Crown Rot of Hosta: Revenge of the South

Say what? What are sclerotia, and is it O.K. to say "sclerotia" in a family newspaper? If you're reading this, the answer must be yes.

The sclerotia story starts with hosta, the reigning glamor queen of shady northern landscapes. Otherwise normal people adore this lovely, wonderfully varied plant with the zeal of cultists. An added bonus has been hosta's relative freedom from serious disease problems.

Enter an invader from the South, the fungus Sclerotium rolfsii. This renegade has long been a scourge of hosta and other plants in the high temperatures of the Deep South. It lives in the soil and attacks the crown (the part of the plant at the soil line). The fallout of its attack is yellowing, browning leaves and mushy, rotted crowns. If you can easily detach unthrifty-looking leaves from a plant, S. rolfsii may be the culprit.

A closer look at the devastation reveals a white mat of fungus fanning out from the infected crown across the soil surface. Look a bit closer and you can see thousands of tiny, spherical sclerotia in the rotted crown. Sclerotia of S. rolfsii are somewhere between BB's and pepper grounds in size and vary in color from white (newly developed) to brick red. Sclerotia, the survival pod of the fungus, allow it to hang on patiently in hostile environments, then wake up and attack when a likely victim (hosta and many other plants) appears.

Until recently, this carnage rated little more than a polite yawn from Northern gardeners. S. rolfsii was a problem of hotter climates than ours, went the common wisdom.

But something happened. Northern gardeners woke one day to find their hostas, some more beloved than children or pets, succumbing to S. rolfsii. What went wrong?

We got invaded, that's what. Hostas shipped from the souytheastern U.S., and infested with sclerotia of S. rolfsii, apparently found their way into gardens in Iowa. Hosta crowns started dying, and "hot spots" of several sick plants appeared in gardens. Awhile later, another hot spot popped up. Heartbroken gardeners had to remove some of their most prized specimens.

Even more worrisome, this Dixie invader showed no signs of intimidation by Iowa winters. The fungus turns out to cope with our winters quite well, and has survived in plant debris for several years in some Iowa gardens.

An especially sobering aspect of crown rot is that no registerted fungicides are effective and labeled against it. Control measures tend toward humble common sense:

• Dig up and remove hostas showing crown rot symptoms. To test your suspicions, look for sclerotia in the crown ares.
• Thoroughly scrape and/or wash soil from all tools used in infested soil. That way, you'll be unlikely to carry the fungus from "patient zero" to other parts of your garden. You can follow up by sterilizing your tool in 10% solution of household bleach, but removing soil is likely to have more impact than bleaching.
• Buy disease-free plants. Big duh, you may say. But now that you can recognize sclerotia, you can avoid plants with symptoms of crown rot. Carefully scouting new plants for sclerotia and other crown symptoms provides a degree of insurance. With luck, vigorous inspection can keep this scourge out of your garden.
• Plant nonhost species into holes from which sick hostas have been excavated. The list of host plants susceptible to S. rolfsii is long, so double-check this list against prospective replacements. If your garden center lacks such a list, give me a call (515-294-0579) and I'll mail or fax you the list.

It's painful to realize that, down there among our hosta crowns, those ornery little Southern sclerotia are having the last laugh on us. If only they wouldn't leave their tiny Moon Pie wrappers and RC Cola cans all over the garden...

This article originally appeared in the August 22, 1997 issue, p. 130.
Prepared by Mark Gleason, Department of Plant Pathology Iowa State University, Ames, Iowa.