Tomato diseases

By John Fulton

[June 29, 2011]  Tomatoes seem to be a favorite with gardeners, as they produce an abundance of fruit. Some people grow large amounts, while others plant one or two in containers. At any rate, the calls and samples have been trickling into the office for a few weeks already. Most of the samples have spots, brown leaves and dropping leaves, or all of the above. Several diseases hit tomatoes, but two of the more common ones are early blight and septoria leaf spot. Blossom-end rot seems to have been common on early tomatoes as well.

Early blight, also known as Alternaria leaf spot, can affect plants at any stage of development. All above-ground parts are susceptible. The most characteristic symptom of early blight is spreading spots, one-fourth to one-half inch in diameter, that form on lower or older leaves. These spots have dark edges, and they are usually brown to black in the center. The spots frequently merge, forming irregular blotches. Concentric rings often form, creating a "target" or "bull's-eye" effect. Affected leaves develop yellow areas around the lesions. Spotted leaves soon turn yellow, wither and drop off. The fungus may cause lesions on the fruit around the stem end and shoulder. The lesion is usually dark brown to black, up to an inch in diameter and with distinct concentric rings.

Septoria leaf spot can also affect plants at any stage of development. Numerous small, water-soaked spots first appear on the lower leaves. These spots soon become circular to angular with dark margins and grayish centers often bearing one or more tiny black bodies called pycnidia, which are spore-bearing structures. Individual lesions are seldom more than one-eighth inch in diameter and are usually quite numerous on an infected leaf. Heavily diseased leaves turn yellow, wither and drop off in large numbers, starting at the base of the plant. Defoliation can be severe during prolonged periods of warm, wet weather.

As for what to do, here is the checklist: First, keep ripe fruits picked off the plants. Second, don't work around tomatoes when they are wet. Next, you can try to improve air circulation, but if your tomatoes are severely affected, you won't want to lose any more leaves. And the final step for this year is to try a fungicide. Mancozeb is probably the recommended one, but it is very hard to find. The other options are Daconil and maneb, which are easier to find but probably won't give you as good a control. The final step for future years is to practice at least a three-year rotation, with good sanitation in the garden.

Blossom-end rot is a non-pathogenic disease that is very common during extended dry periods. It also seems to be worse on tomatoes grown in containers. It begins as a light tan water-soaked lesion on the blossom end of the fruit. The lesions enlarge and turn black and leathery. This can drastically lower the yield and lower marketability of the fruits. The major causal factors are fluctuating soil moisture supply during the dry periods and low calcium levels in the fruit.

Control of blossom-end rot consists of providing adequate moisture from fruit formation to maturity, and use of mulch (grass clippings, plastic, straw, shredded newspapers or plastic) to conserve moisture and even out the moisture supply. Avoid frequent shallow watering. Water deep and then wait five or more days before watering again.

[By JOHN FULTON, University of Illinois Extension]