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White mold in the garden

Quick facts

  • White mold is a disease that causes stem rot, wilt and death of many common flowers.
  • Hard, resting structures, called sclerotia, allow the fungus to survive for many years. This lets the fungus reinfect gardens each year.
  • Follow these critical steps to manage white mold:
    • Inspect your garden often for signs of white mold.
    • Remove and destroy infected plants right away.

How to identify white mold

Zinnias killed by white mold
Zinnias killed by white mold

Symptoms

  • 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 structures (called sclerotia) form on the surface of and within infected stems. These structures are: 
    • Black in color. 
    • Oblong to irregular in shape. 
    • About the size of a broken pencil tip.
Cottony, fungal growth on stems
Cottony, fungal growth on stems
Sclerotinia in zinnia stem
Sclerotia in zinnia stem

Plants affected by white mold

The white mold fungus infects over 400 plant species.

Commonly affected flowering annual plants

  • Petunia
  • Zinnia
  • Marigold
  • Nicotiana
  • Sunflower
  • Salvia

Commonly affected  garden vegetables

  • Tomato
  • Squash
  • Bean
  • Carrot

Commonly affected perennials

  • Chrysanthemum
  • Columbine
  • Delphinium
  • Peony
  • Many common garden weeds

How does white mold survive and spread?

  • White mold is caused by the fungus Sclerotinia sclerotiorum.
  • The white mold fungus forms hard, black, resting structures called sclerotia. These structures are about the size of a broken pencil tip. Sclerotia allow the fungus 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, sclerotia produce a few tiny mushrooms. These mushrooms 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 above the stem infection suddenly wilt and die.
  • New sclerotia will begin to form on and within killed plant tissue.

How to manage white mold

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.

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Michelle Grabowski, Extension educator

Reviewed in 2019

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