Cotton

Bollworms are among the most serious cotton pests. They feed in the bolls, damaging lint and seed and causing considerable reduction in yield and quality. The main bollworms are the African bollworm (Helicoverpa armigera), spiny bollworm (Earias spp) and the pink bollworm (Pectinophora gossypiella). Caterpillars of the false codling moth (Cryptophlebia leucotreta) are sometimes found boring in the bolls.

1) African bollworms (Helicoverpa armigera)

It bores into the boll often with the hind part of the body exposed outside the boll. If younger bolls are attacked they normally show a yellowish colour and the bracteoles open out (flared square). One caterpillar can damage a number of bolls and buds by moving from one to the other. Infested bolls and buds drop prematurely.

2) Spiny bollworms (Earias biplaga / Earias insulana)

The adults are moths, about 12 mm long with a wingspan of about 20-22 mm. The forewings are white, peach, metallic green to straw yellow in colour according to the species. Fully-grown caterpillars are up to 18 mm long. Their body bears numerous fleshy spines. Caterpillars vary in colour from brown and deep orange to grey and yellow. Caterpillars bore into flower buds, young shoots and maturing bolls. Hollowed tender shoots whither and die.

3) Pink bollworm (Pectinophora gossypiella)

The moths are small (about 8-9 mm long) with grey brown or brown forewings covered with dark spots. The hind wings are whitish, broader than the fore wings, and have a long fringe on the posterior margin. The pink caterpillars are up to 15 mm long and have 2 broken transverse red bands on each segment of the body. This is usually a late season pest.

The caterpillars bore into the flower buds and young bolls causing shedding. Caterpillars feeding on flowers spin the petals together, causing the formation of what is called "rosette flowers" which do not open up. The bolls open prematurely and may also rot or drop to the ground. The most important damage is caused by caterpillars penetrating bolls, where they feed on the seeds and soil the lint with frass and excrements. The pest is carried over to the next season and crop as a diapause-caterpillar mainly within the cottonseeds.

The disease causative agent is still unknown but it is associated with whitefly-transmitted geminiviruses, based on symptomatology typical of begomoviruses and the demonstration of its transmissibility by the whitefly Bemisia tabaci. The disease is widespread in Africa south of the Sahara. Disease symptoms include irregular leaf mottling (chlorotic patches), reduced flower production, boll shedding and stunting of infected plants.

It is caused by the fungus Glomerella (Colletotrichum gossypii). It is the primary cause of seedling blight, boll rot and fibre deterioration (staining) in all humid cotton growing areas. Seedlings affected by blight are killed before or after they emerge (damping-off).

Symptoms of seedling blight on cotyledons are usually diseased areas on margins or small reddish or light coloured spots. Diseased seedlings show reddish-brown lesions below ground. The lesions may be on one side of the stem or they may surround it and extend down the root. On the bolls the disease appears as small, round, water-soaked spots, which enlarge, become sunken and brownish in colour. In wet weather, the spots are covered with a sticky mass of fungal spores pinkish in colour.

Boll infection is often associated with wounds made by boll weevils. The lint and seeds are rapidly invaded once the disease gets through the husk of the boll. Affected lint is stained.

The disease survives on old rotten bolls, crop refuse in the field and on the seeds. It is spread on diseased seeds. Infection is favoured by moderate temperatures and high moisture.

Cotton aphids are about 1-3 mm long, and variable in colour from yellow to yellowish green or very dark (almost black) with two cornicles (projections) on the rear end and long antennae. They suck sap preferably from tender shoots and the underside of young leaves. Feeding by aphids may result in crinkling and cupping of leaves, defoliation, square and boll shedding and stunted growth.

Honeydew (a sugary liquid) excreted by aphids accumulates on the upper surface of leaves and sooty mould develops. Honeydew can contaminate the fibre if the bolls are open, causing problems in processing the lint.

The disease is caused by fungus Ascochyta gossypii. It may manifest as seedling blight, a leaf spot, a stem canker, and as a boll spot. The first symptoms are small, round, white, purple-ringed spots on the cotyledons and lower leaves. The spots become somewhat elongated and raised on the upper surface. Later they change to a light brown, the purple ring around the outside disappears, and the diseased tissue often falls out. The upper small leaves, petioles and buds are often infected, and the plant dies. The almost bare stems, with a few small leaves at the tip, are characteristic of the disease at the later growth stages.

Stem infection, which occurs only during consecutive days of cloudy, wet weather, leads to the formation of lesions, which may reach several centimetres in length, with cracks and ragged edges. The centre of these lesions becomes pale, liver-coloured and covered with tiny black dots (bodies of fungal spores, conidiomata). Cankers may encircle the stem and kill the distal parts. Flowers are not attacked, but mature lint can be destroyed. Lint may show a grey discolouration with conidiomata in half-opened bolls.

The disease is favoured by long periods of rain and cool weather. It is also seed-borne.

This disease appears as tan to brown angular leaf spots with yellow margins on leaves, pods, and stems. It may cause severe defoliation during periods of high humidity. It is seed-borne.

Leaves of infected cotton curl upward or downward often accompanied by foliar discoloration and mosaic. These leaves may bear leaf-like enations (growth) on the underside along with vein thickening. Plants infected early in the season are stunted and yield is reduced drastically. The virus is transmitted by whiteflies (Bemisia tabaci).

