Tomato Wilting

Question: Some of my tomato plants have wilted and died quickly and I noticed a white powdery substance on the base of the stem - any ideas?

Answer: The description you gave is very typical of "southern blight", also known as white mold and stem rot, and it is caused by the soil-borne fungus Sclerotium rolfsii. We have had numerous reports about this disease in the last few days. The disease is a common problem on vegetables, especially tomatoes, as well as most other broadleaf crops such as pepper, eggplant and beans. Plants of any age can be attacked if environmental conditions are suitable. Development of southern blight is favored by moist conditions and high temperatures (more than 85 degrees F).
Generally, the first above-ground symptoms are leaf yellowing and wilting of infected plants. The stem at the soil line often appears soft and sunken (cankered) and develops a brown to black discoloration both internally and externally. Under moist conditions, a white fungal growth can be seen on the lower stem near the soil surface; on fruit in contact with the soil; and on crop debris on the soil around the base of the plant. Spherical, light brown, mustard-seed size ( 1 to 2 mm) objects called sclerotia often form in the mycelium. Under dry conditions, fungal mycelium and sclerotia may not be visible. However, if southern blight is suspected, placing a section of the lower stem and a moistened paper towel in an enclosed plastic bag for 24 hours will stimulate formation of a white mat of fungal growth. This would be diagnostic for southern blight.

The fungus is spread as mycelium in infested organic matter or as sclerotia in infested soil. Infection usually takes place at the soil surface but may also occur below the soil line. The fungus may spread more than 3 feet through the soil and from plant to plant within a row. It is common to see five or six infected plants within a row killed. Sclerotia, produced on crop debris and dying plants, serve as inoculum for the next crop.

Control of southern blight is difficult to achieve when inoculum levels are high and environmental conditions favor its development. Rotations with crops such as corn, and grain sorghum will reduce disease. Rotations are most effective when tomatoes or other susceptible crops are not planted in the same area more than once every 3 to 4 years.

Deep-plowing the soil to bury crop debris and the fungus will also help reduce inoculum. Wider plant spacing to improve air movement and roguing infected plants will aid in reducing disease development and spread within a field. Adding copious amounts of organic matter may help as well. The information for this blog camer from the Alabama Cooperative Extension System - Tomato Wilt Disease publication.