Okra

The African bollworm feeds on leaves, flowers and pods of okra. The main damage occurs on flowers and pods. Attack on flowers results in flower abortion. The caterpillar usually bore clean, circular holes in pods, causing extensive damage and promoting decay from secondary infection by diseases.

The cotton aphid (Aphis gossypii), has been identified as one of the major pest problems of okra in Kenya. They cause damage by sucking plant sap weakening the plants, and by excreting a sticky substance (honeydew), which results in growth of sooty mould affecting photosynthesis. Contamination of pods with aphids, aphid skins, honeydew and/ or sooty mould may result in their rejection in the market.

The cotton aphid transmits the Yellow Vein Mosaic Virus in okra. Cotton aphid (Aphis gossypii) is a small aphid. Adults range from just under 1-1.5 mm in body length.

The bacterium is transmitted through seed and water splash. Symptoms consist of spots on leaves and pods. Leaf spots are initially soaked with water and are more obvious on the underside of leaves. They are restricted to the veins and are angular in shape. Spots on petioles are elongated, sometimes slightly sunken and blackish. Spots on pods are initially water-soaked and round. Later, they join together and turn oily black. All spots appear waxy and shiny.

Black mould can be serious in humid warm areas. It causes leaf spots of various shapes. It grows as a sooty to dark oily (olivaceous) mould on the underside of leaves, but when infection is severe and conditions are very humid it also appears on upper surface of infected leaves. Seriously infected foliage rolls, wilts and falls to the ground.

They feed on developing pods. These bugs are small (4 to 6 mm), and blackish in colour. Their wings are transparent. They attack open or damaged pods mainly at the end of the growing season. Nymphs and adults suck from immature seeds, preventing them from ripening. Groups of bugs are usually found between flower buds, flowers and pods. These bugs are minor pests of okra and usually no control measures are needed.

Several species of bugs attack okra. Some of them occur from the early stages of the crop, but are more common during the mature stages, feeding on developing pods. Their feeding causes small, dark, raised blister-like spots on pods.

These bugs are very common on okra plants at the fruiting stage. They are conspicuously red with black bands. Both adults and nymphs feed on developing pods and seeds, piercing them to suck sap, thereby reducing yield. Attacked young fruits shrivel and then dry. Feeding on mature fruits results in damage to the seeds. Stainers also attack cotton and they live on baobab and silk cotton (kapok) trees.

Stainer bugs are between 14 and 24 mm long.

Cutworms are the caterpillars of some moths. They destroy whole plants by girdling and cutting off young seedlings at the soil level during the night. They hide in the soil during the day.

Damping-off diseases affect the initial establishment of a crop. Their main features include poor emergence and death of seedlings leading to poor stands in seedbeds and fields. Seeds may rot before germination. Affected seedlings that have emerged from the soil show water-soaking, browning and shrivelling of the stem at the soil level. They eventually fall over and die. Damping-off diseases are favoured by excessive wetness of the soil and low soil temperatures.

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.

These are common pests of okra. They are tiny to small beetles, with enlarged hind legs that enable them to jump long distances when disturbed. The colour of the adult beetles varies from black, brown, black and yellow striped or metallic blue-green depending on the species. The adults feed on cotyledons, stems and leaves. They make many small holes on okra leaves, known as "shot-holes". Larvae of flea beetles live in the soil and feed on okra roots, but the damage caused is not of economic importance. Flea beetles are particularly damaging to young plants. Seedlings may wilt and die under heavy flea beetle attack or may stunt if injury is not severe. Damage to cotyledons and young leaves is the major cause of crop loss, generally leading to uneven crop stand. When large numbers of flea beetles are present they will also feed on flower buds and pods, causing yield loss by injuring the pods. Damaged pods are not marketable. Some species of flea beetles are reported to transmit the Okra Mosaic Virus in West Africa.

Flower beetles are also known as pollen beetles or blister beetles. Adult beetles feed on flowers reducing pod set. They also feed on foliage biting off irregular patches on leaves. Larvae of these beetles do not feed on plants but on eggs of grasshoppers and locusts. Therefore, they are beneficial and should not be destroyed.

The beetles lay eggs in the soil. Upon hatching, young larvae are very active and mobile searching for eggs of grasshoppers and locusts. The larvae then burrow underground and start feeding on the eggs. Older larvae become sluggish and change shape turning fat, and getting a large body and reduced legs. These beetles are difficult to control because of their mobility. The damage caused is usually not of economic importance.

This fungus is seed- and soil-borne. Diseased plants are stunted. Leaves of diseased plants turn yellow, wilt and later are shed. Usually the lower leaves are first affected. When a stem or main root of diseased plant is cut crosswise, brown discolouration is usually found in the ring just beneath the bark. Wilting of plants is mostly gradual. Water stress, warm soil temperatures, acidic soils and high content of nitrogen in the soil favour the disease. Infestation by root-knot nematodes aggravates Fusarium wilt.

