Wilt symptoms are caused in a large number of broadleaf plants by several species of Fusarium and Verticillium fungi. The fungi differ one from another but the symptoms which they cause are very similar. The only reliable method for separating these diseases is laboratory culture and identification of the fungus isolated from diseased plants.
Plants first show a wilted appearance. Individual branches or even single leaves may be affected at first. Leaves develop a yellow color, often in V-shaped sectors between the major veins. Leaves eventually die and fall.
Discoloration or brown streaking is often found in vascular tissues. The fungus can be readily isolated from the diseased stems. Diseased plants may die soon after first symptoms or they may sprout at the base after the top dies.
The wilt fungi have different general characteristics which will aid in identification and control. These are:
- Verticillium thrives in alkaline soil whereas Fusarium grows best in acid soils.
- Fusarium is more prevalent in sandy soil; Verticillium in heavy soils.
- Fusarium causes more injury when root-knot, reniform or sting nematodes injure the roots; Verticillium does not require injury for infection.
- Fusarium can be transmitted internally in seed while Verticillium is not.
- Verticillium prefers cooler soil than Fusarium.
The two diseases have some characteristics in common. These are:
- Both thrive with high nitrogen fertilizer, excessive soil moisture, thin stands, and deep cultivation during the growing season.
- Both fungi survive long periods in soil in the absence of a cultivated host.
Control of wilt diseases is difficult. Carefully follow suggested cultural practices. Rotation with tolerant plants and clean tillage to destroy infected tissue will help.
Plant breeders have selected many sources of genetic resistance to each wilt disease. Acceptable tolerant varieties have been developed in tomato, cotton, watermelons, and other crops. Growers should plant these improved varieties when they are equal to the susceptible varieties in yield, adaptation, and other cultural characteristics. Seed catalogs and other descriptive literature will give specific information on the tolerance of varieties to wilt diseases. Since new races of the fungi may develop that attack resistant varieties, it is necessary that growers keep up with the latest information on resistant varieties and control measures.
