- Summer squash plants are prone to infection from the Pseudoperonospora cubensis fungus that causes downy mildew disease. The disease is common in regions with high humidity and rainfall. Downy mildew is often referred to as wildfire given the rapid rate of spread. The fungus affects only the foliage of the plants. The damage to leaves causes secondary damage to the fruit, such as sunscald from reduced leaf cover.
- The disease starts with the appearance of light green areas on the upper sides of the foliage. These discolored patches gradually turn yellow to tan with white to gray mold on foliage undersides. Dead leaves often curl at the edges and stay on the plants. The mature crown leaves are infected first, usually at fruit-set time. Eventual defoliation leads to stunted plants and poor fruit growth.
- The fungal spores causing infection spread by wind, insects, rain or infected tools. Disease development is most rapid under moderate to warm temperatures ranging between 61 to 72 degrees F. Extended periods of fog, dew and rain favor fungal growth. In case of dry, hot weather conditions, the rate of disease spread and severity of symptoms is reduced considerably.
- Cultural practices like ensuring that moisture does not persist on foliage for long periods of time help to reduce chances of infection. Avoid the use of overhead irrigation methods for watering plants and do not water late in the day as foliage does not have enough time to dry completely. Plant zucchini in a full sun area with optimal soil conditions and good air ventilation. Recommended fungicides include chlorothalonil, metalaxyl, maneb or copper based products. Cover foliage well with chemical, especially foliage underside, for best effect.
Disease Identification
Symptoms
Favorable Conditions
Management
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