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Common garden tomato problems
by Martha A. Smith
July 8, 2018

Now that summer has arrived, gardeners are noticing problems with their tomatoes. There are a number of diseases affecting tomatoes, says Extension educator Martha Smith, but the most common are early blight, Septoria leaf spot, and late blight.

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 are spreading spots, a quarter to a half inch in diameter that form on lower or older leaves,” Smith says. “These spots have dark edges and they are usually brown to black in the center. They frequently merge, forming irregular blotches. Concentric rings often form creating a 'target' or 'bulls-eye' effect.”

Affected leaves develop yellow areas around the lesions. Eventually leaves become entirely yellow, then 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, 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 having one or more tiny black bodies called pycnidia, which are spore-bearing structures.

“Individual lesions are seldom more than an eighth inch in diameter and are usually quite numerous on an infected leaf,” Smith says. “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.”

Late blight in garden tomatoes usually appears in mid- or late August. “A primary source of this disease can be from leftover potatoes from last year’s garden. The fungus that causes late blight needs living tissue to survive over the winter, so it can't overwinter on tomato cages or supports. However, infected potatoes (the other plant that gets late blight) can carry the disease through the winter. Be sure to destroy any volunteer potato plants that come up. If you plant potatoes again, be sure to buy seed potatoes that are certified as disease-free,” Smith says.

Ideal conditions for late blight development are warm, humid days followed by cool night temperatures with heavy dew, fog, or light drizzly rain that persists through morning. Heavy overcast skies during the morning prevent temperatures from rising rapidly and the foliage remains wet. In moist weather, this fungus can be carried 20 miles or more by strong winds and rain.

On older plants, the fungus causes irregular, rapidly enlarging, water-soaked, pale green to greenish-black lesions, which usually start at the margins or tips of the leaves. In dry weather, these lesions turn dark brown, dry, and wither. A pale green 'halo' often surrounds affected leaf areas. The spot may enlarge until entire leaflets are killed.

Lesions can expand rapidly and result in extensive, if not complete, defoliation within 2 weeks. Severely affected plants may appear as if damaged by frost. Infection of both green and ripe fruit starts at the stem-end or the side of the fruit, and soon spreads over the entire fruit. Infected areas are dark green, brown, or brownish black and greasy-looking, with a firm but slightly wrinkled surface. Successful disease control involves several steps. Smith offers the following guidelines to insure a healthy crop.

1. Crop rotation is recommended every year. Don't plant solanaceous crops (tomatoes and potatoes) in the same area more often than once every three or four years. Destroy any volunteer plants. This will prevent buildup of disease organisms in the soil.

2. Purchase only disease-free plants from a reliable grower.

3. Allow adequate space between plants to increase the rate of evaporation of water (rain or dew).

4. Harvest all ripe fruit at each picking. Ripe fruit left in the garden may decay and infect the remaining fruit.

5. Don't cultivate or work plants when foliage is wet with dew or rain. The organisms spread under these conditions.

6. Apply recommended fungicides according to label directions where the above measures fail to provide adequate control. Contact your local University of Illinois Extension Office for recommended fungicides.

7. After harvest is complete, spade or plow under, compost, or burn all tomato vines. Destroy all potato cull piles, volunteer plants, and solanaceous weeds such as groundcherry, horse nettle, nightshade, and Jimson weed.

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