August 3, 2011

Yard and Garden: Tomato Disorders

By Richard Jauron
Iowa State University Extension Scott County

Tomatoes are the most popular vegetable crop in Iowa. Many diseases and disorders can affect tomatoes during the growing season. Iowa State University Extension specialists describe the symptoms and management of common problems. To have additional questions answered, contact the experts at or call (515) 294-3108.

Why are my tomatoes cracking?

Fruit cracking is a common problem on tomatoes. Cracks usually appear at the top or stem end of the fruit. Cracks radiate out from the stem (radial cracks) or circle the fruit in concentric rings (concentric cracks). Fruit cracking is associated with wide fluctuations in soil moisture levels. A heavy rain or deep watering after a long, dry period results in rapid water uptake by the plant. The sudden uptake of water results in cracking of ripening fruit. Generally, fruit cracking is most common on the large, beefsteak-type tomatoes.

Fruit cracking can be prevented by supplying the tomato plants with a consistent supply of moisture during the summer months. During dry periods, a thorough soaking once every seven days should be adequate for most tomato plants. Conserve soil moisture by mulching the area around tomato plants with dried grass clippings, straw, shredded leaves or other materials. Also, plant tomato varieties that possess good crack resistance. Tomato varieties that possess good to excellent crack resistance include Jetstar, Mountain Spring and Mountain Fresh.

Several of my tomatoes are misshapen. Why?

Misshapen (catfaced) fruit may be due to cool weather during fruit set. Exposure to 2,4-D or similar broadleaf herbicides is another possibility. Catfacing is most common on large-fruited tomato varieties. Affected fruit show leathery scars, bulges, or holes at the blossom end of the fruit. The incidence of catfacing is typically highest on the early maturing fruit and declines during the remainder of the growing season.

A blackish spot develops on the bottom of my tomato fruit. What is the problem and how can it be prevented?

Blossom end rot is probably responsible for the blackish spots on the tomato fruit. Blossom end rot is a common problem on tomatoes. It appears as a brownish black spot on the blossom end (bottom) of the fruit. Secondary organisms invade the brownish black spot and cause the fruit to rot. Blossom end rot is most common on the earliest maturing fruit that ripen in July and early August.

Blossom end rot is caused by a calcium deficiency in the developing fruit. Wide fluctuations in soil moisture levels impair calcium uptake by the root system of the tomato plant. Excessive nitrogen fertilization may also contribute to blossom end rot.

To reduce blossom end rot, water tomato plants on a weekly basis during dry weather to provide a consistent supply of moisture to the plants (tomato plants require about 1 to 1? inches of water per week during the growing season). Mulch the area around the tomato plants to conserve soil moisture. Avoid over-fertilization. There is no need to apply calcium to the soil as most Iowa soils contain more than adequate levels of calcium.

Pick and discard fruit affected with blossom end rot. The removal of the affected fruit will allow the tomato plant to channel all of its resources into the growth and development of the remaining fruit.

Blossom end rot can also occur on pepper, eggplant, summer squash and watermelon.

My tomatoes are developing white or yellow areas on the sides of the fruit exposed to the sun. Why?

The white or yellow areas on your tomatoes are due to sunscald. Sunscald occurs on fruit exposed to the sun during periods of extreme heat. Initial symptoms of sunscald are the development of shiny white or yellow areas on the fruit. Later, the affected tissue dries out and collapses, forming slightly sunken, wrinkled areas. Secondary organisms invade the affected areas causing the fruit to rot.

Losses due to sunscald can be reduced by growing tomatoes in wire cages. Cage-grown tomato plants provide good foliage protection for the fruit. Also, control Septoria leaf spot and other foliar diseases which defoliate the plants and expose the fruit to direct sunlight.