The Homeowners Column

The Homeowners Column

Summer Brings Tomato Problems

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

Tomatoes are the most commonly grown vegetables in home gardens. The taste of home grown tomatoes is alluring and ripe tomatoes are easy to spot among three feet tall weeds. However weeds may not be the only problem for tomatoes.

If you look out into your garden and see nothing but stems, hornworms are probably the culprits. Tomato or tobacco hornworms when fully grown are large 2 to 3 inch long green caterpillars with white stripes. A horn projects from the top rear end of the caterpillar.

Hornworms are voracious eaters and a few can quickly eat the leaves and developing fruit. They are difficult to see and often hang on the underside of leaves or stems. Adult hornworms are large attractive moths called sphinx moths that feed on the nectar of flowers at dusk.

Control of hornworms includes handpicking or using the bacterial agent Bacillus thuringiensis kurstaki sold as Bt, Dipel, Thuricide, or Caterpillar Attack. If you see hornworms with small white cocoon structures attached to their backs, leave them alone. These hornworms are already doomed because the cocoons are a developing stage of a parasitic wasp that will help to naturally keep down the hornworm population.

Tomatoes are having some leaf spot disease problems: septoria leaf spot and early blight. Both cause spots and can cause the leaves to quickly yellow and drop starting at the bottom of the plant. Septoria causes small water-soaked spots. These spots become circular to angular with dark margins and greyish white centers. Early blight causes small brown leaf spots with a target-like series of concentric rings in each lesion.

To control leaf spots: improve air circulation in the garden, mulch, stake or cage vines, remove infected leaves and remove and destroy tomato vines and unharvested fruit at the end of the season. Weekly fungicide sprays such as maneb can be helpful. Be sure to read and follow all label directions especially the days between application and harvest.

Uniform watering will help alleviate blossom end rot that causes the ends of peppers and tomatoes to turn dark brown to black. Blossom end rot results from a calcium deficiency caused by major changes in soil moisture and humid weather. There may be plenty of calcium in the soil, but the plant isn't absorbing it.

Applications of calcium rich fertilizer to control blossom end rot have had mixed reviews. If calcium nitrate or other fertilizers are used, they should be applied to the leaves and tomatoes when the tomatoes are about grape size. Be sure the fertilizer is meant to be used as a leaf spray.

In addition to watering plants thoroughly with one inch of water a week, mulching plants with straw, newspapers, or thin layers of grass clippings can also help to control blossom end rot. When hoeing be sure to cultivate shallowly to protect roots from damage. Caged plants seem to be less prone to blossom end rot than staked plants.

Dry windy days with temperatures above 95 degrees F can cause tomatoes and peppers to not set fruit and for the flowers to fall off. Once weather conditions become more favorable, fruit will develop.

Poor color, mushiness and sunscald can also be problems in hot weather. Color may not develop well in tomatoes exposed directly to hot sun. Sunscald can appear as large whitish areas on the fruit. During hot summer weather tomatoes should be picked as color starts to develop and ripened further indoors at 70 to 75 degrees F. Tomatoes do not need sunlight to ripen.

It's amazing even with all the problems that tomatoes can have, you still have to lock your car doors to make sure the neighbors don't leave you another batch.

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