July 17, 2002
Woodbury Bulletin
Betsy Halden

Crown rot of hosta, Missing monarchs, and non-blooming peonies

We have a number of hostas in the front yard of our new house. They are in full bloom now, and I’m learning to appreciate the beauty of these attractive plants. I was reminded recently of what can happen to our plants during hot and humid weather. A fellow gardener reported spotting the dreaded symptoms of crown rot on her hostas – bronzing, wilting, and collapse of the outermost leaves. She inspected the stems near the soil line and found the white threadlike strands of the fungus and small tan sclerotia, resistant structures that resemble mustard seeds. She remembered that the causal fungus, Sclerotium rolfsii, causes a decay of the stem near the soil line, allowing the leaves and petioles to be easily pulled from the plant.

Researchers at Iowa State University have reported that Sclerotium rolfsii 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 unhealthy-looking leaves from a plant, S. rolfsii may be the culprit.

A closer look at the problem 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 in size between BB’s and pepper grounds. They vary in color from white (newly developed) to brick red. Sclerotia, the survival pod of the fungus, allows it to hang on patiently in hostile environments, then wake up and attack when a likely victim (hosta and many other plants) appears.

Crown rot is also known at “southern blight.” Plants shipped from the southeastern U.S. and invested with sclerotia of S. rolfsii brought the fungus to this area when they were planted in Minnesota gardens. Unfortunately Sclerotium rolfsii is capable of causing disease on many different perennials, vegetables, and annuals.

Crown rot is not easily intimidated. Minnesota winters seem to have no effect on it. There are no registered fungicides that are effective and labeled against it. Control consists primarily of careful inspection before planting and sanitation measures that prevent the spread of the fungus to other plants. Here are some tips:

• Dig up and remove hostas showing crown rot symptoms. Look for sclerotia in the crown areas.

• Thoroughly scrape and/or wash soil from all tools used in infested soil. You can sterilize your tool in 10% solution of household bleach, but removing soil is likely to have more impact than bleaching.

• Buy disease-free plants. Avoid plants with symptoms of crown rot. Look for the tag that identifies the plant as being resistant to crown rot.

Missing Monarchs? A number of Twin Cities gardeners have commented that they have not seen many Monarch butterflies this summer. Dr. Karen Oberhauser, University of Minnesota, Department of Ecology, conducts research with monarchs. She reported that the monarchs are around, but the numbers are low and their distribution seems to be spotty. “I have a lot in my garden, but I know people that haven't seen a single one all summer. You can look at our website (www.mlmp.org) and go to results to see what the numbers of larvae and eggs are in different locations.

“There are probably 2 reasons for their scarcity: the big freeze in Mexico last winter that killed up to 80% of the population, and a very dry spring in Texas that was bad for the milkweed. All the monarchs that come here in the spring/early summer were larvae in the south, and a tough spring is hard on them. We should start seeing a few more soon when our first northern population starts emerging as adults.”

Non-blooming peony? Did your peony fail to bloom this spring? In response to a question about a non-blooming peony, Dr. Mary Meyer, Department of Horticultural Science, University of Minnesota, suggested that the gardener check the planting depth of the peony. “Peonies are notorious for not tolerating too shallow or too deep a planting depth; they like 1 1/2"-2" deep.”

If you would like more information about growing hostas or peonies, go to www.extension.umn.edu and enter hostas or peonies in the search box.

The Minnesota Landscape Arboretum in Chanhassen is a wonderful place to visit on a summer day. The annual and perennial gardens are now in full bloom. For hours and general information call 952.443.1400 or visit the website www.arboretum.umn.edu.

Betsy Halden is a resident of Woodbury. University of Minnesota Extension Service and Iowa State University Extension Service materials were consulted in the preparation of this column. You can reach Betsy Halden at eahalden@comcast.net