Released July 29, 2010
MANHATTAN, Kan. – Recent weather has wrapped the central United States in heat, humidity and sweat. It’s been simply perfect for a fungus that causes crown and root rot in a number of popular plants.
The Sclerotium rolfsii fungus used to be confined to the South. Since the mid 1990s, however, it’s been an increasing problem for gardens in the Midwest, said Megan Kennelly, plant pathologist with Kansas State University Research and Extension.
“In just one week, the rot disease can severely damage hostas – the nation’s No.1 herbaceous perennial. It can make ajuga, daylily and phlox plants simply collapse,” Kennelly said. “Unfortunately, once it’s in your garden, the fungus becomes very difficult to eradicate. We’re lucky only in that it does not spread via airborne spores, as other fungal diseases do.”
When S .rolfsii senses that a potential host plant is nearby, it starts to exude droplets that contain oxalic acid and tissue-destroying enzymes. This witch’s brew starts at ground level, attacking and softening the little stems (petioles) that attach leaves to plants. The infected leaves wilt, turn yellow and then brown.
“At that stage, damaged parts often are easy to pull from the soil or plant,” Kennelly said. “Plus, you start to see the fungus itself on the rotted tissue and surrounding soil. It looks like rapidly growing tangles of fluffy white threads that sometimes resemble swan’s down. These filaments are called the mycelium.
“Within days, the fungus also starts depositing a soil-sprinkling of hard-shelled, seed-like survival structures. They’re about the size of a mustard seed, and they look like tiny orbs that gradually change color from white to tan or red-brown. Called sclerotia, these little time bombs can stay dormant in the soil for several years or immediately start producing new mycelium.”
Whether kicked, rain-washed, or carried by shoe or transplant, sclerotia can help the rot disease spread, she said. The mycelium they produce will then slowly grow across the ground, going from plant to plant.
“The best management comes down to preventing the fungus from entering your garden in the first place,” Kennelly said. “Before buying new plants, inspect them carefully for rot-related symptoms. Replant sites that have had crown rot with plants that aren’t a known host.”
Details on symptoms, management and susceptible plant names are available on the Web at http://www.extension.iastate.edu/publications/sul8.pdf in an Iowa State University Extension publication with color photos of crown rot.
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