Open-pollinated or heirloom tomatoes tend to be more resistant to diseases than hybrid varieties. Testing for disease resistance costs at least $200 per variety, according to a Missouri State article by Carolyn Male. Although many hybrid varieties list their resistance, heirloom varieties often do not because they were never formally tested. Growers should take heed of the fact that heirloom varieties have been passed down for hundreds of years and, as such, are considered hardy against many tomato diseases.
Septoria Leaf Spot
Septoria leaf spot is the most common type of fungus in tomato plants. The first signs of this disease are small water spots on the plant's leaves. These water spots soon turn into brown circular spots and then spread into splotches. The circular splotches on the leaves will be light colored and occasionally have a black speck in the middle of them. Eventually the lower leaves of the tomato plant will rot and fall off. This usually happens after the plant has begun to bear fruit. Remedies for septoria leaf spot include planting the tomato plants far apart, watering only in the morning and rotating crops.
Early blight is caused by a fungus. The first signs of this disease are the loss of the lower leaves of the tomato plant. Brown or black spots may appear on the leaves just prior to them falling off. These spots will be light or medium brown with dark edges. When early blight strikes the tomato, the fruit will become black at the top and sunken in. Gardeners should remove infected plants from the area as soon as this disease is discovered in order to prevent affecting other tomato plants. Rotating crops annually will help minimize the outbreak of this disease.
Anthracnose often affects tomato plants after they have begun to bear fruit. A gardener may notice small circular spots on the skin of the tomato. They will often have dark centers or dark rings around the spot. This disease is often spread by rainwater that carries spores of the fungus to the tomato plant. Tomato growers are advised to pick all ripe fruit often in order to combat the spread of this disease.
Verticillium wilt is a disease that affects tomatoes as well as peppers, watermelon, radishes, potatoes, eggplants and strawberries. This disease begins at the bottom of the tomato plant and works its way to the top. Telltale signs are yellow splotches on the lower leaves followed by the leaves turning yellow and dropping off the plant. This disease does not kill the plant nor keep it from producing fruit; however, the yield will be greatly affected. Growers are advised to plant varieties that are resistant to this disease. A "V" will be listed on the seed package if the variety meets this criteria. Gardeners should also rotate tomatoes into areas where peppers, eggplant and potatoes have not previously been grown
Late blight is more common during periods of heavy rain. The first sign of this disease is a patch on a leaf that appears to be water soaked. These water-soaked splotches will spread quickly into irregular-shaped blotches that are green to black in color. They may even appear to be frost damaged. White growth will often appear on the underside of the leaves. Late blight is common at the end of the growing season. Crop growers should avoid rotating tomato plants with potatoes in order to prevent this disease during the next growing season.