Symptoms and Signs
Fusarium wilt causes infected seedlings to wilt and possibly die. Inner tissues of affected seedlings are red or brown. In older plants, leaves turn yellow and develop tipburn. Internally, the vascular system is brown to black in the leaves, and a reddish-brown discoloration develops in the cortex of the crown and upper root. Plants may be stunted and fail to form heads.
Comments on the Disease
In California, Fusarium wilt of lettuce occurs in Fresno County and in the southern region of coastal Monterey County. The pathogen causes a disease only in lettuce but may be sustained on the roots of many plants. Once introduced into a field, the fungus will probably remain indefinitely. Fusarium wilt is favored by high soil temperatures. It exists in the form of three races, but only Race 1 is currently found in California. Fusarium wilt symptoms may resemble those caused by ammonium toxicity and Verticillium wilt.
Avoid infested fields. Do not move soil from an infested field into a clean field.
Organically Acceptable Methods
Use cultural controls in an organically certified crop.