Tomato

Fruit borers such as the African bollworm (Helicoverpa armigera) and leaf-eating caterpillars such as the cotton leafworm (Spodoptera littoralis) attack the developing and mature fruit of tomato. The African bollworm, also known as the tomato fruit worm, is one of the most destructive insect pests of tomato, causing yield losses as high as 70% due to fruit boring. They usually bore into the fruit from the stem end, and feed on the inner parts of the fruits, causing extensive fruit damage and promoting decay caused by secondary infections.

They prefer green fruit. Caterpillar of the cotton leaf worm feeds on the leaves of tomato and bores into the fruit, especially those growing down the plant near to the soil. They are olive-green, dark grey or brown in colour with large, black, triangular spots on their back. The fully-grown caterpillars are 3 to 5 cm long. The eggs are laid in batches in one or more layers, usually on the underside of the leaves and covered with hairs. Both caterpillars have many natural enemies such as predatory ants, spiders, damsel and robber flies, and parasitic wasps and flies

Spots of fruits are sunken. When weather is moist, the spots get tan-coloured (growth of fungal spores). There may be several spots on affected fruit and they may join up and cover the whole fruit. Severely affected fruits drop. Eventually, affected fruits dry and become black. Most often, soft-rot bacteria invade affected fruits and cause a soft watery decay.

The fungus usually attacks fruits on plants that are weakened or over-ripe. Infection is favoured by relative humidity close to 100% and temperatures between 21 and 30⁰C.

Aphids such as cotton aphid (Aphis gossypii) and the green peach aphid (Myzus persicae) suck plant sap, which can reduce plant growth; they also secrete honeydew, on which sooty moulds growth. Sooty mould on fruits reduces their market value. These aphids may also transmit virus diseases during feeding such as the cucumber mosaic virus. In Kenya, aphids are occasionally found on tomatoes, but they are not considered an important pest.

The first symptoms are yellowing, curling and wilting of the leaflets, often on one side of the leaf. The affected leaflets turn brown and dry but remain attached to the plant. The whole plant may also show a one-sided disease development. Long brown strips may appear on stems and shoots. These strips dry and crack open to form cankers from which the disease gets its name. When an affected stem is cut lengthwise, there is a creamy white, yellow or reddish-brown line inside the woody tissue. The pith is easily separated from the wood along this line.

The vascular bundles within the pith are destroyed and cavities are formed in the pith. Young affected fruits are small, deformed and seeds are aborted. Also spots may develop on older fruits. These spots are circular, up to three mm wide, with slightly raised brown centre surrounded by a pronounced white halo resembling a 'bird's eye". The halo is flat. The bacterium is seed-borne and can survive up to three years in soil in crop debris. Spread within a crop is by rain splash, sprinklers and pruning knives.

It is another serious disease, occurring during the rainy season. It is most noticeable on fruits, but also causes damage to the foliage and stems. On green fruits, the initial spot is very small and water soaked. It eventually enlarges to about 6 mm. As the bacterial spot matures, it becomes brown and scabby without extending deep into the fruit. On foliage, irregular greasy, dark green spots, 2 to 3 mm wide, are observed. The spots eventually dry and the tissue often tears. The disease is transmitted through the seed.

Bacterial wilt has often been reported as the most serious handicap for tomato in the tropics. The disease causes rapid wilting and death of the entire plant without any yellowing or spotting of leaves. Total collapse of the plant usually occurring when temperatures reach 32⁰C and above. Plant wilts while still green. When the stem of a wilting plant is cut across, the pith has a darkened water-soaked appearance, and on squeezing the cut stem, a white, yellow or greyish, slimy exudate may appear. In later stages of the disease, decay of the pith may cause extensive hollowing of the stem.

It is caused by calcium deficiency and irregular watering. The disease occurs at the blossom-end of the fruit. It starts as a water-soaked spot that enlarges to become dark brown and sunken. The surface of the spot becomes dark and leathery. The rot is dry.

In addition to the cotton leaf worm (see fruit borers), other caterpillars such as the cabbage looper (Trichoplusia ni) feed on tomato leaves. However, generally they are not of economic importance.

The cabbage looper is a green caterpillar with faint white stripes along the body. Caterpillars are about 3.5 to 4 cm long when fully-grown. As they move, they arch their back in a looping fashion, hence the common name looper. Larvae (caterpillars) chew holes in the leaves, and larger caterpillars consume great amounts of plant material.

The adult is a moth, about 2.5 cm in length and mottled, greyish-brown.

Cutworms are the caterpillars of certain moths. They are serious pests of tomato seedlings. They cut stems of newly transplanted or emerged plants at the base. Cutworm damage is more critical after thinning or transplanting.

Damping-off can occur when seedlings die before they have pushed through the soil, resulting in patches that appear to have germinated poorly. Alternatively, seedlings may emerge but fall over and die some time afterwards. The base of stem of affected seedlings become wet and brown. These disease-causing fungi are common in moist soils. They can also be transmitted on seeds.

Leaf spots of early blight are circular, up to 12 mm in diameter, brown, and often show a circular pattern, which distinguishes this disease from other leaf spots on tomato.

The lower leaves of the plant usually turn yellow and die. One or more branches may exhibit such symptoms. Leaflets on one side may be affected while those on the other side are symptomless. Diseased leaves readily break away from the stem. When affected stems just above ground level and petioles are cut diagonally, a reddish-brown discolouration of the water conducting tissues will be observed. The fungus is both seed- and soil-borne. It causes most damage on light, sandy soils. It is most active at temperatures between 25 and 320 C. The fungus can survive in the soil indefinitely even when no tomatoes are grown. It can also survive in fibrous roots of weeds (e.g. Amaranthus, Digitaria and Malva species). Acidic soils (pH 5.0 to 5.6) and excessive nitrogen fertilisation promote disease development. Infestation by root-knot nematodes encourages the disease.

Symptoms of late blight are irregular, greenish-black, water soaked patches, which appear on the leaves. The spots soon turn brown and many of the affected leaves wither, yet frequently remain attached to the stem. Infection of the fruit occurs at any stage of growth. It is most common on the upper half of the fruit. The spots are greasy brown and have a firm corrugated (rough) surface that occasionally shows narrow zonate markings.

Symptoms of leaf mould are pale-green or yellowish areas with undefined margins that appear on the upper leaf surface. Under humid conditions the lower leaf surfaces of the spots become covered by an olive-green to greyish-purple velvety growth of the fungus.

Leafmining flies make tiny punctures on the side of tomato leaves when feeding and laying eggs. These punctures may serve as entry point for disease causing organisms such as bacteria and fungi.

The most destructive stages are the immature stages (maggots). Maggots measure up to 3 mm in length. The maggots feed on leaf tissue inside the leaf leaving a wandering track in the form of irregular mines. Heavy mining of leaves may reduce photosynthesis affecting development of flowers and fruits. Heavy attack may cause leaf drop. This is particularly serious for tomato seedlings which may die due to defoliation. Defoliation of tomato plants may also expose fruits to sunburn and affect the market value. Leafminers attack a wide range of cultivated vegetables.

The spots of powdery mildew appear on older leaves and are light chlorotic to bright yellow. These spots later run together and become necrotic. Whole leaf blades may collapse and dry up. On the upper leaf surface of green leaves, a fine talcum-like powder is observed. This is fungal growth.

Root-knot nematodes are very destructive to tomatoes. Damage is most serious on light sandy soils in furrow-irrigated crops. The nematodes cause small lumps known as root knots or galls on the roots. Heavily infected roots are severely distorted and swollen. In time the roots rot. The affected plants are either stunted or have tendency to wilt or even die in hot weather. The affected plants, in most cases, just wilt with the foliage still green - without yellowing. The nematode problem is primarily due to improper crop rotation.

