Tomatoe Diseases

.. se, only the leaves near the tips of the branch remain alive. When the stem is cut lengthwise, the base shows a discoloration of the woody tissue similar to Fusarium, but is usually darker, and generally it occurs only in the lower part of the stem. The fungus enters the plant through the feeder roots and grows into the stem in the woody conducting vessels just under the cortex. The fungus lives in the soil for a long time and it is exclusively the source of infection.

Progress of the disease is favored by cooler temperatures and is retarded by the high temperatures that are favorable to Fusarium wilt. Locating seedbeds and fields in Verticillium-free soil, and using resistant varieties are the most effective means of controlling the disease. Gray Mold Causal Agent(s): (fungal – Botrytis cinerea) Plants become more susceptible to this disease as they become older. It is mostly a problem in greenhouses, but it can also affect tomatoes in the field. The fungus first becomes established on dead leaves at the base of the plants. A heavy, gray growth of the fungus covers these, and numerous spores are soon found, giving the affected area a cottony appearance.

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Affected leaves collapse and shrink. The fungus progresses into the stem, producing cankers. Affected fruits first show a watersoaked, soft area in the points of infection. The dark gray growth of the fungus soon is seen on these spots. Regular fungicide applications should help in controlling this disease.

Botryosporium Mold Causal Agent(s): (fungal – Botryosporium sp.) This fungus can often be found on greenhouse tomatoes. It superficially resembles gray mold. Septoria Leaf Spot Causal Agent(s): (fungal – Septoria lycopersici) Infection usually occurs on the lower leaves near the ground, after plants begin to set fruit. At first, small watersoaked spots are observed, which under ideal conditions will become numerous. Large areas of the leaves may be affected but the individual spots can be recognized.

The watersoaked spots become roughly circular, with dark margins surrounding a light gray center. With time, black specks which are spore producing bodies can be seen in the center of the spots. If the spots are numerous, the lower leaves will turn yellow, die and progressively drop from the plant until only a few leaves remain on the top of the plant. The fungus is most active when temperatures range from 60 to 80 degrees F., and humidity conditions are high. The disease is usually not serious during periods of hot, dry weather.

The fungus can overwinter on crop residue from previous crops, decaying vegetation and some tomato-related wild hosts. Crop rotation, plowing under crop residues, and clean cultivation will reduce the amount of inoculum in tomato fields. Repeated fungicide applications will keep the disease in check. Southern Blight Causal Agent(s): (fungal – Sclerotium rolfsii) The first symptom is dropping of leaves suggestive of other wilts. Wilting progresses and plants die quickly. Stems show decay of outer tissues at the ground line. Frequently, they are covered by a white fungal mat in which are embedded numerous small, light-brown bodies about the size of cabbage seed. The fungus can also attack fruits where they touch the soil. The fungus does not grow at temperatures below 68 oF.; it requires abundant moisture for growth. Infection takes place below the soil line or close to ground level.

Control is obtained by sanitation, crop rotation, and by treating infected soil with a soil fungicide prior to planting. Seedling Disease Causal Agent(s): (fungal – Rhizoctonia sp., Pythium sp.) Seedlings fail to emerge or small seedlings wilt and die soon after emergence. Surviving plants may have infected root systems and watersoaked areas on the stem close to the soil line. As the plants mature, they become more resistant to damping-off. Avoid excessive moisture in the seedbed, plant seed treated with fungicides and use sterilized media for growing transplants.

Tobacco Mosaic Causal Agent(s): (viral – TMV) Symptoms are light and dark green mottling of the tomato foliage, and curling and slight malformation of the leaflets. Plants may be somewhat stunted if infected when small, but the plants and fruit are not much reduced in size if plants are not infected until they reach the fruiting stage. Several strains of the virus are known that can cause different symptoms. The virus is highly infectious and readily transmitted by any means that introduces even a minute amount of sap from infected to healthy plants. The most common means of transmission is by handling contaminated plants. The virus may also be present in certain types of tobacco; therefore, smokers may transmit the disease. Control measures are: avoiding handling plants more than necessary, washing hands before handling plants, and protecting healthy plants from infection.

Double Streak Virus Causal Agent(s): (viral – viral) Caused by a combination of viruses. Leaves show a light-green mottling, accompanied by the development of numerous small, grayish-brown, dead spots which have a thick, paper appearance. Numerous narrow, dark brown streaks develop on the stem and leaf petiole. Fruits are often rough and misshapen and on the surface they have small, irregular, greasy, brown patches which render them unfit to market. The virus is transmitted by workers handling the crop. Avoid infection by tobacco mosaic virus, wash hands before starting to work and remove infected Spotted Wilt Causal Agent(s): (viral – viral) This disease is similar to streak in that it causes streaking of the leaves, stems and fruits. Numerous small, dark, circular spots appear on younger leaves. Leaves may have a bronzed appearance and later turn dark brown and wither. Fruits show numerous spots about one-half inch in diameter with concentric, circular markings.

On ripe fruit these markings are alternate bands of red and yellow. The virus also affects other vegetables and many wild hosts and ornamental plants. Thrips can transmit the disease from the wild hosts. For control, eliminate weeds around field edges and turn rows; remove infected plants when small, and control insects in the field. Curly Top Causal Agent(s): (viral – viral) Pronounced upward rolling and twisting of the leaflets that expose their under surfaces, stiff and leathery foliage, and a peculiar dull yellowing of the entire plant are typical symptoms of the disease. There is also some purpling of the veins and the plant is usually very stunted.

Very few fruits are produced after infection. The virus is not transmitted through the seed or soil, nor is it spread by mechanical means. The main vector is the beet leafhopper that becomes infected by feeding on wild or cultivated plants having the disease. The disease is difficult to control. Keep field surroundings free of weeds.

Controlling insects may effect some control. Science Essays.


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