TOMATO PLANT DISEASES

Common Diseases

Considerable progress has been made in breeding disease resistance or tolerance to the more commonly occurring tomato plant viruses and diseases, such as the verticillium and fusarium wilts, tobacco mosaic virus, altenaria stem canker, stemphylium gray spot, septoria leaf spot, and bacterial speck.

Most cultivars are not resistant to anthracnose, bacterial cancer, bacterial spot, late blight, and Southern stem blight, while many tomato cultivars are resistant to alternaria stem cancer, bacterial speck, bacterial wilt, early blight, fusarium wilt, gray leaf spot, tobacco mosaic, and verticillium wilt.

Gray mold (Botrytis cinerea) is a commonly occurring disease in warm and wet environments that primarily affects older leaf tissue. Best control is to keep the foliage dry by maintaining movement of dry air within the plant canopy.

Seed Borne Diseases

Those seed borne diseases that can be controlled by heat treating the seed are bacterial canker, bacterial spot, bacterial speck, and anthracnose.

Phytium aphanidermatum is a serious root disease that occurs in warm wet conditions, frequently in those hydroponic growing systems in which the nutrient solution is recirculated. When occurring, the disease can quickly kill plants, and its control requires the dismantling of the entire hydroponic growing system for complete sterilization. In soilless medium systems, pinebark as an ingredient in the soilless mix, offers some control of this root disease.

Disease control requires constant plant observation and evaluation, and the use of control measures to prevent the introduction of disease organisms from outside sources.

Soil Borne Diseases

A soil borne disease that can be severe in warm wet weather conditions is Rhizotonia which is best controlled by soil sterilization, and by keeping soil from making contact with plant foliage.

Sterilization is required when tomato plants are grown in soil to control such diseases as bacterial wilt, Southern blight, fusarium wilt, and fusarium crown rot. Field soil sterilization with methyl bromide has been the commonly used procedure. For greenhouse soils, steam sterilization is the common method. Since use of methyl bromide is being phased out, alternative methods of chemical sterilization are being sought

Although most diseases produce characteristic visual symptoms, proper identification by a skilled plant pathologist before a corrective chemical treatment is made is essential. Many colleges of agriculture or their agricultural extension services within the land-grant university system in the United States and some soil and plant analysis laboratories offer pathological services plus pest chemical recommendations. In addition, there are crop consultants in most of the major crop-producing areas of the United States who can field identify plant diseases. Most are usually familiar with current pest control regulations, and some crop consultants are also licensed pest chemical applicators.

TOMATO INSECTS/PESTS

Common Insect Pests

Most tomato cultivars are not resistant to the following insects: aphids, cabbage looper, Colorado potato beetle, fall army worm, corn earworm, leaf miners, thrips, and tomato pinworm. Other insects that will attack the tomato plant are Colorado corn earworms, flea beetlcs, fruit flies, hornworms, spider, stick bugs, and whiteflies, insects that are becoming increasingly difficult to control, particularly in the field and in those areas of the United States where tomato crops have been produced for a long period of time. In addition, insects can carry diseases, such as various gemini viruses that are just now being specifically identified.

Another insect that is become increasingly difficult to control is the thrip, a very small insect that can damage the tomato plant as well as carry virus diseases. This insect, as well as others, should be prevented from entering the greenhouse by screening ventilation openings.

Insect Population Monitoring

The insect population in the greenhouse is best monitored by placing yellow- or blue-colored sticky boards at intervals within the plant canopy, and then daily examining these boards, noting the numbers and species of insects found on the board. As insect numbers accumulate, procedures for insect population control can be instituted by using chemical procedures or by introducing predator insects into the greenhouse. The use of these sticky boards, however, is not a means for controlling insect populations.

Insect Control Using Predator Insects

The use of predator insects to control various insect pests has a fairly long history of development and use. Some of the plant-damaging insects that can be controlled by predators are aphids, mealy bug, scales, spider mites, thrips, and whiteflies. Successful predator insect control is based on careful monitoring and introduction of predators before the target insects become out of control. Air temperature and humidity are important factors that can influence both the target insect and its predator, and therefore must be maintained at optimum levels. As with any pest problem, a combination of control factors becomes essential for success in keeping insect populations from reaching damaging levels. Supplies of predator insects and instructions in their use are available from a number of sources.

Biopesticides

A new line of pesticide chemicals is being developed containing naturally occurring fungus organisms that can invade the insect’s body as a means of controlling insect pests in the tomato greenhouse. Some products are quite new and use experience by growers has not been well documented.

Insecticidal Soap

Insecticidal soap is another control chemical that will kill aphids, mealy bugs, whiteflies, and mites. The plant must be completely saturated with the insecticidal soap in order to be effective, and repeated applications may be needed to control an insect infestation. Another form of an insecticidal soap is neem oil insecticidal soap, a new product, with possible control of a wide range of insects commonly occurring in the greenhouse. Neem-containing materials have had a long history of use in various developing countries around the world. Neem oil is extracted from the fruit of the neem (Azadiruchta indica) tree.