Southern Blight on Vegetables and Herbs
When a plant or part of the plant (leaf, flower, fruit) rapidly collapses and dies this is called blight. Many bacteria, fungi and viruses can cause blights. Most blights are favored by certain weather conditions, such as hot and humid or cool and moist.
Southern blight is caused by the fungus Sclerotium rolfsii. This fungus can attack a wide range of vegetable crops and is seen with some frequency in Maryland on pepper and tomato. 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 dark brown lesions, 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 often unaffected. Stem decay at the soil line is common during hot, humid weather.
|Close inspection of the stem at the soil line reveals dark brown lesions.||Decay of stem from southern blight.|
|Southern blight on thyme.||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 can be seen on the lesion.|
Management: 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 wilted and blighted plants and the mycelium clinging to stems and mulch. Deep plowing can provide good control by burying the sclerotia. Keep the soil pH above 6.5 and add compost to garden beds on a regular basis.
The sclerotia are very resistant to heat and are not easily killed in compost piles. Throw out infected plants with household garbage or seal the plants in a clear plastic bag, expose to full summer sun for 4 weeks and then put the plants through a hot compost process.