By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver County
Looking forward to the flavor of a home-grown tomato after months of the tasteless fare offered in grocery stores? Unless you are vigilant, diseases, pests, and environmental problems can rob you of that perfect tomato.
Fusarium and Verticillium are soil-borne fungi that cause vascular wilt diseases in tomatoes. They grow in the water-conducting tissues of the plant. A close look will reveal brown discoloration in the tomato's stem. Unable to receive nourishment through the damaged vessels, leaves turn yellow and begin shriveling from the ground upward and the plant dies (see above photo). There is no treatment, but vascular wilt diseases can be prevented. The incidence of these diseases has increased with the growing popularity of heirloom, non-resistant, varieties. When choosing seeds or plants for your garden, buy resistant varieties (labeled VFN). Rotate crops. Avoid planting tomatoes, potatoes, or peppers in the same location for 3-4 years. Be sure to remove and destroy all diseased plants.
Brown discoloration of stem with fusarium wilt
Tomato spotted wilt virus (TSWV) is an increasing problem in the home garden. The disease begns with dark spots that form on leaves and spread to stems, forming cankers. The affected side of the plant develops bronzing of the leaves. The growing tips die back and the growth of the plant is stunted. The fruit develops yellow spots and rings in a mosaic pattern. There is no treatment for the infected plants and they should be discarded.
TSWV is spread by western flower thrips. This insect is very difficult to control. As weeds are often host plants to thrips, they should be kept to a minimum in the areas surrounding the vegetable garden. When planting your garden, be sure to buy certified virus-free seed and plant varieties that are labeled for TSWV resistance.
Tomato Spotted Wilt Virus
Infection by the tobacco mosaic virus causes crinkling and light to dark mottling of foliage. When cut open, the fruit shows browning inside. This disease is easily spread by direct contact with tobacco products or on the hands, clothing and tools of those who have handled tobacco products. Be sure to wash your hands carefully before working in the garden if you are a smoker. Choose plants that are labeled with a 'T' after the variety. This indicates a higher resistance to the tobacco mosaic virus. Infected plants and debris must be removed and destroyed to prevent spread of the disease to healthy plants.
Tobacco mosaic virus symptoms (middle and right) compared with normal foliage (left)
Septoria leaf spot and early blight occur less commonly in our dry climate. These diseases result when soil is splashed up on the leaves by rain or sprinklers. They can be prevented by mulching and avoidance of overhead watering.
There are a number of insects that can cause tomato problems. Most noticeable are the hornworms. These 3-4 inch caterpillars devour the foliage near the top of the plant. They are easily picked off and disposed of. Bacillus thuringiensis, rotenone or Sevin insecticides can control them when they are small.
Aphids suck the juices from the plant. They can be found on the underside of leaves and identified by the sticky honeydew they produce. Aphids can be effectively controlled with insecticidal soap.
Psyllids, another small insect that sucks plant juices, are more of a problem some years than others. They cause the foliage to turn yellow and curl. Plants are stunted and often produce no fruit. Insecticidal soap is also effective against psyllids.
Flea Beetles chew small holes into the leaves, leaving it looking as though it had been "blasted" with fine shot.
Distortion of the tomato is often the result of an environmental problem.
With blossom end rot, a small water-soaked spot appears near the blossom end of the tomato. As it enlarges, the spot becomes dark brown to black, sunken and leathery. This happens when calcium is not readily available to developing fruit. Calcium imbalance can result from fluctuations in soil moisture caused by improper irrigation or prolonged dry weather. Other causes are high nitrogen levels from fertilizer, or a disruption of the root system. You can prevent blossom end rot by the correct application of nitrogen, and keeping the plants mulched to maintain moisture. Mulching also helps to control weeds and eliminate the need for cultivation that can damage roots.
Blossom End Rot
The sudden exposure of fruits to direct sunlight in hot, dry weather can cause sunscald. This results in white or yellow patches on the side of the tomato exposed to the sun. To avoid sunscald, limit pruning and keep foliage healthy to provide shade and protection for the ripening fruit.
Misshapen or malformed fruit can be caused by cool weather occurring during fruit set or from herbicide exposure.
Catfacing is an abnormality that develops on the blossom end of susceptible tomato varieties. It results from cool weather at blossom time and causes the fruit to pucker and have deep crevices.
Growth cracks occur as a result of the rapid growth stimulated by wet weather following a dry period. Two types of growth cracks affect the stem end of tomatoes: concentric and radial. Concentric cracking produces circular cracks around the stem end of the fruit. Radial cracks spread outward from the stem scar.
Leaf roll, or leaf curl, is a physiologic distortion that may develop with periods of cool, rainy weather. It cause the lower leaves to roll upward and become thick and leathery. Leaf roll does not affect plant growth or fruit production and requires no treatment.
Herbicides can distort the foliage and fruit of tomatoes. They are especially sensitive to 2,4-D. Damage can bend the leaves down, causing cupping and thickening. New leaves are narrow and twisted and do not fully expand. Fruit may be catfaced and fail to ripen. Exposure can occur when herbicides are applied to lawns for weed control and the spray "drifts". Resultant fumes can also effect the plants for several days after treatment. Clippings from grass that has been sprayed with a herbicide should not be used as mulch in the vegetable garden. If the exposure is minimal, the plant will outgrow the injury. Be sure to water the affected plants thoroughly and often.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010