Tomato ‒ Southern Blight

Southern blight is a common disease which has been observed on over 500 different plant species. Gardeners often observe the disease in tomatoes, beans, herbaceous annuals and various ornamental species. Southern blight is also referred to as southern stem blight and/or white mold. It is caused by the fungal pathogen known as Sclerotium rolfsii. It is classified as a fungus which reproduces asexually (anamorphic) and is placed in the fungal group “fungi imperfecti” by scientists. Southern blight usually infects the lower stem of plants near the soil surface. Some plant species roots may become infected, but generally, the disease infects plants at the soil surface. Infected plants often decline, wilt, and die rapidly as a result of damage caused by the infection and reduction of nutrient flow and water movement within the plant.



Visual specification:



- Field (naked eye) view:

The disease is most often noticed when plants begin to wilt and/or when a white thread-like (mycelium) mat or growth is observed on the soil and/or plant. This usually occurs when the weather is warm and humid. The fungal growth extends over the soil surface at the base of the plant and commonly covers the plant stem, a few inches above the soil line. After initial infection, mycelium begins to create multiple, rounded sclerotia (an overwintering structure) which are pinhead, carrot or mustard seed in size (1‒2 mm in diameter). The sclerotia may vary in color from dark brown, tan, orange, and/or white. When environmental conditions are favorable with a susceptible host present, the fungus may repeat the infection process. Fruit of plants at or near the soil surface may become infected and appear water-soaked, followed by a sunken slightly discolored lesion, which will eventually collapse and deteriorate.



- Microscopic view:

Under the microscopic, a thread-like growth (hyphae) is usually observed. The hyphae do not exceed 10 µm in width (5‒9 microns in diameter). Hyphal cells are not swollen and the hyphae are regular with septations, cells which are mostly uninuculate (having one nucleus). Older hyphae generally bear widely spaced apart clamp connections (240 µm distance).

Under normal conditions, sclerotia can only survive in the soil for a few weeks to months. However, in dry laboratory environments, they have been reported to survive 5 or more years.

## Keywords
angiosperms plants eudicots Solanales Solanaceae tomato