White mold in the flower garden
Figure 1. Zinnia bed with several plants killed by white mold.
Mid to late summer this year, in various gardens across Minnesota, flowering annuals that had been thriving and blooming earlier in the season suddenly wilted and died back completely (Figure 1). A wide range of annuals were affected including zinnia, marigold, sneezeweed (Helenium), nicotiana and petunia. In some of the annuals with multiple branches, one branch might wilt and die while the others remained healthy. Upon closer examination of these failing plants, gardeners noticed that the infected stems were tan to off-white colored in a section close to the base of the plant. The stem above the discolored area would be a normal green color, and the roots below would be a healthy white or beige. Fluffy white cotton-like fungal growth was often seen at the discolored area of the stem especially if there was high humidity at the time.
What was happening? These plants were falling victim to white mold, a plant disease caused by the fungus Sclerotinia sclerotiorum. Including annual flowers, this disease affects over 360 species of plants, most of which are herbaceous (non-woody stems) and have broad leaves. Other susceptible plants include common garden vegetables like tomato, squash, bean and carrot, perennials like chrysanthemum, columbine, delphinium, and peony, and many common garden weeds.
Figure 2. White cottony fungal growth on an infected zinnia stem.
Figure 3. Hard black sclerotia, and white balls of mycelia preparing to become sclerotia.
Although white mold appears to kill plants quickly, it typically infects plants in early spring or summer and then develops unnoticed for most of the growing season. Whenever temperatures are cool (around 51 to 68°F) and the soil is wet, the white mold fungus will release spores that can travel on the wind for up to a mile or more. Spores that land on wounded or aging plant tissue, like old petals or leaves, will germinate and start an infection. At first the infection will look like a water soaked spot on the stem. It will then turn tan or off-white in color and continue to grow. Through this part of the infection, the plant will continue to look healthy from above. Eventually however the fungus will rot through the entire stem. When this happens the leaves suddenly wilt and die, much to the unsuspecting gardener's surprise.
The best way to recognize white mold is to look for a tan to off-white section at the base of the plant that has cottony white fungal growth on it when humidity is high (Figure 2). In older infections, hard black sclerotia (fungal resting structures), about the size and color of a broken-off pencil tip can be seen either clinging to the outside of the infected stem, or embedded inside the dead stem. Small white balls of fluff may be seen in the same area. These are sclerotia just beginning to form (Figure 3).
Sclerotia are the form in which the white mold fungus survives Minnesota's harsh winter. New infections start the following season from spores produced by these sclerotia. It is therefore very important to remove all plants infected with white mold, taking care not to knock off any sclerotia in the process. These plants should not be composted, but destroyed through burning or placing them in the trash. It is very difficult to get rid of all of the sclerotia once the disease has started and cultural control practices should be used in the spring to reduce the number of infections the following year.
Although there is no one control strategy available to gardeners to completely rid the garden of white mold, using several cultural control practices can help reduce the number of plants affected. The white mold fungus needs moisture on the plant tissue to start an infection, so any practice that reduces humidity and helps plants dry off quickly will help. Mulch the garden with organic mulch like woodchips or bark. Use drip irrigation or soaker hose instead of overhead watering systems. Space plants apart from one another so air moves between them and dries them quickly. Chose plants with an upright and open form because they will dry more quickly than plants that lie along the ground or grow in dense clumps.