The causative organism of Southern Blight, Sclerotium rolfsii Sacc. (= Athelia rolfsii), is a fungus that lives on the ground and does not form spores, similar to Rhizoctonia.
High temperatures help it to grow quickly, optimal temperatures are between 27°C and 30°C. The fungus needs a lot of oxygen, infestation starts in the upper substrate layers at the root neck or stem base. Roots in the substrate are not infested. Sclerotium can hardly grow in cool, humid substrates.
Damage
The upper roots and the stem base of infested plants start to rot, the rotted areas are mushy and bad, the plants become lurid and dry.
A characteristic of the disease is the quick breakdown of the plants. In high humidity, a white to yellowish mycelium shows in the infested areas spreading quickly in a flabelliform way.
Later on dark brown round sclerotia form in the mycelium. Sclerotia which are only about 1.0 mm in size are a major characteristic of the disease.
Control
Fungicide treatment is only possible in the initial phase of the disease (Iprodione, Cyprodinil + Fludioxonil). As soon as mycelia and small round sclerotia occur, direct treatment is no longer possible.
Sclerotia in substrates, pots and transportation boxes can survive for a long time and are easily spread in greenhouses in the water.
Infested plants should be immediately removed from the stand, because of the high risk of infestation, and destroyed.
In case of widespread Sclerotium infestation in greenhouses, the greenhouses must be thoroughly disinfected and hygiene measures have to be carried out.
The use of organic amendments, cotton gin trash, and swine manure was found to manage southern blight through the improved colonization of soil by antagonistic Trichoderma spp. Along with species of Trichoderma, other biological agents, such as Gliocladium virens, Bacillus subtilis, and Penicillium spp., were found to antagonize S. rolfsii and may aid in disease suppression. Gliocladium virens was found to reduce the number of sclerotia in soil to a depth of 30 cm, resulting in a decreased incidence of southern blight on tomato. Trichoderma koningii also reduced the number of sclerotia and the plant-to-plant spread of southern blight in tomato fields. More recently, an isolate of Streptomyces philanthi was shown to be effective at protecting chili pepper plants from infection by S. rolfsii.
