Silvery Spots on Zucchini Leaves

Causal Agent

• An infestation from the sweetpotato whitefly, also referred to as the silverleaf whitefly, leads to an appearance of large, silver patches on zucchini foliage. The disorder affects all varieties of summer and winter squash except cucumber, muskmelons and watermelons. The discoloration is the result of a physiological imbalance in the plants caused by the toxins introduced through pest feeding.

Pest Description

• The adult silverleaf whitefly is about 0.9 to 1.2 mm long with a light yellow body and white wings. Females lay clusters of 50 to 400 eggs on foliage undersides. Eggs are two-thirds female and hatch into wingless larvae. The larvae change to pupa and eventually into adults. In warm weather, it takes 18 days from the eggs to adult stage while under cooler temperatures, this duration may be two months.

Damage

• The whiteflies feed on the plant juices or plant sap from tender plant areas, leading to yellowing or silver discoloration that initially starts at leaf veins. As feeding continues, this silver color spreads across the entire upper foliage areas. Foliage undersides remain unaffected. In cases of mild silvering of foliage, the fruit is not affected. However, in case of severe damage, the fruit color also lightens to a light green to yellowish green shade. Yields and fruit quality are reduced.

Management

• Given the high level of resistance of eggs and larvae to a large number of insecticides, the control of silverleaf whitefly is not easy. The adult flies are very resistant to dried pesticides. Best management requires chemical application to foliage undersides where eggs and larvae are concentrated. Repeat applications until all pests have hatched in order to ensure optimal control. Recommended pesticides include pyridaben, insecticidal soaps, bifenthrin, acephate and imidacloprid.