Powdery Mildew Management in Cucurbits and Grapes

Powdery mildew is caused by obligate fungal pathogens (mainly Podosphaera and Erysiphe spp. in vegetables; Uncinula necator in grapes). It is one of the most common and easily recognisable fungal diseases, causing significant quality and yield losses in cucurbits (cucumber, pumpkin, squash), grapes, and many other crops.

Identification

Powdery mildew appears as white to grey powdery fungal growth on leaf surfaces, stems, and fruit. Unlike most foliar diseases, it does not require leaf wetness to infect — it spreads readily in warm, dry conditions with high humidity.

Key symptoms:

• •White powdery patches on upper (and later lower) leaf surfaces
• •Yellowing of affected leaves
• •Premature leaf death in severe cases
• •Reduced fruit size and quality

Disease Cycle

Conidia (spores) are produced in vast numbers under dry, warm conditions (20–28°C) with relative humidity of 50–90%. Infection occurs without free water on leaves — this distinguishes it from most other fungal pathogens.

Chemical Control

Application Strategy

• •Begin applications at first sign of disease or at crop growth stages known to be high-risk
• •Apply on a 7–14 day interval during disease pressure
• •Do not apply sulfur when temperatures exceed 30°C — phytotoxicity risk
• •Ensure good spray coverage of both leaf surfaces
• •Alternate between FRAC groups to delay resistance

Cultural Control

• •Choose resistant or tolerant cultivars where available
• •Ensure adequate plant spacing for air circulation
• •Avoid excessive nitrogen — it promotes lush vegetative growth susceptible to powdery mildew