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.
What to do: Keep tomato fields free of weeds. Do not grow brinjals (eggplant) or tomatoes in succession.
Powdery mildew on Beans (Erysiphe polygoni)
French bean pods with powdery mildew. (c) GTZ – IPM Horticulture Project. Kenya.
Powdery mildew on French beans (c) GTZ – IPM Horticulture Project. Kenya.
A white powdery mould appears on the upper leaf surfaces. Severely diseased leaves turn yellow and die. Leaf petioles, stems and pods can also be affected.What to do:
Plough under bean debris after harvest.
Practise a 2-3 year crop rotation without legumes.
Damage of bacterial wilt on tomato. (c) David B. Langston, University of Georgia, Bugwood.org
A row of tomatoes with individual plants infected with bacterial wilt (Ralstonia solanacearum). Note that the wilt often starts next to a trellising pole because the pathogen can overwinter on the pole. Tomatoes in South Africa are typically cultivated with wire trellising to hold up the plants. (c) Courtesy of EcoPort, www.ecoport.org
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.
What to do: Use resistant varieties (e.g. “Fortune Maker”, “Kentom” and “Taiwan F1”) If only a few wilted plants are found, immediately remove them from the field. Practise long-term crop rotation. A method called “bio-fumigation” is under development from the Australian Center for International Agricultural Research for the reduction and long-term elimination of bacterial diseases from the soil by incorporating especially mustard or radish plants in large amounts into the soil immediately before planting tomatoes or potatoes. This practice is reported to reduce incidence of bacterial wilt by 50-70% in the Philippines (ACIAR 2005/6).
Anthracnose(Colletotrichum coccodes) on tomato (c) Clemson University – USDA Cooperative Extension Slide Series, www.insectimages.org(www.ecoport.org)
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.
What to do: Use resistant varieties, if available. Use certified disease-free seeds. Practise crop rotation. Destroy infected crop residues.
Damage of Bacterial spot (Xanthomonas campestris pv. vesicatoria) on tomato leaf. (c) Clemson University, www.insectimages.org
Bacterial spot (Xanthomonas vesicatoria) on tomato fruit: Typical sunken, black spots on the fruit and malformation due to the bacterial infection. (c) Courtesy EcoPort (http://www.ecoport.org): Plant Protection Service Archives, Wageningen, NL
Bacterial spot (Xanthomonas vesicatoria) on tomato. Older spots are blackish and slighly raised, central spots disintegrate and sink (c) A.M. Varela, A.A. Seif, icipe
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.
What to do: Use certified disease-free seed. Remove and destroy crop residues after harvest. Avoid working in the fields when plants are wet. Spray with copper fungicides; copper can control this disease fairly well except under heavy infection. For more on copper fungicide click here Grow resistant cultivars; this is the best control method for both bacterial diseases, but resistance is not universal owing to variable strains of the pathogens.
Tomato leaf mold symptoms (c) Elizabeth Bush Virginia Polytechnic Institute and State University, 2013
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.
What to do: Prune and stake to reduce humidity. Avoid excessive shading by providing adequate plant and row spacing. Avoid wetting leaves when watering.
Cabbage looper on cabbage. The first instar is white and almost clear with a black head capsule. Later instars are green with a thin white line on each side. Mature larvae reach 3 to 4 cm in length. (c) A.M. Varela, icipe
Cabbage looper (adult moth). The adult is ca 2.5 cm in length and mottled, grayish-brown. The wingspan is ca. 4 cm. (c) Joseph Berger. Courtesy of Ecoport
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.
What to do: Conserve natural enemies. These caterpillars are attacked by a large numbers of natural enemies, including parasitic wasps and flies. Birds and bats feed on the adults (moths). Pick caterpillars by hand and destroy them. When control is necessary use biopesticides such as neem-based products or Bt. Neem products control cabbage looper by interfering with the growth of the young caterpillar. Bt and neem should be applied when caterpillars (larvae) are still in the early growth stages.
Damage on tomato fruit and leaves caused by the Tomato russet mite, Aculops lycopersici (c) Courtesy EcoPort (http://www.ecoport.org): L. Steynberg
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.
What to do: Conserve natural enemies. Few natural enemies of the tomato russet mite are known. Predatory mites are considered to be the most important natural enemies. However, the effect of these natural enemies is hampered by extensive use of pesticides.
