Source: Cucumber (Cucumis sativus)-Powdery Mildew | Pacific Northwest Pest Management Handbooks
Symptoms First, tiny white superficial spots appear on leaves and stem. Spots become powdery white and expand to cover all portions of the plant. Sometimes the mycelium becomes covered with small black structures (chasmothecia) about the size of a pinhead but these chasmothecia are rarely present. Usually the disease is in scattered spots in the field but can sometimes be widespread. Severely infected leaves may turn brown, exposing the fruit to sunscald.
