White mold in the flower garden
Figure 1. Zinnia bed with several plants killed by white mold.
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.
The fungus Sclerotinia sclerotiorum
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.
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.
- All leaves on one stem wilt and die.
- Infected parts of the stem are tan to off-white, dry, and brittle. Stem tissue just above and below the infection often remain green.
- Fluffy white fungal growth may be seen on infected stems or leaves when humidity is high.
- Hard, black oblong to irregular fungal resting structures (called sclerotia), about the size of a broken pencil tip, form on the surface of and within infected stems.
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.
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
- Choose 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.
- Space plants far enough apart so air moves through them and dries them quickly.
- Use drip irrigation or soaker hose instead of sprinklers
Clean up infected plants
- Remove all plants infected with white mold, as soon as the disease appears. Take care not to knock off any sclerotia in the process.
- Infected plants should be burned or buried in an area of the yard that will not be used for vegetable or flower gardening in the future.
- Infected plants can be composted only if the compost heats up to 148 to 158°F for a minimum of 21 days. If your home compost pile does not meet these standards, consider bringing infected plants to a municipal compost facility that does.
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.
- New Guinea Impatiens (Impatiens hawkeri)
- Pentas (Pentas lanceolata)
- Sweet Flag (Acorus granimeus)
Resistant – Very minor disease symptoms; no wilting or plant death.
- Purple Millet Grass (Pennisteum glaucum)
- Elephant Ear (Colocasia esculenta)
- Canna (Canna x generalis)
Highly resistant – No symptoms of disease.
- Fiber Optic Grass (Scirpus sp.)
- Ornamental Reed (Juncus effusus)
- Ornamental Sedge (Carex flagellifera)