Spider mites are important pests of tomatoes. They are more prevalent in dry areas. Infested leaves show a white to yellow speckling and then turn pale or bronzed. High spider mite infestation causes defoliation, which leads to production of smaller fruits with lower content of ascorbic acid (vitamin C). Spider mite feeding on fruits causes speckling of the fruits.

The tobacco spider mite Tetranychus evansi, is one of the most damaging spider mites on tomato. This spider mite can kill plants very rapidly under hot and dry conditions. Plants with high numbers of this mite can be covered with webbing and an orange cloud of mites.

Thrips may also be a problem in tomatoes in Kenya. Thrips are small (about 1 to 2 mm long). They usually feed on the lower surface of leaves puncturing them and suck the exuding sap. They also attack buds, flowers and fruits. Attacked leaves have a silvery sheen and show small black spots (thrips excrements). Under heavy infestation attacked buds, and flowers usually fall off. Attacked fruits show speckling and small necrotic patches on the surface affecting fruit quality. Fruits may become deformed.

Thrips feed on tomatoes at all stages, but their feeding on seedlings is particularly damaging. Heavy infestation can reduce stands of young seedlings in hot weather. Thrips of the genus Thrips and Frankliniella are vectors of viruses such as the Tomato Spotted Wilt Virus (the most economically important virus in tomato production) and the Tomato Chlorotic Spot Virus.

It is slender, greenish in colour with a dark brown tint on the forewings. Eggs are laid singly on the growing points, petioles and stems of the tomato plant. The young bugs (nymphs) resemble the adults, but initially have no wings. Older nymphs have wing pads. The tomato bug feed on stems, leaves and flowers of tomatoes, but prefer young leaves and growing points. Sucking by nymphs and adults results in the formation of brownish rings on stems, petioles, growing points and leaf veins, which become brittle. Repeated feeding by the bugs cause crinkling (rolling, puckering and unevenness) of leaves.

Attacked stem bases become swollen with narrow brownish rings at the apices of the swollen areas. Shedding of flowers may be partly due to feeding by this bug. Other major crops attacked include eggplant, sesame and bottle gourds. The tomato bug also exhibits predatory habits, and has been reported preying on other insect pests, such as aphids, jassids (leafhoppers), caterpillars and whiteflies. It has been identified as an important predator of eggs and young caterpillars of the African bollworm and a potential control agent of whiteflies in Europe. This bug was found frequently and in large numbers on tomatoes, throughout NE and NW Somalia (Seif et al. 2003). It has also been reported as a tomato pest in Ghana (Youdeowei, 2002). In Kenya, large numbers of this bug have been observed in tomato fields under heavy and frequent spraying of synthetic pyrethroids.

Affected plants show light and dark green mottling and distortion of the youngest leaves. The latter may be stunted or elongated resembling fern leaves. Under high temperature and light intensity, mottling can be severe. Under low temperature and low light intensity, stunting and leaf distortion are severe. Internal browning of the fruit sometimes occurs; this symptom is most common when fruits become infected at mature green or pink stage.

The disease can be seed-borne but can also survive on plant debris in the soil and so re-infect newly planted crops. The virus is easily mechanically transmissible by contact between plants or through human activities (transplanting seedlings or pruning).

They are very tiny, approximately 0.2 mm long, and cannot be seen with the naked eye. They are yellowish, brown or pink. They have a ringed conical body with the head and two pairs of legs at the large end. They complete the lifecycle in 1 week; this explains the rapid increase of this mite in tomato fields. They prefer high temperatures and low humidity. They feed on all above-ground parts of the tomato plant, causing spotting, twisting or folding of leaves and fine cracks on the fruits. Attacked leaves and stems develop a greasy appearance and turn bronzed. The plants can drop their leaves, especially in hot weather. Fruits are then exposed to sunburn. Damage to the plant typically begins near the ground and spreads upwards. Damage can develop very rapidly, and the mites can kill plants in a few days in dry hot weather. Since the mite cannot be seen, the symptoms are easily confused with diseases. The small size of the mites makes monitoring difficult. The first signs of mite presence are the curling and bronzing of the lower leaves of the stem.

