August 31 Vegetable Production Update | NC State

Powdery Mildew on cucumber and squash in Western North Carolina

Powdery Mildew is caused by the fungal pathogens Podosphaera xanthii and Erysiphe cichoracearum. Disease can occur on all cucurbit crops. However, due to the incorporation of resistance genes, most commercial cucumber and melon cultivars are less susceptible. High-humidity (50-90%), moderate temperatures (68 to 80 ℉), dense foliage, and low light conditions favor disease development. However, dry conditions favor colonization, sporulation, and dispersal of the pathogen. Signs and Symptoms: White, powdery fungal growth first appears on older or shaded leaves and on the underside of leaves. Yellow spots may form on upper leaf surfaces opposite of powdery mildew colonies. The white, powdery growth can also appear on stems and petioles. Infected leaves will senesce (deteriorate) early, and fruit yield and quality can be affected. Due to plant stress induced by powdery mildew infections, speckling of fruit can occur in some cucurbits. Management: Provide adequate spacing between plants and remove any debris, weeds, and volunteers that can harbor the pathogen. When possible, the use of resistant cultivars is recommended. Fungicides should be applied to protect the crop from infection. For the latest fungicide recommendations for cucurbit powdery mildew see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels.