Powdery Mildew

Powdery mildew occurs on many different flowers, woody ornamentals, and trees including roses, snapdragons, African violets, kalanchöe, English ivy, zinnias, photinia, oak, lilac, and begonias. Several different genera of fungi cause powdery mildew. Although usually one genus specifically attacks one or two different plants, some species of powdery mildew (such as Erysiphe cichoracearum) attack a wide range of plants. All the powdery mildew fungi are obligate parasites, requiring live tissue to grow and reproduce. In greenhouses, the fungus survives by spreading from the diseased plants to the new plants of that same crop. If that crop is not grown for several weeks, the fungus dies out and diseased plants must be brought into the greenhouse to establish the fungus again. Outdoors, fungal structures form on leaves and twigs that allow the fungus to survive winter conditions.

• White powdery fungus grows on the upper leaf surface of the lower leaves and flower parts.
• Leaves may be twisted, distorted, then wilt and die.
• On some plants such as kalanchoe, infected leaves have dry, corky, scab-like spots and fungal growth is not obvious.

• High relative humidity at night
• Low relative humidity during day
• 70-80 degrees F (22-27 degrees C) temperatures (these conditions prevail in spring and fall)

The spores are carried by air currents and germinate on the leaf surface. Liquid water on leaves inhibits spore germination. The fungus grows on the leaf surface but sends fine threads (haustoria) into the cells to obtain nutrients. From the time a spore germinates to the time new spores form may require only 48 hours. High humidity favors spore formation while low humidity favors spore dispersal.

Some powdery mildew are inhibited by free moisture on leaves while others are favored by wetness on leaf surfaces.

• When conditions are favorable for 3-6 consecutive days, heat and ventilate in late afternoon to reduce night humidity.
• Antitranspirant materials such as Vapor Gard* or Wilt Pruf* applied to coat the leaf can prevent infection. In the landscape, application remains effective up to 30 days. Treat plants such as lilac on June 15, July 15, and August 15 (* Trade name).
• Apply a fungicide on a regular schedule until conditions change. Be certain the crop is on the label. Fungicides for powdery mildew control:
  
  

  COMMON NAME	TRADE NAME
  Ampelomyces quisqualis	AQ-10 (biological control)
  azoxystrobin	Heritage, Quadris
  chlorothalonil	Daconil, PathGuard
  fenarimol	Rubigan
  horticultural oil	SunSpray Ultrafine oil
  kresoxim methyl	Cygnus
  myclobutanil	Eagle, Systhane
  paraffinic oil	SunSpray Ultra-Fine Oil
  piperalin	Pipron
  potassium bicarbonate	Remedy, Arbicarb, Kaligreen, FirstStep,
  potassium salts of fatty acids	M-Pede
  thiophanate methyl + mancozeb	Zyban, Duosan
  triadimefon	Bayleton, Strike
  triforine	Triforine, Funginex
  triflumazole	Terraguard

Chemicals are not available for use on many susceptible plants. Since liquid water inhibits spore germination for most powdery mildews, syringing the leaves greatly inhibits infection and can protect the plant. Syringing is done during the day, especially at times of low humidity. Heat and ventilate in late afternoon to keep night humidity low. Only syringe if other leaf diseases are not a problem since almost all other pathogens require liquid water on the surface to infect.

Powdery mildew on most deciduous trees does little damage and does not require fungicides.