Disease guide

Sclerotium rolfsii (teleomorph: Athelia rolfsii)

This fungus infects emerging seedlings below or at the soil level and causes damping-off. Under favorable environmental conditions, Sclerotium rolfsii is able to infect any part of the plant. The first symptoms of disease in mature plants are dark-brown lesions on the stem at or just beneath the soil line. First foliar symptoms are progressive yellowing and wilting. Later, the fungus produces fan-like webs of whitish mycelium around the rotted stem. Small brown sclerotia form within the mycelial mass. As the disease progresses, infected plants wilt and die.

Sclerotium rolfsii has a wide host range. The fungus overwinters as mycelium or sclerotia in and on infected plant debris. Sclerotia can survive in soil for many years. Rainfall or irrigation following a period of drought generally stimulates germination of sclerotia and initiates the infection process. High humidity and warm temperatures generally favor rapid fungal growth and disease development. A soil pH between 3 and 5 is best for fungal growth. At a soil pH of 7 or above, germination of sclerotia is inhibited. Sclerotia spread short and long distances in infected transplants, plant debris, soil, surface water, and on farm equipment and poor quality seed.

Rotate with grasses and deep-plow to bury sclerotia to help reduce soil inoculum level. Grow plants in raised beds to promote soil drainage. In small-scale plantings, rogue infected plants when symptoms are first visible to reduce disease spread. Soil fungicides and biological control using Trichoderma spp. and Gliocladium virens offer some protection.