Powdery Mildew
A fungal disease that produces white or gray powdery growth on the surface of leaves, stems, and fruit. Unlike most fungal diseases, it thrives in warm, dry conditions rather than wet ones, and is caused by host-specific obligate fungi in the order Erysiphales.
Powdery mildew produces the characteristic white or gray powdery coating on leaf surfaces, stems, and occasionally fruit. It’s one of the most visually distinct plant diseases in the garden. If the white powder wipes off with your finger, leaving visible surface damage below, it’s powdery mildew.
Unlike most fungal diseases, which spread in wet conditions, powdery mildew fungi thrive in warm (60-80°F), dry conditions with moderate humidity. The spores actually require dry leaf surfaces to germinate - rain washes spores away and high humidity (above 90%) inhibits germination. Hot, dry late summer is the most common development period.
The Pathogens
Powdery mildews are obligate parasites in the order Erysiphales - they can only grow on living plant tissue, not in the soil or on dead material. They are also highly host-specific: the powdery mildew on squash (Podosphaera xanthii and Sphaerotheca fuliginea) will not infect tomatoes; the mildew on tomatoes (Leveillula taurica, Oidium neolycopersici) will not infect cucumbers. You can’t spread it between unrelated crop families by contact.
Common powdery mildew pathogens by host:
- Cucurbits: Podosphaera xanthii, Sphaerotheca fuliginea
- Grape: Erysiphe necator (also called Uncinula necator)
- Apple and pear: Podosphaera leucotricha
- Tomato: Leveillula taurica
- Pea: Erysiphe pisi
- Squash and pumpkin: particularly susceptible late season
Damage
On cucurbits, powdery mildew typically appears first on older leaves and progresses to younger foliage. Heavily infected leaves turn yellow, then brown, and drop prematurely. Severe early-season infection reduces fruit production; late-season infection in mature plants has less impact on yield but can reduce fruit sugar content and accelerate plant decline.
On grapes, powdery mildew affects leaves, shoots, and berries. Infected berries crack and are vulnerable to secondary rots.
Management
Resistant varieties. Many cucurbit varieties are bred with powdery mildew resistance. Checking seed catalog disease resistance codes (PM = powdery mildew resistance) is the most reliable prevention.
Air circulation. Dense planting reduces air movement and increases the moderate-humidity microclimate that powdery mildew favors. Adequate spacing is a prevention measure.
Spray options:
- Potassium bicarbonate (KHCO3): raises pH on leaf surface, creating unfavorable conditions for spore germination. Available as commercial products; effective as preventive and early intervention. OMRI-listed for organic use.
- Baking soda (sodium bicarbonate): less effective than potassium bicarbonate, with potential phytotoxicity at higher concentrations. Use at no more than 1 tablespoon per gallon.
- Neem oil: Contains azadirachtin; some efficacy against powdery mildew. Works best as preventive. Can cause leaf burn in hot, bright conditions.
- Sulfur-based fungicides: Effective and organic-approved. Do not apply within 2 weeks of oil sprays (sulfur + oil can damage leaves) or when temperatures exceed 90°F.
- Milk: A diluted milk spray (1 part whole milk to 9 parts water) has shown efficacy in multiple trials (including a trial published in Crop Protection). Whey proteins appear to inhibit fungal growth. Not a joke; the evidence is reasonable.
Remove infected tissue. Cutting off heavily infected leaves reduces sporulation and slows spread, particularly early in an outbreak.
Powdery mildew on cucurbits in late August, when harvest is mostly done, warrants minimal intervention. Powdery mildew on young squash plants in June or July warrants action.