|Powdery Mildew -
Erysiphe, Sphaerotheca, Phyllactinia, Microsphaera,
or Uncinula spp.
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.
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.
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.
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.
- Sulfur: Do not apply in hot weather (>80º F).