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Gail Ruhl, Plant Disease Diagnostician, Purdue University
The cream-colored structures at the base of the rotted Hosta leaf are fungal "fruiting" bodies known as sclerotia. The basal rot, ropy or fan-shaped white mycelium, and the presence of sclerotia are diagnostic for this destructive fungal disease known as Southern Blight (causal organism: Sclerotium rolfsii). This soil-borne fungal pathogen is an aggressive stem rotter of a wide range of species, including not only flowers and vegetables, but also some field crops and fruit trees. It is favored by high temperatures and high humidity. Controlling Southern Blight is difficult because the pathogen has a wide host range and because the sclerotia persist in the soil.
It is important to avoid moving soil from the affected area to other areas of the garden since sclerotia can be moved with the soil. Infected plants and surrounding soil should be removed. Replace with species not reported to be susceptible to the disease. Some plants from which you can choose include Abutilon, hyacinth, Alyssum, bells of Ireland, lavender, Cleome, Mertensia, cockscomb, Portulaca, four o'clock, primrose, English daisy, statice, Fritillaria, tansy, globe amaranth, Freesia, Gypsophila, and Geranium (wild geranium).
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Southern Blight of Hosta
Plant and Pest Digital Library and Digitally Assisted Diagnosis, Purdue University, West Lafayette, Indiana.