin and squash. Symptoms include damping-off, root rot, crown rot, stem rot, wilting/collapse of plants, and lesions on leaves and fruit. Lesions on stems are constricted, darkened, and water-soaked, often extending a few inches above the soil line, similar to black shank of tobacco. Lesions on leaves tend to be circular and initially water-soaked in appearance, turning tan to dark brown color. Circular lesions are common on fruit and will appear water-soaked and sunken. Lesions can be wet or slimy with a thin layer of mycelium and sporangia, particularly when weather is wet. 

Phytophthora blight crown rot on yellow squash.

(Photo: Kenny Seebold, University of Kentucky)

Phytophthora blight fruit rot on yellow squash.

(Photo: Kenny Seebold, University of Kentucky)

Phytophthora blight leaf lesion on yellow squash.

(Photo: Kenny Seebold, University of Kentucky)

Phytophthora blight fruit rot on watermelon fruit.

(Photo: Gerald Holmes (Strawberry Center), California Polytechnic State University, Bugwood.org)

Management:

• Improve soil drainage.
• Select resistant cultivars when available.
• Space plants for air circulation and rapid drying. 
• Apply protectant fungicides.
• Promptly remove and destroy diseased plant material during the growing season.
• Avoid movement of infested soil to clean fields.
• Promptly destroy crop residues after harvest.
• Rotate with nonhost crops for at least 4 years.

Return to Diseases