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Tomatoes: Planning for 2002

September 12, 2001

It's never too early to begin planning next year's garden. What worked well this year? What problems did you encounter? Which varieties will you choose for next year? Here, I'll point out some common diseases of tomato and discuss some key management issues. Now is a good time to evaluate your tomato varieties for diseases and begin planning for next year.

Early blight: The most characteristic symptom is brown spots on the older leaves. The circular to angular spots enlarge until they are about 1/4 to 1/2 inch in diameter and soon develop dark, concentric rings or ridges, giving a targetlike appearance. The fungus may also cause depressed but similar lesions around the stem end and shoulder of tomato fruit. Fruit lesions are often covered with a dark brown, velvety layer of spores.

Septoria leaf blight: Although the disease can appear on tomato leaf petioles, stems, blossoms, and flower stalks, it is most commonly found on the lower leaves. Symptoms first appear as tiny, water-soaked areas, but soon enlarge to form circular or angular lesions about 1/8 to 1/4 inch in diameter. Mature lesions have a dark margin with a grayish white center that contains tiny, black fruiting (spore-producing) bodies. Heavily diseased leaves turn yellow, wither, and drop off in large numbers, starting at the base of the plant.

Bacterial spot and speck: Bacterial leaf spot appears as small (1/8-inch diameter), water-soaked, translucent lesions that later turn brownish black and may have a yellow halo. The lesions are somewhat irregular and appear greasy on the upper leaf surface, with a translucent center and a black margin. The centers of the lesions dry out and frequently tear. Only a few spots may cause a leaflet to turn yellow, wither, and drop prematurely. Spots on green fruit first appear as small, black, raised pimples surrounded by a narrow, water-soaked border. As they age, spots are slightly raised, superficial, and up to 1/3 inch in diameter, with lobed margins and water-soaked borders. Eventually the raised center sinks, forming a brownish black crater that usually does not penetrate the seed cavity. Foliar symptoms of bacterial speck are virtually identical to those of bacterial spot but can be differentiated by the symptoms on immature fruit. With bacterial speck, fruit symptoms appear as black, slightly sunken stippling, which eventually result in lesions less than 1/16 inch in diameter. With both diseases, only immature fruit are infected.

Fusarium wilt: Symptoms on mature plants generally appear between blossoming and fruit maturation. The first symptoms include yellowing of the older leaves (usually beginning on one shoot or on one side of the plant), which progresses until most foliage becomes yellow and wilts during the hottest part of the day. Eventually, the plant collapses and dies. Besides these symptoms, the fungus also causes the vascular system to turn dark, chocolate brown, beginning below the soil-line and extending for some distance up the main stem. This discoloration is especially evident where the leaf petiole joins the stem.

Verticillium wilt: Often the first symptoms are mild wilting during the day. As the disease advances, lesions develop along the edges and between the veins of lower leaflets. Unlike the targetlike early blight lesions, lesions due to Verticillium wilt are V-shaped and may be either tan or brown, with diffuse yellow borders. Wilt symptoms can easily be confused with Fusarium wilt and drought. Like Fusarium wilt, the symptoms are caused by the soil-borne fungus invading and plugging the vascular system. However, vascular discoloration due to the Verticillium wilt pathogen is typically lighter tan than Fusarium wilt.

Viral diseases: A wide range of viruses has been identified in Illinois, including tobacco mosaic virus (TMV), cucumber mosaic virus (CMV), and tobacco etch virus (TEV). Symptoms vary, depending on which virus or viruses are involved. In general, you should suspect viral infection when you observe the following symptoms: light and dark green mottled areas on the leaves, puckered leaves, rough or wrinkled (rugose) leaves, improper unfolding of leaves, extremely distorted leaves, bunched shoot growth, mottled or warty fruit, and overall stunted plant growth.

Management suggestions

Plant resistance is a simple and inexpensive way to manage certain diseases. Many varieties are available with resistance to one or more diseases. When purchasing tomato varieties, go armed with the knowledge of which diseases have caused problems in your garden and select varieties with the appropriate resistance code (for example, V = Verticillium wilt; F1 = Fusarium wilt (race 1); F2 = Fusarium wilt (race 2); N = Nematodes; T = Tobacco mosaic virus; St = Stemphyllium (gray leaf spot); A = Alternaria stem canker; L = Septoria leaf spot). Keep in mind that even resistant plants may still succumb to disease if stressed or if disease pressure is very high. Consult page 119 of the Home, Yard, and Garden Pest Guide (University of Illinois Extension circular 1374) for a list of varieties with resistance to one or more diseases.

Foliar diseases such as early blight tend to be more problematic where plants are stressed, as from poor fertility, drought, insect damage, or heavy fruit. Proper sunlight, fertility, and irrigation go a long way toward promoting healthy plants. If you find you need to apply a pesticide, consult page 121 of the Home, Yard, and Garden Pest Guide and follow the product label carefully.

No pesticides are available to control Fusarium wilt, Verticillium wilt, or viral diseases, so it is important to discourage these diseases from building up and becoming a problem. Your garden may be small, but do your best to practice crop rotation. In other words, don't plant tomatoes in the same area year after year. When planning your rotation, keep in mind that Verticillium wilt is also a disease of plants closely related to tomato (such as potato and pepper).

Because many foliar diseases begin on the lower, shaded leaves, remove them preventively to reduce early infection. Should individual plants become heavily diseased and unthrifty during the season, remove them to avoid disease buildup (for soilborne diseases such as Verticillium wilt or Fusarium wilt) or spread. For the same reasons, remove and dispose of the leaves, stems, and large roots in the fall.

For more information about tomato culture and pest management, consider the following sources: Vegetable Gardening for the Midwest (University of Illinois Extension Circular 1331); Urban Programs Resource Network—Tomato, http://www.urbanext. uiuc.edu/veggies/tomato1.html; Report on Plant Disease (RPD) fact sheets, http://www.ag.uiuc.edu/~vista/rpd.html. These and other publications are available through your local University of Illinois Extension office.

Author: Bruce Paulsrud


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