Tomato

Insects of Tomato

Insects, such as white fly and aphids cause physical damage only when they occur in large numbers. However, they may transmit viruses, which can cause much greater damage. These insects can come from outside your field, and may cause entire crop to become infected. Also, leaves damaged by insects become more susceptible to fungal and bacterial diseases.

White fly(Bemisia tabaci)

The adult fly is white in color and 1-2 mm long. It feeds, just like the larvae, on the leaf sap. When plant leaves are turned over, a whole swarm of white fly may fly up. They lay eggs on the underside of the leaves. The eggs hatch after about 1 week. After 2 to 4 weeks the larvae form a cocoon and metamorphosis takes about one week. White flies are especially a problem in the dry season. Once the wet season starts they disappear. Some measures to combat white fly:

  • Encourage the presence of natural predators of white fly, by planting shrubs or other plants between the crop rows (inter-planting) or along pathways between borders.
  • Use resistant cultivars (hairy leaves make it difficult for the white fly to lay its eggs).
  • Spray a solution of kerosene and soap to control white fly.

Aphids (Aphidae)

Aphids are soft, oblong insects about 2.5 mm in length. Direct damage occurs when they attack the crop in large numbers, especially the youngest leaves and stems. In addition to causing direct damage, aphids also transmit several viruses. Measures to control aphids:

  • Remove old crop debris before sowing new crop.
  • Inter-crop with other crops.
  • Use nitrogen fertilizer in moderate amounts.
  • Spray a solution of soap, cow urine or neem extract (Azadirachta indica).

Diseases of Tomato

Tomato plants are susceptible to several fungi, bacteria and viruses. Fungi and bacteria cause foliar (leaf), fruit, stem or root diseases. A virus infection often leads to dwarfed growth and decreased production. Damage caused by diseases can result considerable yield losses for a farmer. Some diseases commonly found in tomatoes are discussed below.

Bacterial Wilt (Ralstonia solanacearum)

This bacterium is especially common in humid tropical lowlands, where temperatures are relatively high. It causes bacterial wilt, which is a soil-borne disease. The first symptoms in infected plants are wilting of terminal leaves, followed in 2-3 days by a sudden and permanent wilt, but there is no yellowing. Adventitious roots may develop on the main stems. The vascular system in the stem of infected plants appears light brown in transverse or longitudinal section; it becomes a darker brown at a late stage of infection. The pith and the cortex near the soil line also become brown when the plant is completely wilted. A white, milky stream of bacteria will ooze from xylem elements when stem sections of infected plants are suspended in water. The bacteria survive in the soil and enter roots of young plants through wounds made by transplanting, cultivation, insects or certain nematodes. The bacteria are spread through irrigation water, soil movement, or moving infected plants (e.g. when transplanting). The following measures will help to control bacterial wilt:

  • Use tolerant/resistant varieties.
  • Avoid infested fields. Once the soil has been infected, do not grow Solanaceae for at least 7 years. Rotate with cereal crops.
  • Do not injure roots or leaves, so be careful during transplantation and prune as little as possible.
  • Make sure the field is well drained.

Fusarium Wilt (Fusarium oxysporum)

From the bottom up, leaves wilt, turn yellow and curl at the edges. A brown stain can be seen if the stem or roots are cut. The plant may wilt on only one side or on a leaf, while the other half or rest of the plant remains healthy for a long time. Pink fungus fluff is found on dead plant parts. Measures that can help control Fusarium wilt.

  • Use resistant or tolerant varieties.
  • Adopt crop rotation.
  • Remove and burn affected plants.
  • Minimize the watering schedule. To prevent the soil drying out apply mulch on the seedbed.
  • Decrease the acidity of the soil by applying calcium or marl.

Verticillium Wilt (Verticillium albo-atrum)

This disease is most common in cooler climates (e.g. highlands). Signs of infection are similar to those of Fusarium, but they appear more slowly. The plants wilt, and leaves become yellow. Many side roots may form at the base of the plant. The fungus spreads through crop debris, especially in slightly acidic soils (low pH). This disease also affects other Solanaceous plants. Measures to control this wilting disease:

  • Use resistant/tolerant varieties.
  • Weed thoroughly.
  • Plough and clear crop remains.
  • Use healthy seed.
  • Rotate with plants other than Solanaceae.
  • Apply calcium or marl in the soil.

Early Blight (Alternaria solani)

This fungus can be found everywhere, and its effect is most serious in humid and hot climates. It is spread via seed, wind, rain and infected plant remains. Plants that have been damaged are more susceptible to this fungus. Round, brown spots (with concentric rings) appear on the leaves, reaching a diameter of 1.5 cm. Sometimes small lumps can be found on the stem or on leaves, causing leaves to turn yellow and wilt. Flowers and small fruit fall off. Major control measures:

  • Use tolerant varieties.
  • Remove and burn damaged plant parts.
  • Weed regularly and thoroughly.
  • Use pathogen-free seeds.
  • Adopt crop rotation.
  • Do not plant young plants near older plants.
  • Apply effective fungicides

Late Blight (Phytophthora infestans)

This fungus can be found in all regions of the world, but is more common in highlands or in cool humid conditions in lowlands. The fungus is usually spread via crop remains. Dark, watery marks with a yellow spot on the inside are visible on the leaves. Sometimes the marks start at the edge of the leaf and spread inward, sometimes the spots spread from the centre of the leaf outward. On the underside of the leaves, the spots are white. The stems and fruit can be affected also. Fruit gets brown spots and the leaves wilt. The signs of late blight become visible early in the growing season. Measures that can prevent late blight:

  • Use tolerant varieties.
  • Weed regularly and thoroughly.
  • Remove and burn affected plants and plant debris.
  • Do not plant young plants near older plants.
  • Apply mulch on seedbeds, so that less watering is needed.
  • Avoid planting tomato near potato crops.
  • Increase aeration by staking and removing affected leaves.

Virus of Tomato

Cucumber Mosaic Virus

CMV causes stunting in tomato plants. Leaves may show a mild green mottling or more shoestring symptoms in which the leaf blades are greatly reduced. Fruits are small in size and often misshapen. CMV is transmitted by different aphid species. Aphids usually introduce the virus into a tomato crop from weeds or neighboring crops. Control of the vector is important to prevent CMV epidemics:

  • Grow resistant varieties.
  • As CMV has a broad host range, it is important to eliminate weeds and ornamental plants that harbor the virus.
  • Remove and destroy infected individual plants as this helps to limit the virus spread within the field.

Tomato Yellow Leaf Curl Virus

TYLCV occurs worldwide. Infected plants are erect and stunted. Leaves are yellow and curl upward or downward. An entire yield can be destroyed if plants are infected in the nursery. Whitefly transmits TYLCV.

Common Control Measures

  • Use tolerant varieties.
  • Use reflective plastic mulch.
  • Protect seedlings with a net in the nursery.
  • Control the insect vector.

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