Southern blight is a fungal disease caused by Sclerotium rolfsii. It is also known as southern stem rot. The disease is a problem primarily in the Piedmont apple growing region in the southeastern United States. S. rolfsii is a widespread pathogen that affects several hundred plant species and the list of host plants includes azalea, black walnut, crabapple/apple, forsythia, hosta, hydrangea, rose, viburnum. The fungus affects the lower stems and roots of apple trees, killing the bark and girdling the trees. Common symptoms are yellowing leaves, sudden wilting, stems turning brown and dying near the soil line, and plant death (Figure 1). White mycelium may appear on the soil surface, on the lower stem/trunk, including just underground and on plant debris. Clusters of spherical structures called sclerotia may also appear on the host plant and plant debris close to the soil line (Figure 2). Infected trees can usually survive for a month, but plant death often occurs more rapidly shortly after symptoms are noticed. Most herbaceous plants wilt and die in a few days or less. Tree death usually occurs rapidly. Sclerotium rolfsii overwinters in the soil and in plant debris as sclerotia. The light brown or pink sclerotia serve to spread the fungus. This fungus can be spread in runoff water, eroding soil particles and on items such as tools, shoes and plant containers. Generally, the fungus doesn't survive at depths greater than 2 to 3 inches; however, it can survive for years in the soil. It prefers a wet environment and acidic soil conditions.
Figure 1. Yellowing leaves, sudden wilting, stems turning brown on infected apple tree. Image: Dr. Elina Coneva
Figure 2. Clusters of sclerotia appear on the infected apple tree. Image: Dr. Elina Coneva
The disease is most severe on 1- to 3-year-old trees. Root, crown, and collar tissues become resistant to infection as the bark thickens with age.
Typically this disease appears during the hot months of June, July and August. A grower should look for scattered dead plants in the orchard. To manage the disease, avoid planting apple trees on sites where the disease has been severe on previous crops such as peanuts, clover, tomatoes, and soybeans. Remove infected plants and soil surrounding the plant. Keep the soil around the bases of trees free of dead organic matter that serves as a food base for S. rolfsii. Some differences exist in rootstock susceptibility. The most resistant rootstock currently used is M.9. There are no chemicals registered for control of southern blight on apple trees.
Extension Specialist, ACES
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