Gardeners may think of white mold mainly as a disease on snap bean, cabbage, and other spring vegetables. In mild, rainy winters like 2019-2020, however, white mold is already active. The fungus can be found on winter annuals like stock (Matthiola incana), snapdragon, diascia, delphinium, and calendula. Perennials that haven’t frozen back, like chrysanthemum, may also have symptoms.
White mold is the common name for the disease and the fungal pathogen, Sclerotinia sclerotiorum. The fungus survives long term in the soil as sclerotia, which are dried masses of mold covered with a black rind.
On diseased annuals with tall flower stalks, a straw-colored or water-soaked lesion runs up and down the stalk and spreads onto the attached blossoms or leaves. White mold grows on the diseased tissues. Small white clumps of mold are the beginnings of new sclerotia that turn gray and then black when they are mature.
Freezing temperatures break sclerotial dormancy, i.e., “wake-up” the fungus, like cold treatment breaks seed dormancy. About a month later, tiny mushrooms grow out of sclerotia buried within 4 inches of the soil surface. Once the mushrooms push out of the ground, they release hundreds of airborne spores. Spores float up into the air and land on stock, delphinium, or calendula petals that have dropped and landed on leaves. The spores germinate and grow on senescing leaves or dying petals, which provide a high-energy “snack” for the fungus before it spreads to the rest of the plant. In the photo, notice the rose-colored petal of stock stuck in the middle of the stem lesion.
Diseased flower stalks should be cut off as soon as white mold is found so the fungus doesn’t spread to the rest of the plant. Remove diseased plants before sclerotia form. Fallen diseased leaves also should be gathered and removed, as sclerotia can form on any diseased plant part. Put diseased plant parts in the garbage, not into compost piles, where the fungus could still form sclerotia.
Thiophanate-methyl, sold as Thiomyl, is the only conventional fungicide available to home gardeners that helps manage white mold. Spray susceptible winter annuals to prevent white mold or to slow its spread.