Extension Communications 3614 Administrative Services Building Ames, Iowa 50011-3614 (515) 294-9915
5/22/00 Contacts: Brooke Edmunds, Extension Plant Pathology, (515) 294-0589, bedmunds@iastate.edu Elaine Edwards, Extension Communication Systems, (515) 294-5168, eedwards@iastate.edu Yard and Garden Column for the Week Beginning May 26 Crown Rot Poses Danger for Hosta Lovers By Brooke Edmunds Research Assistant, Plant Pathology Iowa State University Extension Hostas are prized for their attractive leaves, assortment of shapes and sizes, tolerance of shade and minimal maintenance. But in the last few years, some gardeners have noticed a problem. Each year in midsummer, the outer leaves of the same planting of hostas turn yellow and wilt. These leaves are easily pulled off the plant, seemingly severed at the base. Gardeners are confused because hostas are supposed to have few problems in Iowa. So what's going on? Yellow, wilted leaves that easily detach are symptoms that something has caused rotting of the crown. The crown is the area of the plant at or just below the soil. A fungus, Sclerotium rolfsii, can cause such crown rot on hosta. This disease is usually introduced into the garden as hardened round masses of the fungus called sclerotia. Sclerotia are similar in color to soil particles and very small, often compared to the size of mustard seeds. The crown rot fungus is most active during the humid, hot weather in June and July. First, the small round sclerotia break open. Fluffy white strands of the fungus, called mycelium, are produced and grow across the soil to the base of the plant. The mycelium exudes a poisonous chemical called oxalic acid. This chemical makes short work of the petioles (leaf stalks) of hostas, causing a brown soft rot to occur. The fungus invades and breaks down the crown tissue. This destruction causes water to stop flowing to the leaves, causing them to yellow and wilt. If you tug at a discolored leaf, it will easily separate from the plant and show a brown mushy texture at the end. Only the outer leaves of the hosta are usually damaged, but small plants may be killed. A close look at soil around an infected plant shows sclerotia scattered over the surface. These resistant structures are white when young but mature to a brick red or dark brown color. Sclerotia can infect more plants right away, or can survive over the winter. What can you do to avoid crown rot problems? Start by inspecting plants before buying them. Take a close look at the base of the petioles checking for sclerotia, white fluffy mycelium or any brown, rotten areas. Examine leaves for yellowing or wilting. Hold the petioles and give a little tug -- healthy leaves will remain attached to the crown. Remember other problems, such as drought, excessive sun or inadequate fertilizer can cause discoloration of the leaves, mimicking crown rot symptoms. What if you find evidence of crown rot at home in your yard? First, put the suspected area in quarantine. Avoid spreading soil that may be contaminated with sclerotia to other areas. As you know, mud and debris are easily stuck in the treads of gardening shoes and on tools, so rinse everything well before leaving the quarantine area. For extra assurance, dip tools in a 10 percent bleach solution. The crown rot fungus seems to be especially fond of mulch and grows rampantly through it. Maintaining a mulch-free ring several inches wide around the base of hostas may help reduce the chance of crown rot. If small areas are infected they can be excavated and replaced. This is hard work, requiring that all contaminated soil to a depth of 8 inches be removed. New soil can be added and the area replanted. Hostas can be put back in, but it's best to replace these plants with less susceptible species. Coral bells, lamb's ear and lady's mantle are all suited to shady conditions and are less susceptible to crown rot than hosta. For more information and lists of plants susceptible and tolerant to this disease, check out the Iowa State University Extension publication: Crown Rot on Hosta (SUL 8). This publication is available from your local Extension office or the Extension Distribution Center, 119 Printing and Publications Building, Iowa State University, Ames, IA 50011-3171. -30- ml: isugarden
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