Looking for Regional Information?

Decoding Tomato Disease Resistance Codes

By Charles Hopkins Published 05/27/2007 | Food & Drink

If you have had problems growing tomatoes, you may be interested in some disease resistant varieties and hybrids. But codes like "VF", "VFFA", and "VFNT" can be somewhat cryptic. What kinds of problems are common to tomatoes, and how can you tell if a particular variety is resistant to the disease you are encountering in your backyard garden? The capital letters that follow the names of some tomatoes should tell you what disease resistances that type has:

"V" indicates the plant is resistant to the fungi that cause Verticillium wilt, Verticillium dahliae and V. albo-atrum. Symptoms of Verticillium wilt don't usually appear until the plant is bearing fruit or it experiences a dry period. Symptoms include V-shaped lesions on the leaves.

"F" or "FF" means the plants are resistant to the Fusarium oxysporum fungi that cause Fusarium wilt. This is a serious disease that manifests itself in the wilting and yellowing of leaves starting at the base of the plant. Left unchecked, Fusarium wilt can kill tomato plants well before harvest time. Some Fusarium fungi have overcome the initial "F" resistance tomatoes. If your tomatoes are afflicted with this second type, choose plants with the "FF" designation.

"N" means that the tomatoes are resistant to the parasitic round worms, nematodes that often lie dormant in the soil. Symptoms of nematode infestation include root galls that are up to an inch in diameter. Afflicted plants are less vigorous, do not respond to fertilizer, and tend to wilt more easily in hot weather.

"A" indicates resistance to the Alternaria alternata fungus that causes Alternaria stem canker. Tomato stems, leaves and fruit are afflicted with cankers that range from dark brown to black. If the cankers appear on the stem they can be accompanied by brown streaks. Stem cankers can girdle and kill a plant before harvest.

"T" means that the plants are resistant to the Tobacco Mosaic Virus (TMV). TMV causes a mottled appearance in tomato leaves and reduced tomato size and yield. Sometimes unripe fruit of plants afflicted with TMV will brown.

"St" designated plants are resistant to Stemphylium or gray leaf spot. Afflicted plants develop brown to black spots on older leaves. As the disease progresses, the spots get bigger while the center become gray. The gray centers eventually drop out, leaving behind small holes. The Stemphylium solani fungus causes gray leaf spot.

"TSWV" indicates resistance to the Tomato Spotted Wilt Virus. Symptoms of infection vary widely between tomato varieties, but include yellow and brown rings and brown streaks on plant stems, dead leaf spots and tips, and severely stunted growth. Fruit may have red and orange discolorations at maturity.

Remember that resistance is not the same as immunity. Tomatoes such as Early Cascade VF Hybrid can still be infected with Verticillium and Fusarium Wilt. They just won't be as susceptible to damage as their non-resistant cousins.

In addition to growing the appropriate resistant strains, you should use good gardening practices to keep these fungal, viral, and parasitic pests under control. Rotating your tomato crops with annual rye, alfalfa, peas or clover will prevent most nematodes and fungi from multiplying. Just make sure that you choose species and cultivars that do not harbor the particular pest you're trying to control. Soil solarization will kill off many more nematodes, fungi, and disease vectors (aka, weeds and larval insects). By keeping your tomato plants as healthy and vibrant as they can be, good watering, drainage, and mulching also help control pests. Finally, do NOT re-incorporate diseased plant matter into the soil at the end of the season --- compost from a diseased plant will just infect your next crop, so it is best to either burn it or throw it away.

The trick to growing delicious tomatoes in your backyard is to find out what pests it already harbors, choose tomato varieties specifically resistant to those pests, and follow good gardening/pest-control techniques. Once you have grown a couple of good crops and have your pests down to a manageable level, you can even move on to grow more exotic tomato varieties.