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Out, out darn spots: How to prevent, treat powdery mildew

  • Barbara Robertson
  • Powdery mildew, one of the most easily identifiable fungi, first appears as white spots on plants and then spreads. In some plants, the leaves eventually turn yellow or brown and fall off, leaves or shoots can be dwarfed, twisted or distorted, and the plant can become covered with the white powdery growth.

    The spreading spots are as difficult to cure as Lady MacBeth’s conscience, but fortunately they’re rarely as fatal as the treacherous queen’s dagger. Powdery mildew can weaken plants if heavily coated leaves reduce photosynthesis, and the fungus is about as attractive as dust on a drinking glass, but affected plants usually don’t die.

    There are several species of fungi broadly called powdery mildew, but most grow as thin layers of mycelium, that is, a network of fine white filaments. If you look at powdery mildew with a hand lens, you’ll see the spores, which grow in chains. These spores can be lifted by the wind and flown to a new host, or transported by aphids and thrips; however, the fungi tend to be family or species specific. Powdery mildew from a rose won’t infect your squash plant.

    Vegetables affected by powdery mildew include artichoke, beans, beets, carrot, cucumber, eggplant, lettuce, melons, parsnips, peas, peppers, pumpkins, radicchio, radishes, squash, tomatillo, tomatoes and turnips. Powdery mildew usually does not grow on vegetable fruits, but infected plants may have reduced yields and fruit with little flavor. On artichokes, onions, peppers and tomatoes, powdery mildew produces yellow patches rather than white powder.

    Unlike some fungi, this fungus doesn’t like moist conditions; in fact, spring moisture inhibits growth. Optimum growing conditions for the disease are temperatures from 60 to 80 degrees, shade and over-fertilization with nitrogen. Thus, to make plants less susceptible, grow them in sunny locations, prune excess foliage for air circulation, and don’t over-fertilize with nitrogen. Or, pick resistant plants.

    Fortunately, some highly susceptible plants have resistant or less-susceptible varieties: crape myrtle, rose, London plane tree, rhododendron, zinnia, apple, raspberry, peach, melon, pumpkin, squash cucumber, beans and peas.

    Yet here’s a spot:

    If you already have powdery mildew in your garden, you have some choices for controlling it. Overhead sprinkling that washes spores off the plant may reduce the spread. Do this midmorning so the moisture dries quickly, and consider adding soap to the water to increase control. Prune plants with small infestations and quickly bag the cuttings being careful not to spread the spores while you do so. Unfortunately, powdery mildew can survive from one season to the next.

    You can try preventing it from spreading by applying fungicides that protect healthy tissues and/or eradicate existing infections. However, know that the fungus has been developing resistance to many of the fungicides. And as always, read labels and be cautious when using any product that can be toxic.

    Among the least toxic fungicides that protect plants are those formulated with beneficial microorganisms such as bacillus subtilis. Apply protective fungicides thoroughly in the earliest stage of disease development and in approximately seven- to 10-day intervals as new growth appears. If the mildew is mild to moderate, use eradicants such as horticultural oils like neem and jojoba, which may provide some protection. Sulfur has traditionally been used; however, it is effective only if applied before the disease appears and can damage some cultivars.

    Or, consider milk. There are no published scientific studies yet for ornamentals, but studies have confirmed that when applied early, milk is as effective as a fungicide in protecting the leaves of pumpkin grown in fields and greenhouses, and zucchini and cucurbits in greenhouses. One study used a concentration of 30 percent whole milk to water; another used 40 percent. Milk can smell unpleasant as it breaks down, but leaves coated with a milk spray may be less vulnerable to aphid attack, and milk sprays might reduce virus transmission.

    So, with a little preventive action, you might free your garden from powdery mildew. Or at least keep those darn spots under control.