Spray neem extracts. Neem oil and aqueous neem kernel have been reported to give good control of this mite in Costa Rica. For more information on Neem extracts click here. Ensure proper irrigation during early stages of the crop. This can help prevent mite build-up later in the growing season since tomato russet mite infestation is higher on tomato plants under water stress. No resistant varieties are available. In Kenya, tolerance to leaf damage was observed in two varieties, namely “Early Stone” and “Beauty”.
Tomato bug adult (Nesidiocoris tenuis) on tomato plant, they are about 4mm long (c) A. M. Varela, icipe
Tomato bug adult (Nesidiocoris tenuis) on tomato plant, they are about 4mm long (c) A. M. Varela, icipe
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.
What to do: Inspect crops for nymphs and adults feeding on buds, developing flowers and leaves, and for signs of bug feeding, including discoloured spots, necrosis, stunting, and early fall of flowers and fruits. Avoid volunteer crops. Assure proper disposal of crop residues. Avoid indiscriminate use of broad-spectrum pesticides
Tomato fruit damaged by spider mites (c) A. M. Varela, icipe.
Spider mites on tomato. Note the mites and their webbing visible beetween the leaves. (c) Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org
Severe spider mite damage to tomato plants. (c) A.M. Varela, icipe
Initial infestation of spider mites, note mites on underside of leaflet. (c) A. M. Varela, icipe
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.
What to do: Conserve natural enemies. Natural occurring predators are in many cases capable of controlling the two-spotted spider mite and the carmine red spider mite. However, this is not the case for the tobacco spider mite.
There are few predators known to feed on this mite. In Kenya, ICIPE has released predatory mites for control of the tobacco spider mite in early 2007.
Inspect the crop regularly to determine the presence and level of infestation of spider mites. A recommended monitoring method is to select randomly 20 tomato plants and access the level of mite damage of three leaflets per plant using a leaf index ranking from 1 to 5 (1 is few yellow spots, 5 is leaf totally covered with spots, dry patches occur). Once the average damage level excess the first rank, control measures should start. When detected early, initial infestations are usually concentrated on a few plants, in many cases in the borders of the field. In this situation, remove and burn or bury infested plants to prevent the problem from spreading and becoming serious. Since mite populations are initially restricted to some plants, spot spraying (spraying only attacked plants) is usually effective. Avoid the use of broad-spectrum insecticides. They may kill natural enemies. In addition some synthetic pyrethroids can actually enhance spider mite reproduction leading to mite outbreaks.
Spraying neem extracts, soapy solutions and water are reported to help control red spider mites.
Avoid water stress. If necessary, irrigate the crop regularly, and apply mulch to reduce water evaporation.
Practise good field sanitation to minimise the risk of infestation. Keep the field free from weeds, remove and burn or compost crop residues immediately after harvest. Avoid planting new crops next to an already infested field (tomato or other crops).
Fruit borers (Helicoverpa armiguera, Spodoptera littoralis)
Credit: Biovision-Infonet
The African bollworm or tomato fruitworm (Helicoverpa armiguera feeding on tomato fruit (c) A. M. Varela, icipe
African bollworm(Helicoverpa armigera) damage on tomato (c) Anne Bruntse
The fruit borer or cotton leafworm (Spodoptera littoralis) feeding on tomato fruit. (c) A. M. Varela, icipe
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
What to do: Avoid planting tomato near maize or cotton to prevent heavy pest infestations.
Monitor the crop regularly. Detection of eggs and small caterpillars before they enter into the fruit is very important. Once the caterpillars have entered the fruit they are protected from insecticidal sprays and will have caused damage. Check also for natural enemies and parasitised eggs. Healthy eggs are white with a reddish ring, but they turn black when parasitised. It has been recommended to randomly select 30 tomato plants and examine the leaves immediately below the topmost open flowers to look for eggs of African bollworm (AVRDC, 2000).
Conserve natural enemies. A very rich variety of natural enemies of fruit borers have been recorded. The most important are parasitic wasps (egg and larval parasitoids) and predators such as anthocorid bugs, ants, lacewings and ladybird beetles. Although these natural enemies cannot always prevent economic damage, they play a significant role in controlling the pest populations. For more information on natural enemies click here. Spray selective biopesticides such as Bt or neem extracts. Pyrethrin and rotenone are also used for control of this pest, but they are harmful to natural enemies and not allowed in organic agriculture. Control measures should start when more than five healthy eggs are found or when large numbers of small caterpillars are found on leaves. For more information on Neem click here. For information on Bt click here. Hand pick and destroy eggs and caterpillars. This helps when their numbers are low and in small fields.
