Yard and Garden: Tomato Pest and Disease

July 12, 2018, 10:20 am | Richard Jauron, Willy Klein

ripe tomatoes.AMES, Iowa -- Tomatoes are the most popular vegetable crop in Iowa - popular with gardeners, green caterpillars and growing season plant diseases. Iowa State University Extension and Outreach horticulturists describe the symptoms and management of common problems found in home gardens. 

There are large, green caterpillars on my tomato plants. What should I do?

The large, green caterpillars are probably tomato hornworms. Tomato hornworms are bright green, up to 4 to 5 inches long, and have red or black, horn-like projections on their rear ends. After feeding, hornworms move to the soil where they pupate and spend the winter. The following summer the pupae transform into five-spotted hawk moths and repeat the cycle.  

Tomato hornworms feed on the leaves and fruit of tomatoes and other vegetables including eggplant, potatoes and peppers. They can quickly defoliate portions of the plant and damage the fruit.

Often the best control option for home gardeners is to pick off the caterpillars by hand and destroy them. Finding them can be challenging, especially when they’re small, as they are well camouflaged. Another control option is to use a biological insecticide, such as Bacillus thuringiensis (Bt) or a synthetic home garden insecticide.  

Brownish black spots have developed on the bottoms of some of my tomato fruit. What is the problem?  

Blossom end rot is probably responsible for the brownish black 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 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 need 1 inch 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 garden soils contain 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 also can occur on pepper, eggplant, summer squash and watermelon.


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