Warm weather can be the perfect condition for several diseases of vegetables.

Most growers and consultants think of cold wet weather as the ideal condition for plant diseases. There are a few plant diseases that only become an issue when the weather becomes hot. Hot conditions are common in California’s Central Valley.

Southern blight is a disease that becomes an issue every summer. Potato growers with late-growing fields are familiar with this disease. It can occur on all vegetable crops growing in the summer months. Lately the disease is impacting more and more tomatoes.

Southern blight is caused by a soil borne fungus called Sclerotium rolfsii.

The disease is also called “rolfsii.” It survives in the soil as small hard bodies of fungal tissue call sclerotia which resemble alfalfa seed. The sclerotia germinate under warm moist conditions when a host plant is nearby.

The fungus primarily attacks the plant at the soil line which makes identification fairly easy. A tan mass of fungal growth can be seen with a mass of alfalfa seed-sized sclerotia. The sclerotia are initially white in color, but turn brown during maturation. Numerous sclerotia will occur.

As the infection progresses, the plant wilts and eventually dies. The inspection of infected plants shows fungal growth and numerous sclerotia on the plant at the soil line. Tomato fruit on the soil surface also become infected as well as potato tubers.

Southern blight has a very wide host range and can infest many vegetable plants including onions, garlic, beans, peppers, and others vegetables.

Southern blight is often not noticed until it is widespread in the field. The disease likely was in the field for years. Once the sclerotia become numerous in a field then the disease becomes noticeable.

Each infected plant can literally produce tens of thousands of sclerotia. In one to two seasons Southern blight can result in entire crop loss.

Under warm conditions (86 F and higher) the disease can progress very rapidly.

Control of Southern blight can be difficult. Deep plowing will bury the sclerotia and move it away from attacking plants at the soil line. Crop rotations to non-host plants including small grains will significantly reduce sclerotia levels in the field.

Fungicide use at planting helps manage the disease. If sclerotia levels are high in the field, fumigation should be considered. Metam sodium will control Southern blight, but the costs of fumigation may limit its use.

Scouting the fields during the summer months to determine if Southern blight is present allows for efforts to be undertaken before the sclerotia levels become too numerous.