Key to Tomato Disease Codes
F=Fusarium Wilt
Fusarium wilt symptoms begin in tomatoes as slight vein clearing on outer leaflets and drooping of leaf petioles. Later the lower leaves wilt, turn yellow and die and the entire plant may be killed, often before the plant reaches maturity. In many cases a single shoot wilts before the rest of the plant shows symptoms or one side of the plant is affected first. If the main stem is cut, dark, chocolate-brown streaks may be seen running lengthwise through the stem. This discoloration often extends upward for some distance and is especially evident at the point where the petiole joins the stem.
FF=Fusarium Wilt Races 1 and 2

V=Verticillium Wilt
Verticillium wilt symptoms on tomatoes are similar to those of Fusarium wilt. Often no symptoms are seen until the plant is bearing heavily or a dry period occurs. The bottom leaves become pale, then tips and edges die and leaves finally die and drop off. V-shaped lesions at leaf tips are typical of Verticillium wilt of tomato. Infected plants usually survive the season but are somewhat stunted and both yields and fruits may be small depending on severity of attack.

N= Nematodes
Plant parasitic nematodes, are small microscopic roundworms which live in the soil and attack the roots of plants. Yield reductions can be extensive but vary significantly between plant and nematode species. In addition to the direct crop damage caused by nematodes, many of these species have also been shown to predispose plants to infection by fungal or bacterial pathogens or to transmit virus diseases, which contributes to additional yield reductions.

T=Tobacco Mosaic Virus
The foliage shows mosaic (mottled) areas with alternating yellowish and dark green areas. Leaves are sometimes fern-like in appearance and sharply pointed. Infections of young plants reduce fruit set and occasionally cause blemishes and distortions of the frui. The dark green areas of the mottle often appear thicker and somewhat elevated giving the leaves a blister-like appearance.

A= Alternaria Stem Canker
Symptoms appear on stems, leaves, and fruit. Dark brown to black cankers with concentric zonation occur on stems near the soil line or above ground. Cankers enlarge, girdle the stem before harvest, and kill the plants. Vascular tissue about 2 inches above and below the cankers exhibit brown streaks. Dark brown to black areas of dead tissue between leaf veins are caused by a toxin produced by the fungus.

St=Stemphylium Grey Leaf Spot
The disease is limited to the leaf blades. The fungus infects plants from the earliest seedling stage to maturity. Initial symptoms include one to several minute brownish black specks that appear on both surfaces of the leaf. The area changes in color from brownish black to grayish brown, as the spots enlarge and these centers crack and partially drop out to give the leaf a shot-hole appearance. The entire leaf then turns yellow, droops, and eventually dies.