Violas with powdery mildew
Photo: Margery Daughtrey

1. Powdery mildew is a very conspicuous disease, caused by an assortment of fungi that are often host-specific. The fungus grows across the leaf surface (most often, but not always, on the top) and uses absorbing structures called haustoria in epidermal cells to obtain nutrients.

2. Most crops can host one or more powdery mildews. In the greenhouse, we see powdery mildew often on begonia, torenia, verbena, calibrachoa, gerbera, petunia, African violet, poinsettia and rose. Herbaceous perennials inclined to powdery mildew include monarda, garden phlox and aster. A savvy grower learns to sidestep this disease by choosing less-susceptible species or cultivars. Epidemics build quickly when environmental conditions are favorable: massive quantities of spores are produced from the fungal network on the plant surface, and are moved around on air currents to start new colonies.

Dahlias with a thin coating of powdery mildew
Photo: Margery Daughtrey

3. Stems and petals are at times infected, as well as leaves. These fungi don’t require a sopping wet leaf to start a new infection. Merely the high humidity typical of a greenhouse will foster powdery mildew. The fungus grows across the leaf surface (most often, but not always, on the upper surface). Powdery mildew is sometimes regulated by temperature. For example, we know from Mary Hausbeck’s research that powdery mildew will not become epidemic on poinsettias until fall, when greenhouse temperatures stay below 86 degrees Fahrenheit. Skilled scouting for powdery mildew is critical to management. In the case of phlox and poinsettia, it helps to know that white colonies will first show on the undersurfaces of leaves.

4. Different powdery mildew diseases may affect different plant parts. For roses, infections occur on immature leaves, pedicels and petals. For verbena and petunia, infection starts on lower leaves. Heavy infestations of powdery mildew can keep plants stunted by sapping their energy. In the world of ornamentals, the white polka dots or coatings of powdery mildew are considered so unsightly that there is no tolerance for the disease.

Examples of powdery mildew on various crops: African violet
Photo: Margery Daughtrey

Monitoring

  • On poinsettia leaves, look for pale spots that might indicate where a powdery mildew colony is hiding below.
  • On sedum or kalanchoe, watch for scabby spots. Under the microscope, you may be able to see powdery mildew.
  • On petunias, verbenas and calibrachoas, check for yellowing, browning and death of lower leaves: powdery mildew growth is often very thin, and hard to see.
  • Examples of powdery mildew on various crops: Rose
    Photo: Margery Daughtrey
  • On begonias and pansies, scan for dark spots. On these and other plants, watersoaked or purplish spots may be powdery mildew.
  • On hydrangeas, reddish discoloration is a signal to look more closely.

Prevention

  • Scout crops that are known to be powdery mildew hosts. Detect infestations early so that you can initiate a management program before the problem has spread widely.
  • Keep greenhouse humidity down, below 85 percent relative humidity.
  • Space plants adequately.
  • Use fans to circulate air and avoid dead air pockets where inoculum can build up.
Choose to grow cultivars or species of your crops that are less vulnerable to powdery mildew (keep notes on ones to avoid).

Treatment

  • Start a fungicide management program as soon as powdery mildew appears.
  • Rotate fungicides among different FRAC (Fungicide Resistance Action Committee) groups to avoid repeatedly using materials with the same mode of action.
  • Utilize biofungicides with, for example, Streptomyces or Bacillus species.
  • Examples of powdery mildew on various crops: Peony
    Photo: Margery Daughtrey
  • Utilize contact materials such as potassium bicarbonate, horticultural spray oil, piperalin or polyoxin-D.
  • Utilize systemic materials according to label directions (or else resistance will develop in the powdery mildew population). Strobilurin (FRAC 11) and DMI (FRAC 3) materials are two of the most effective groups. Some fungicide combination pre-mixes are now available as well.

Margery is a plant pathologist specializing in ornamentals at Cornell’s Long Island Horticultural Research & Extension Center. She aims to help growers outwit diseases.