The virus is transmitted by thrips (Thrips tabaci and Frankliniella spp.). The virus has a wide host range including many ornamental plants, vegetable crops and weeds. Diseased leaves have purplish-brown spots. Diseased leaves later turn brown, die and droop. Brown streaks occur on leaf petioles and stems. Infected plant becomes dwarfed, and with its drooping leaves, it resembles a plant affected by wilt. Symptoms on fruits consist of circular zones with shades of yellow or brown alternating with green and later with pink or red. Fruit symptoms are very characteristic of the disease.

The virus is transmitted by Whiteflies (Bemisia tabaci). It is not seed-borne. Tomato plants infected early in the season are normally stunted and excessively branched. Affected leaves are chlorotic and are curled upward. Flower drop is common, and therefore, infected plants have a reduced number of flowers and fruits.

Tuta absoluta is a new insect pest of tomato plant in sub Saharan Africa. The moth pest originated from Peru and has spread to Europe, Africa and Asia. In Africa, T. absoluta was detected in Tunisia (2009), Ethiopia (2012), Senegal (2013), Sudan (2010) , Tanzania and Kenya (2014) and in Uganda (2015). Although tomato plant is the main host for T. absoluta, the pest also attacks other plants in the Solanacea family including potato, nightshade, eggplant, pepino, tobacco and solanaceous weeds such as Datura.

The life cycle of T. absoluta is completed within 30-35 days. T. absoluta can have 10-12 generations per year. Females can lay up  250-300 eggs on host plants. Eggs are small, cylindrical, creamy-white to yellow and approximately 0.35 mm long. Eggs hatch in 4-6 days after egg laying. The larva is cream-colored with a characteristic dark head. T. absoluta has four larval instars. Pupation may take place in the soil, on the leaf surface or within mines. The adults are small moths with 5-7 mm body length. The adults have thread-like antennae and forewings with grey scales and black spots.

The larvae feed between the leaf layers causing irregular mines. They eventually tunnel into the stems or burrow into the fruits, leaving black frass as it tunnels.

T. absoluta is a very challenging pest to control because of its fast reproductive rate (10 -12 generations per year), short life cycle, nocturnal adults, and concealed larval stage inside the plant parts. It can cause up to 100% loss of tomato fruits if left uncontrolled.

Soil solarisation can provide control of many soil-borne diseases, nematodes, and weeds. Preventing weeds from producing seed helps reduce weed populations in subsequent crops; this also applies to areas adjacent to the cropped fields. A soil cap (5 to 10 cm mound of soil) over the seedline at planting can reduce the first flush of weeds competing with the crop seedlings. The cap is removed just after tomato seedlings germinate, but before rapid elongation of the hypocotyls. This mound also reduces soil moisture loss and emergence problems that result from soil crusting after periods of rainfall, and is formed by disks or other implements.

When tomato seedlings are about 10 cm tall, cultivation can create a dry layer of soil (dry mulch) on the seedline to prevent weed seeds from germinating and to smother small weeds that have already emerged. Flaming is also used for weed control in large-scale commercial tomato production.

Tobacco whitefly is a serious pest of tomatoes. Whiteflies attack tomatoes from seedling to mature stage. They suck sap from the leaves, and may weaken the plants. Feeding of whiteflies cause yellowing of infested leaves. Immature stages (nymphs) produce honeydew on which sooty mould grows. Heavy honeydew or mould coating reduces plant growth and fruit quality.

The main damage caused by whiteflies is indirect as vectors of virus diseases. Bemisia tabaci transmits several virus diseases in tomato. In particular it is an efficient vector of the Tomato Yellow Leaf Curl Virus (TYLCV), a major disease in Kenya.

Small numbers of whiteflies do not cause major direct plant damage. However, even small numbers of whiteflies may need to be controlled in areas where TYLCV is common.