Fusarium oxysporum f. sp. radicis-lycopersici
Typically, the first symptoms are observed when the plant is at the mature green fruit stage. These include a yellowing of the oldest leaves that gradually progresses to the youngest leaves. When the disease is severe the plants may quickly wilt and die, however, a diurnal wilting during hot sunny days is more common. As the disease progresses the entire root system turns brown and the tap root often rots away. Chocolate brown lesions develop at or near the soil-line and extend into the vascular system. This brown vascular discoloration typically does not extend more than 25 cm (10 in. ) above the soil-line, which helps to distinguish this disease from Fusarium wilt. When the humidity is adequate, fungal sporulation may be observed on the exposed lesions.
Conditions for Disease Development
The fungus can survive in the soil, on infected plants and on the roots of alternative hosts (eggplant, pepper and a number of legume crops) for several years. Fungal conidia can be spread in soil, on farm machinery, in irrigation water and through the air. Infection takes place through feeder roots and wounds caused by secondary root formation. Disease development is favored by cool (20°C, 68°F) soil temperatures.
The use of varieties resistant to Fusarium crown and root rot is generally the most effective way to control this disease. Soil steaming in combination with fungicide drenches may also reduce losses. Fumigation does not appear to be an effective control measure due to rapid colonization of the sterilized soil by Fusarium oxysporum f. sp. radicis-lycopersici.