Symptoms and Signs
Initial symptoms of Verticillium wilt appear at the rosette stage when the lower leaves wilt. As an infected plant develops, the outer whorl of leaves turns yellow, wilts, and dies. Those leaves closest to the lettuce head can yellow, die, and remain closely appressed (attached) to the head. Discolored streaks occur in the internal vascular tissues of the taproot and crown, and can be a combination of green, brown, or black.
Symptoms may appear similar to those caused by Fusarium wilt, or by severe corky root. The internal root discoloration of Verticillium wilt may also be confused with ammonia toxicity damage, though excess ammonia usually causes a brick-red discoloration of the central root core and not the black vascular streaking caused by V. dahliae.
Comments on the Disease
This disease was discovered for the first time in a few fields in California's Central Coast in 1995. The causal organism, Verticillium dahliae, is a well-known pathogenic fungus that infects and damages numerous crops including artichoke, spinach, and strawberry. As a soil inhabitant, V. dahliae can persist in the soil for an almost indefinite period of time. The pathogen produces small, black resting structures (microsclerotia) that resist weathering and drying.
Control measures are similar to those suggested for Verticillium diseases of tomato, strawberry, cauliflower, and other crops.
To prevent Verticillium wilt:
- Rotate for 3 to 4 years with nonhosts (e.g. small grains) or fumigate.
- Do not move soil from an infested field into a clean field.
Resistant varieties, once they become available, will be a primary strategy for managing the disease.
Organically Acceptable Methods
Use cultural controls in an organically certified crop.
Soil fumigation with effective fumigants is the only chemical control available for reducing soilborne populations of the pathogen.