Photo: Southern blight on sage
Southern blight is caused by the fungus Sclerotium rolfsii. This fungus can probably attack all herbaceous perennials. It is active only during hot weather, so plants can grow well in infested soil during most of the growing season and only become damaged during the hottest part of the summer. The first symptoms seen are wilting and collapse of individual stems or entire plants. Close inspection of the stem at the soil line reveals white mycelium (strands of fungus growing on the stem and mulch or soil surface) and small (1/8 to 1/16 inch), tan spherical sclerotia that resemble mustard seeds. (They are white when first formed and gradually over several days turn brown.) Roots of infected plants are unaffected. Cortical decay of the stem at the soil line is common during hot, humid weather.
Photo: Decay of stem from southern blight
Southern blight is commonly found on Lysimachia, Ajuga, and groundcover thymes. It is capable of blighting most herbaceous perennials, vegetables, annuals, herbs, and even turf and woody plants.
Photo: Southern blight on thyme
Management strategies: The basis for control of Southern blight is to reduce the number of sclerotia surviving in the upper few inches of the soil. During the growing season, remove blighted plants and the mycelium clinging to stems and mulch. Deep plowing can provide good control by burying the sclerotia.
Southern blight on Rudbekia
The cornerstone for control is sanitation both during the growing season and in the fall. Wilted and blighted plants and plant parts should be promptly removed from the garden. Do not compost material killed by Southern blight because the sclerotia of this fungus may survive the composting process.