Powdery Mildew: Identification, Conditions, and Management

Powdery mildew affects a wide range of flowers, woody ornamentals, and trees—including roses, snapdragons, African violets, kalanchoe, English ivy, zinnias, photinia, oak, lilac, and begonias.

Multiple fungal genera cause powdery mildew. Most are host-specific, but some (like Golovinomyces cichoracearum, formerly Erysiphe cichoracearum) infect many plants. All powdery mildew fungi are obligate parasites, requiring living tissue to grow and reproduce. In greenhouses, the fungus spreads from diseased to new plants of the same crop; if that crop is absent for several weeks, the fungus dies out. Outdoors, survival structures on leaves and twigs allow overwintering.

Symptoms

• White, powdery fungal growth on upper surfaces of lower leaves.
• Leaves may twist, distort, wilt, and die.
• On some plants (e.g., kalanchoe), infected leaves develop dry, corky, scab-like spots with little visible fungal growth.

Conditions Favoring Powdery Mildew

• High relative humidity at night
• Low relative humidity during the day
• Temperatures of 70–80°F (22–27°C)—common in spring and fall

Spores travel on air currents and germinate on leaf surfaces. Free water on leaves inhibits germination for most species. The fungus grows on the surface but sends haustoria into cells to absorb nutrients. The cycle from spore germination to new spore production can take as little as 48 hours. High humidity favors spore formation; low humidity favors dispersal.

Managing Powdery Mildew in Greenhouses

• When favorable conditions persist for 3–6 consecutive days, heat and ventilate in late afternoon to lower night humidity.
• Apply a labeled fungicide, biological control, or anti-transpirant. Consult your Cooperative Extension for current recommendations.
• Syringing (lightly misting) leaves during the day when humidity is low inhibits spore germination for most powdery mildews. Use only if other foliar diseases aren’t a concern, since many pathogens require free water. Syringing is a good non-chemical option where pesticides aren’t feasible or no fungicides are registered.

On most deciduous trees, powdery mildew causes minor damage and rarely warrants fungicide treatment.