Grasshoppers are long, slender, winged insects with powerful hind legs and strong mandibles, or mouthparts, adapted for chewing. They range from 1-10 cm in length. They have a front pair of rigid wings and a hind pair of larger, membranous wings, often brightly coloured. When the wings are at rest, the hind pair folds and is covered by the front pair. The rear legs are well developed and usually have sharp spines. Grasshoppers are variable colour; but many are green or brown.

Many grasshoppers lay eggs in pods in the ground, preferably in bare soil; others lay eggs in plant tissue or in crevices. Immature stages are similar to adults but have shorts or no wings. Some grasshoppers have short antenna (short-horned) and other long antenna (long-horned). Grasshoppers can jump well and most of them can fly long distances.

Most grasshoppers are plant-eaters, but some long-horned grasshoppers are predatory with spiny front legs adapted for grasping prey. Grasshoppers feed on okra, eating chunks from the leaves, and stems of young shoots. They can be a problem at the seedling stage, but usually they do not cause damage to older plants.

Leafmining flies are common pests of okra in Kenya. Mining of leaves by larvae may reduce photosynthetic activity, affecting development of flowers and pods. In severe infestation, the leaves might be completely mined. As a result the leaves dry and fall off prematurely, causing loss of vigour and turgidity in the plant. This may eventually result in plant wilting.

The disease is characterised by a white coating resembling a fine talcum powder on lower and upper leaf surfaces. The white coating is the fungal growth. Severe infection causes the affected leaves to roll upward and eventually scorching. The disease also attacks stems, flower buds and okra pods. Infection occurs when humidity is in the range of 52 to 75% and air temperatures are around 26 to 27 °C.

The nematodes are soil inhabitants. They attack a wide range of crops, particularly vegetables. Their infestation is most serious in light sandy soils and in furrow irrigated fields. Diseased plants are stunted, yellow and have a tendency to wilt in hot weather. Diseased plants appear in patches in the field. The roots of diseased plants have small lumps known as galls or root knots.

Various species of caterpillars feed on okra leaves. Semiloopers chew irregular holes on leaves, giving them a ragged appearance. Other caterpillars feed on the lower leaf surface leaving the upper surface intact (this type of damage is known as windowing). Leafrollers spin or roll leaves together, and eat the leaf margins, causing the leaves to curl and droop. Leaf-eating caterpillars cause economic damage only when present in large numbers, especially in young or stressed plants. Well-established, healthy plants can tolerate considerable loss of leaves; however, seedlings may be killed by severe defoliation.

H. derogata larvae are creamy-white or pale yellow, but appear to be green, owing to ingested leaves in the gut. First-instar larvae are 2 mm long and grow to 22-26 mm long. The larvae have dark-brown heads. The adult is a moth with wingspan of 30-40 mm.

Spider mites suck the sap of plant tissues. Infested leaves first show a white to yellow speckling and then turn pale or a reddish bronze colour as infestation becomes heavy. The leaves curl up under severe attack and finally wither and fall off, leading to plant defoliation. In severe infestation, spider mites will also attack pods, causing pod contamination. Major factors leading to spider mite outbreaks are very hot and dry conditions, destruction of natural enemies and the presence of other highly infested crops (e.g. cotton tomato) in the vicinity of the okra crop.

It is a major pest of okra. The full grown larva is 13-18 mm long and 2.5 mm wide. It is stout and spindle shaped. The body colour is variable, ranging from greyish-brown, through grey to green, with a distinctly paler or white median line. The adult is a moth, about 12 mm long with a wingspan of about 20-22 mm. The forewings (front wings) are white, peach, metallic green to straw yellow in colour according to the species.

The larvae/caterpillars bore into terminal shoots of young plants, causing death of the tip and development of side shoots. When pod production starts, the caterpillars move to the flower buds, small pods and eventually mature pods. Damaged flower buds and young pods are shed, leading to yield reduction. Damaged pods ad pods contaminated with insect frass are not marketable. High doses of nitrogen fertilisers have been found to increase spiny bollworm infestation.

Both adults and nymphs of the stink bugs feed on buds, blossoms, pods and seeds. They are 6-15mm long. Their feeding causes local necrosis resulting in small pimples on the pod, and occasionally pod shedding. On very young pods, it causes twisting and distortion of pods, rendering them unmarketable.

Stink bugs are a minor pest and normally do not justify any control. But they can be a problem at podding.

Thrips feed on the lower surface of leaves, flowers and fruits. Both larvae and adults puncture the leaves and suck the exuding sap. Heavily attacked leaves become curled, wrinkled and finally dry up. Any environmental stress that weakens the plants makes them more susceptible to thrips attack. Thrips attack on seedlings and young plants delays crop development. Feeding on the flowers may result in deformed pods. Attack on fruits may scar them. Affected fruits are not marketable.

Adults and nymphs of the tobacco whitefly suck sap from leaves of okra plants. Attacked plants have low vigour and may wilt, turn yellow and die if whitefly infestations are severe or of long duration. Damage may be accentuated when plants suffer water stress. Whiteflies excrete honeydew on which supports sooty mould growth. This mould may cover the leaves during heavy infestation, and as a result the leaves may turn black, affecting photosynthesis.