Overview of hosta petiole and crown rot
Crown Rot of Hosta: Revenge of the South The American Civil War was over a long time ago, but not to some Southerners. They're still mad about losing, and they're fixin' to get even. Their latest weapon? Sclerotia.
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.
Signs and symptoms of hosta petiole and crown rot
Symptoms of the disease include marginal yellowing and browning of the leaves, beginning with the lower leaves. As the disease progresses, the leaves discolor entirely and wilt. In the final stages of the disease, most of the leaves completely collapse and lay flat on the ground. Because the bases of the petioles are rotted, the leaves can be easily pulled away from the crown of the plant.
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.
Type of Sample Needed for Diagnosis and Confirmation
The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents. If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us
Management of hosta petiole and crown rot
An especially sobering aspect of crown rot is that no registered fungicides are effective and labeled 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 area.
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, contact us.