Powdery mildew

    Powdery mildew caused by Erysiphe cichoracearum

    (Sphaerotheca fuliginea), is a serious and common fungal disease during warm and humid conditions with overcast days. Unlike downy mildew, infestation and reproduction may occur under relative humidity as low as 46. Infection can occur at 50F but is optimum at 80F. All cucurbits are susceptible, but the disease is normally not a problem on watermelons. Symptoms develop primarily in 2-3 week old leaves and on stems. Younger leaves are almost immune to powdery mildew. The disease is characterized by white powdery-like growth, especially on the upper side of leaves, and on stems (Figure 2). Affected leaves at first appear normal, then lesions turn yellow to brown and dry out, resulting in early leaf-fall of the older leaves. The disease is spread rapidly in wet weather and a field may be wiped out in only a few days. Fruit infection is rare, but occurs in cucumbers and watermelon. The disease survives on volunteer cucurbit crops or weeds and is carried over long distances by prevailing wind currents. Controls include maintaining a healthy vigorous crop, removing volunteer cucurbit crops and weeds around the field, sanitation, and weekly fungicidal sprays. Recent data indicates that sodium bicarbonate (household baking soda) and potassium silicate sprays may effectively control powdery mildew, as well as other cucumber diseases. Consult with your local county extension agent or pesticide control specialist for current recommendations and for an update on current EPA regulations on the use of these products as fungicide sprays. Use of tolerant cultivar such as Milo, Dasher II, Marketmore 76, Poinsett 76, Sweet Slice, Slice Master, Sprint 440 II, and Gemini 7 offers some control.