Background information on the ecology and management of petiole rot caused by S. rolfsii var. delphinii.
Adapted from: Edmunds, B., M. Gleason, and U. Schuch. 2000. Crown Rot: A serious disease of hosta and other ornamentals. Iowa State University Extension Bulletin SUL-8. 8 pp.
Hostas are the top-selling herbaceous perennials nationwide, thanks to their attractive foliage, endless diversity in shape and size, tolerance of shady areas, and minimal maintenance needs.
Few pests plague hostas in the Midwest. Since the mid-1990's, however, a disease called petiole rot has damaged many hosta plantings. This fungal disease, caused by Sclerotium rolfsii var. delphinii, can severely damage hostas within a week. Once established in an area, S. rolfsii var. delphinii is difficult to eradicate.
Symptoms begin to appear on hostas after prolonged hot, humid weather. The lower leaves turn yellow, then brown, and wilt from the margins back toward the base. The upper leaves may soon collapse, as well. Wilted leaves can be easily pulled from the crown, because they have been infected at the base of the petiole. The bases of these damaged petioles show a brown discoloration and mushy texture. With plants with less succulent stems, such as peony, stem bases may be girdled and then the leaves will become discolored and wilted, but the stems may not collapse. Ropy-textured white threads (mycelium) of the petiole rot fungus typically are present on the rotted tissue and surrounding soil. A closer look often shows small spheres, about the size of mustard seeds, sprinkled on the soil surface. These tiny spheres (0.5-1 mm), called sclerotia, allow the fungus to survive cold winters and other unfavorable conditions. As sclerotia mature, their color changes from white to a light tan or reddish brown.
The petiole rot disease cycle begins with the germination of sclerotia. Mycelium fans out in all directions from the sclerotia, slowly growing across the surface of the soil in warm, wet weather. When a host plant is nearby, the fungus exudes droplets containing oxalic acid and tissue-destroying enzymes. Oxalic acid kills plant cells and contributes to the breakdown of their cell walls.
When S. rolfsii var. delphinii's mycelium comes into contact with hosta plants, the oxalic acid kills the petioles at the base. With their petioles severely damaged, the leaves begin to collapse. The stem bases, and the soil for several inches (centimeters) around the crown, may be covered with fluffy white mycelium, which produces sclerotia. Sclerotia are compact, hardy structures and can survive temperatures well below freezing, either in plant debris or at the soil surface. There is no required dormancy period, so newly formed sclerotia can produce new mycelium at once or remain dormant for several years.
On the positive side, S. rolfsii var. delphinii seldom produces microscopic spores as most other fungi do. Without spores, it cannot spread by moving on air currents but only as sclerotia in soil or on plants, or by mycelium that grows slowly across the ground from plant to plant.
What are the circumstances behind petiole rot's movement throughout the Midwest? The answer is unknown, but one possibility is that S. rolfsii var. delphinii has been spread on contaminated nursery plants. Others point to the frequent exchange of hostas among gardeners and hosta enthusiasts. Once petiole rot has entered a planting, it is easily spread by sclerotia clinging to soil on shoes, tools, plant material, and running water.
S. rolfsii var. delphinii can affect at least 18 herbaceous ornamental species. Many of these susceptible species are often planted in the same beds as hostas, so the pathogen can move from hostas to neighboring plants.
Effective management of petiole rot requires the IPM (integrated pest management) approach of combining several practices. The most effective management practices are a combination of cultural tactics such as careful inspection before planting, sanitation to control spread of pathogen, and choosing plants believed to be less susceptible. These methods will not cure infected hostas but can slow or stop the spread of disease.
A key to avoiding petiole rot during plant propagation and production is careful inspection. Regularly examine plants for telltale leaf yellowing and collapse, sclerotia, softening and browning at the bases of petioles, and white mycelium around damaged tissue. Experienced hosta growers know that leaves can turn yellow for other reasons, such as heat stress or a fungal disease called Rhizoctonia root rot, but only S. rolfsii var. delphinii petiole rot will show other symptoms and signs of the fungus. Once you are convinced that the problem is petiole rot, discard the symptomatic plants as well as the planting medium and the pot.
Deep plowing (by using a moldboard plow, for example) pushes surface residues, including sclerotia of S. rolfsii var. delphinii, well below the soil surface. Since sclerotia decay more rapidly when buried than at the soil surface, deep plowing can lower disease risk by reducing survival of sclerotia.
Pre-plant fumigation of production fields with methyl bromide and chloropicrin was sometimes used to minimize problems with S. rolfsii var. delphinii. However, methyl bromide use has been prohibited in the United States since the year 2005 due to an international treaty to protect the Earth’s ozone layer. Critical use exemptions still allow some use on certain crops in some states when no alternatives are available.
Fungicides are sometimes used to suppress S. rolfsii var. delphinii in hosta as well as other ornamental crops. Flutolanil (sold as Contrast®) azoxystrobin (sold as Heritage®) and fludioxonil (sold as Medallion®) are labeled for use against this fungus on ornamental plants. Fungicides containing pentachloronitrobenzene (also known as quintozene or PCNB) typically are applied preventively to the soil as a drench or granule. Trade names of PCNB products labeled for control of petiole rot include Terraclor®, Defend®, Pennstar®, Revere®, and PCNB®. Note that the labels of these products do not specifically mention hosta because hosta is included within the broad designation "ornamentals" on the labels. Since phytotoxic reactions to PCNB can occur, it is advisable to treat a small bed first, and then check the hostas' reaction, before attempting larger-scale treatments. To our knowledge, flutolanil and PCNB products labeled for use on ornamentals are available only through commercial pesticide dealers. Sanitizing agents such as chlorine bleach have been used against petiole rot, but these products are not legally labeled for this use.
D.F. Farr 1989. Fungi on plants and plant products in the United States. APS Press, St. Paul, MN;B.A. Edmunds, K. I. Gleason, and M. L. Gleason. 2003. First report of Pulmonaria longifolia and Astilbe arndsii as hosts of Sclerotium rolfsii var. delphinii. Phytopathology. 93:S23.