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White mold in the flower garden

Michelle Grabowski

plant group with many dead, leafless sections

Figure 1. Zinnia bed with several plants killed by white mold.

Michelle Grabowski

Importance

White mold is a disease that results in stem rot, wilt, and death of many common flowers. The fungal pathogen that causes white mold makes a hard resting structure that can survive in the garden for many years, allowing the disease to reoccur each year. As a result, early identification and removal of infected plants are critical steps in managing this disease.

Pathogen

The fungus Sclerotinia sclerotiorum

close up of stem in the ground with tiny cottony growth

Figure 2. White cottony fungal growth on an infected zinnia stem.

Michelle Grabowski

close up of wooden stem, one branch split in half

Figure 3. Hard black sclerotia, and white balls of mycelia preparing to become sclerotia.

Michelle Grabowski

Susceptible plants

The white mold fungus infects over 400 plant species. Popular flowering annual plants including petunia, zinnia, marigold, nicotiana, sunflower, and salvia are commonly infected. Other susceptible plants include garden vegetables like tomato, squash, bean, and carrot, perennials like chrysanthemum, columbine, delphinium, and peony, and many common garden weeds.

Identification

Biology

The white mold fungus forms hard black resting structures called sclerotia (about the size and color of a broken pencil tip) that allow it to survive in the soil and plant debris for 5 or more years. In spring and summer when temperatures are cool (51 to 68°F) and the soil is moist, these sclerotia produce a few tiny mushrooms that release spores that can travel up to a mile or more by wind. Spores that land on wounded or aging plant tissue, like old petals or leaves, will germinate and start an infection. Infections move into the main stem and eventually girdle it. When this happens, the leaves suddenly wilt and die. New sclerotia will begin to form on and within killed plant tissue.

Management

Once white mold has been introduced to a garden, the disease often reoccurs each year. Several cultural control practices can help reduce the number of plants affected.

Reduce moisture on and around plants

Clean up infected plants

Use resistant plants

The plants below were evaluated for resistance by the University of Minnesota in 2011-2016.

Moderately resistant – Individual stems or shoots become infected and die back but the plant remains alive through the growing season.

Resistant – Very minor disease symptoms; no wilting or plant death.

Highly resistant – No symptoms of disease.

2016

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