What Causes Zucchini Leaves to Spot?

Powdery Mildew

• Powdery mildew first shows on zucchini plants as pale yellow spots on old leaves, which eventually turn to white or grey felt-like blotches that may cover the entire leaf surface. Lesions later develop on the undersides or shaded sections of leaves and spread to the remaining foliage. Severe infections can lead to withered vines, premature ripening of the fruit and plant death. Apply fungicides for sulfur products as soon as mildew is noticed to control the spread of the fungus.

Anthracnose

• Anthracnose typically affects zucchini plants in mid to late season, causing brown spots on the leaves. The center of these leaf spots may fall out, leaving holes. Anthracnose fungus can survive on infected plant material and spores develop on infected leaves and fruit. The disease typically develops during warm, moist weather. There are several fungicides available for treating anthracnose but good coverage of fruit and leaves is necessary to gain adequate control. Infected vines and leaves should be removed and all dead plant material cleaned away at the end of the season to prevent reinfection.

Angular Leaf Spot

• Zucchini leaves showing small, angular, straw-colored spots with a yellow halo may be infected by a bacterial disease called angular leaf spot. These spots later dry out, leaving holes in the leaves. Tan spots also develop on the fruit, which can lead to bacterial soft rot of the entire fruit. This infection thrives in humid conditions and may infect all cucurbit crops. To control the disease, do not collect or plant seeds from infected plants and rotate infected crops for two or more years with a resistant crop before planting another member of the squash family of plants. Infected plants should be removed and destroyed at the end of the season. Copper may slow the spread of angular leaf spot if treated early, but once the disease is severe, it won't be effective.

Scab

• Scab is an infection caused by the fungus Cladosporium cucumerinum. In zucchini plants, this infection shows as gray spots surrounded by a yellow halo on the leaves. The centers of the spots eventually drop out and the fruit are covered with greenish-black velvety fungus. To eliminate the fungus that causes scab, plant resistant varieties for two or more years before planting any squash plant variety. Remove and destroy infected plants and debris at the end of the season. Preventative fungicide treatment is only necessary in large fields where there is a history of scab infection.