Issue: August 2005

What ’s Killing My Tomatoes?

The popularity of homegrown tomatoes is probably only exceeded by the pests and other problems that plague tomato plants.

Insects and diseases, stress from insufficient water, nutrient imbalances and heat can all affect plant growth.

Curly top virus is a common problem in August. There is no cure for the disease, which is spread by an insect called the beet leafhopper. Curly top stunts plant growth. It turns leaves yellow-green and leathery and makes them curl upward. Veins in the leaves often turn purple. Infected plants usually feel warm to the touch compared with healthy plants. Pull up and discard all infected plants.

Verticillium and fusarium wilt are other common diseases. They are soilborne fungal diseases that cause older plant leaves to turn yellow and die. When the stem of the plant is cut near the soil line, the tissue just below the bark will look tan to dark brown in color. Pull up wilted plants and discard them in the trash. To avoid wilt, plant resistant or tolerant varieties. Look for the letters “V” and “F” with the variety name.

Stunted yellow plants can also be a sign of nematodes, particularly in sandy soils. Nematodes, microscopic worms that live in the soil, feed on roots and eventually cause plants to wilt. Look for varieties with the letter “N” for nematode resistance or tolerance.

Tomato hornworm is often the biggest tomato pest but the hardest to see. The five-inch, green worm is relatively plump and blends well with foliage. It has white diagonal stripes and a horn on its posterior. Look for worms near shredded foliage just above the black droppings they leave on the soil.

Round holes found in fruit are generally caused by tomato fruitworms. They’re also known as corn earworms because they chew around the tips of sweet corn. Striped worms can be brown, green or yellow. They are relatively small, growing up to two inches long. They can also be found chewing on foliage.

August weather can cause several problems for tomatoes. Fruit often cracks because dry, hot weather alternates with rainfall. Lack of water can also cause blossom-end rot, especially if soil is low in calcium. A water-soaked spot appears on the bottom of fruit, eventually turning brown to black with a leathery texture.

Intense sun and poor foliage cover can cause sunscald on tomato fruit. Fruit shoulders will blister, forming a gray, paper-like spot. Training vines to a cage will create more shade for fruit and keep it off the ground, resulting in less fruit rot.

Hot weather, particularly at night, can also result in poor fruit set. Excessive nitrogen in fertilizer can cause the same problem. Tomatoes require fertilizer with a good balance of nitrogen, phosphorus and potassium.

For more gardening information, visit New Mexico State University's Cooperative Extension Service publications world wide web site at http://www.cahe.nmsu.edu/pubs/_h.

George W. Dickerson, Ph.D., is is a horticulturist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.