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How to combat powdery mildew on plants

  • Martha Proctor
  • If you've ever observed grayish-white powdery spots on leaves, shoots and sometimes on flowers and fruit in your garden, you've probably seen evidence of powdery mildew fungi.

    Powdery mildews are more prevalent than many other leaf-infecting diseases in Marin because of California's dry summers. These fungi affect virtually all kinds of plants: cereals and grasses, vegetables, flowers, weeds, shrubs, fruit trees and broad-leaved shade and forest trees. Susceptible vegetable crops include artichokes, beans, beets, carrot, cucumber, eggplant, lettuce, melons, onions, parsnips, peas, peppers, pumpkins, radicchio, radishes, squash, tomatillo, tomatoes and turnips.

    The type and severity of symptoms depend on the variety of the host plant, the age and condition of tissue when affected, environmental conditions, and the specific species of powdery mildew fungi involved. Although there are more than 1,100 fungi species that cause the disease, powdery mildews are host specific; they cannot survive without the proper host plant.

    A powdery mildew infestation can occur all season long, but is less common during the heat of summer. Although not considered to be fatal, in advanced disease, infected leaves may drop prematurely, and leaves, buds or shoots may become distorted, dwarfed, and discolored. Severely infected flowering crops may have reduced yields, shortened production times, and fruit that has little flavor. Damage is more unsightly than harmful on most trees.

    In spring, as daytime temperatures rise above 60 degrees, the fungi responsible for powdery mildew begin to produce and then disperse spores into the air. Wind and splashing rain move the spores to new foliage on an infected plant or to nearby plants. If environmental conditions are favorable and contact is made on a suitable host, infection begins. Initial symptoms are small, circular, powdery white spots that expand and eventually join as the infection progresses. Once present, the fungus survives the winter attached to plant parts or to its fallen leaves.

    Conditions that favor the onset and spread of powdery mildew include shade, warm days (68 to 86 degrees) and cool nights, crowded plantings where air circulation is poor and relative humidity greater than 95 percent. High relative humidity, such as in a greenhouse, favors spore formation while low relative humidity favors spore dispersal.

    Unlike other foliar diseases, water on leaves is not a requirement for infection, as spores will not germinate if immersed in water. Under optimum conditions, powdery mildew will be visible 48 hours after infections are initiated; new infections produce spores in about five days.

    Many plants, such as roses, vegetables and Kentucky bluegrass, are developed to be resistant or tolerant to powdery mildew. Use resistant varieties of these plants whenever possible.

    Moderate levels of powdery mildew can be ignored on most plant species, but prevention and immediate control activities should be taken to avoid damage to highly susceptible plants such as certain cultivars of apple, crape myrtle, euonymus, stone fruit and rose.

    Several cultural practices will reduce the incidence or prevent powdery mildews:

    • Choose slow release fertilizers. Avoid excessive or late-summer application of nitrogen fertilizer to limit new growth, as it is more susceptible to infection.
    • It is best to irrigate in the early morning to let the plant tissue dry as soon as possible.
    • For infected vegetables and other annuals, remove as much of the plant and its debris in the fall. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris, as temperatures typically are not hot enough in home compost systems to kill the fungus. Instead, put the debris into your green waste can so that it will be commercially composted at higher temperatures.
    • Selectively prune overcrowded plants to help increase air circulation. This helps reduce both relative humidity and infection.

    If the above practices fail to prevent disease build-up, an application of fungicide may be necessary. Sulfur products have been used for centuries to manage powdery mildew but are only effective when applied before the disease appears. Sulfur acts as a protectant fungicide, but must be applied prior to infection, usually at the end of spring rains. Several horticultural oils such as Neem oil or JMS Stylet act on existing infections, eradicating them. A protectant fungicide prevents new infections from occurring whereas eradicants can kill an existing infection.

    Thoroughly cover a highly susceptible plant with protectant fungicide (such as sulfur or sulfur combined with soap like M-Pede or Safer applied regularly in 7- to 10-day intervals) before the disease appears. Use eradicants (oils) at the earliest signs of the disease. Once the mildew growth is extensive, controlling the situation with any fungicide becomes more difficult. Do not apply sulfur or horticultural oil when the temperature is 90 degrees or above or to water stressed plants; sulfur should not be applied within two weeks of any oil spray.

    The first line of defense is to promote healthy plants. Plant highly susceptible host plants in good soil in a sunny location that allows good air circulation.