Whiteflies

• Whiteflies are small white pests that, when mature, resemble tiny moths. They are found feeding on the undersides of the leaves of various plants, including zucchini. Whiteflies are sucking insects, which means they drain sap from the plant and cause moderate curl and yellowing; however, whiteflies also transmit leaf crumple virus, which causes extensive curling of the zucchini's leaves. Whiteflies are controllable with regular pesticide sprays, but viral diseases are not treatable. Remove and destroy zucchini plants that exhibit the heavily curled leaves of leaf crumple virus.
  
  

Aphids

• Aphids are small green pest insects that feed on the leaves of zucchini plants. These pests feed on the sap, like whiteflies, by biting small holes in the veins of the leaves and sucking out nutrients. The young, tender leaves of zucchini plants are especially susceptible to damage from aphids; severe aphid infestations cause leaf curl on zucchinis. Some leaves, particularly new leaves, may die as a result of heavy feeding from aphids. Treat aphids with a pesticide spray.
  
  

Powdery Mildew

• Aphids sometimes spread powdery mildew to zucchini plants. Powdery mildew is a fungal disease that causes the leaves of zucchini plants to exhibit a grayish or white covering, from which the disease draws its name. Over time, the disease causes the leaves of the zucchini plant to curl and turn yellow or brown; eventual defoliation occurs if the powdery mildew is left untreated. Use fungicide to keep the fungus from spreading and prune severely damaged leaves.
  
  

Squash Leaf Curl Bigeminivirus

• Although its name implies the disease only affects squash, squash and zucchini are both cucurbits, and so squash leaf curl bigeminivirus affects zucchini as well. This disease is transmitted by insects that feed on the leaves of the zucchini plant, and it causes the zucchini plant to produce stunted growth. Severe leaf curl results from infection with squash leaf curl bigeminivirus. Viruses cannot be treated; destroy infected plants and apply a pesticide to healthy zucchinis to keep them from contracting the disease.