Clemson University, USDA Cooperative Extension Slide Series, www.ipmimages.org

 

Sarah Holden, Missoula County Extension

HOSTS: Annual and perennial flowers, esp. roses, lupines, bee balm and tall phlox; woody shrubs and trees, esp. Apple, Ash, crabapple, lilac, caraganna, raspberry and yellow-leafed spireas. For vegetables, cabbage, cauliflower, brussels sprouts, broccoli, kohlrabi, kale), sometimes tomatoes, eggplant, peppers.

DESCRIPTION: White, threadlike fungal mycelia give NEW leaves and shoots a powdery appearance. Later, light brown to orange pinhead-sized specks form within the mass of white growth. These tiny dots mature and turn black. These black structures are the overwintering stage.

LIFE CYCLE: This disease overwinters on leaf debris. Spores germinate in the spring when rainfall is LOW OR ABSENT, relative humidity is high (90-95%) at night, and days are warm (68 - 80° F.) The fungus grows well only on succulent, new growth. Airborne spores are released on rainless days. Peak spore production occurs by mid afternoon.

CONTROLS: Apply preventative measures in early morning.

Biological: Ampelomycea quisqualis or Bacillus subtilis.

Some compost teas showed efficacy as part of an integrated approach to preventing powdery mildew .

Chemical: Antitranspirants used at a 3% solution v/v (or 7 - 8 Tbsp./gallon of water). Must be applied on a regular basis to keep new growth protected.

• Bicarbonate based products - (Armicarb 100) or (Kaligreen): when powdery mildew is first observed.
• Phosphate salts (KH₂PO₄): Can be obtained from a chemical supply house, have been reported to control powdery mildew at a concentration of 0.5% w/w or 1 Tbsp./gallon of water with 1/4 to 1/2 tsp. of insecticidal soap added . Studies in Missoula on various flowers and on apple gave satisfactory results. Adding 5% wine vinegar to phosphate salts applications increased efficacy to 83% in one study on roses .
• Horticultural oils: 3 Tbsp./1 gallon of water and soaps added to oils at 1/4 to 1/2 tsp/gallon of water or alone at 3 Tbsp/gal. have been used as part of an integrated control for powdery mildew . Leaf burning can occur.
• Neem.
• Sulfur: Do not apply in hot weather (>80º F).