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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. Later, a tan to dark brown color will develop. Circular lesions are common on fruit and will appear water-soaked and sunken. Particularly in damp weather, the lesion’s surface may be covered in a thin, yeasty film made up of mycelium and sporangia of P. capsici.

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: Kenny Seebold, University of Kentucky)

Management:

• Crop rotation
• Irrigation management (avoid excess soil moisture/overhead irrigation)
• Sanitation (avoid movement of contaminated soil)
• Fungicides

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