The moths are about 15 mm long with a wingspan of 25-30 mm. They are yellowish-white with black and brown spots on the head and thorax and a series of dark-brown wavy lines on the wings. Caterpillars are dirty pale green and semi-translucent and up to 30 mm long when fully grown. Moths lay eggs on the underside of leaves. Young caterpillars feed initially on the underside of leaves, but older caterpillars spin or roll leaves together, and eat the leaf margins, causing the leaves to curl and droop. They pupate in the leaf roll or in debris on the ground.

The leaf roller is a common pest, which may cause considerable local damage. In extreme cases, the cotton plants may be almost completely defoliated, as a result the growth of the plants is stunted, and the bolls ripe prematurely leading to yield reduction.

Several types of bugs attack cotton. They are late season pests and they usually attack during the flowering and fruiting stages of the crop. The most important are the cotton stainers.

Stainer bugs are between 14 and 24 mm long. They are bright red, yellow or light grey with an orange tinge depending on the species, and with black bands. Stainer bugs are late season pests. They appear when the bolls are ripening. Female lays whitish yellow eggs in moist soil or in crevices in the ground. They hatch to produce reddish-orange nymphs. Initially the nymphs are wingless, but wings develop gradually as the nymphs grow.

The nymphs are found together in the area where the eggs have been laid and later disperse to look for food. Both nymphs and adults feed on the bolls, but adults cause the most serious damage. They pierce through the boll and suck the seeds reducing germination capability and the quality of the seed oil and the cake.

Furthermore, they cause severe indirect damage by transmission of a fungus (Nematospora sp.), which leads to internal boll rot and stain of the lint with typical yellow colour, hence the name 'cotton stainers'. The nymphs feed mainly on seeds in open bolls reducing the seeds's oil content and their germination capacity. Severe bug attack affects yield, oil content and the marketability of the crop.

Caterpillars cut seedlings at ground level. They can be found in the soil up to a depth of about 5cm around the plant host. Caterpillars of the cotton leafworm are up to 45 mm long. Their colour varies from yellowish white to bluish grey and greyish brown. On the dorsal side of the caterpillars there are usually w rows of triangular black spots. These spots can be absent with the exception of those at the fore and hind part of the body. Although the caterpillars feed mainly on leaves, they occasionally cut plants and attack fruits.

Cotton seedlings are subject to attack by several fungal and bacterial diseases. These include the anthracnose fungus (C. gossypii), Ascochyta blight (A. gossypii), wilt fungus (F. o. f.sp. vasinfectum), bacterial blight (X. a. pv. malvacearum), Pythium spp. and Rhizoctonia solani.

Pythium spp. are particularly serious where soil temperatures are low and wet weather prevails at planting.

Rhizoctonia solani causes sore shin disease of cotton seedlings. Attacked seedlings develop dark to reddish-brown cankers on the stems near the soil line. The cankers encircle the stems or penetrate so deeply that the plants fall over and die.

The disease attacks seedlings, leaves, stems and bolls. Seedling attack, usually called seedling blight, results in small, round, water-soaked spots on the cotyledons as they emerge from the seed coat. The spots furnish inoculum (disease source) for the developing true leaves. Leaf spots are translucent, water-soaked, angular and bordered by veins. Affected leaves become browned and blackened. Spread of the bacteria along leaf veins is commonly called vein blight. Affected veins appear black. Younger leaves are more susceptible to vein blight than mature, old leaves.

In older plants black lesions may develop on the stems. These lesions girdle the stem and may cause breaking of the stem when windy. This symptom is known as black arm. Infected bolls have round, shiny lesions, which later become sunken, browned and finally blackened. Infected bolls become deformed and open prematurely. Fibre in diseased bolls is usually stained.

The disease is carried over season to season on infected crop debris and diseased seeds. Spread of the disease is through use of infected seeds and in the field by wind-blown rain water.

Other bugs feeding on cotton include Helopeltis bugs, blue bugs and cotton lygus and the cotton seed bug.

Cotton Helopeltis (Helopeltis schoutedeni)

Helopeltis bugs are up to 10 mm long and have very long antennae. They are bright red (females) or yellowish-red (males) in colour. The bugs prefer to feed on young plant tissue of leaves, shoots, peduncles and petioles. The toxic saliva injected during sucking causes pale brown necrotic patches. The leaves and the stems of the plants are twisted. The lesions on the leaves often drop out leaving holes as if attacked by chewing insects. The green boll wall is also attacked showing dark, circular, sunken lesion.

Adult leafhoppers are (2 to 3 mm long) and thin. The wings are held roof like over the abdomen. They are pale green to yellowish green in colour, shiny and more or less transparent. The legs are slender with bristles. The nymphs resemble the adults but are smaller and do not have fully developed wings. Adults and nymphs suck sap from the leaves, remaining on the underside during the day, but also moving to the upper surface during the evening. When disturbed they run sideways rapidly to reach a shady part of the host plant.

Feeding by leafhoppers causes discolouration, and leaf curl, the outer zone of the leaf turns yellow to reddish and whiter later. Heavy leafhopper infestation may retard plant growth and may cause severe yield losses. Cotton fields attacked by leafhoppers are easily recognisable from some distance, owing to their reddish or purple colour ("hopperburn").

Other bugs feeding on cotton include blue bugs and cotton lygus and the cotton seed bug.

Blue bugs (Calidea degrii, Calidea bohemani)

The adults are large bugs, 8 to 17 mm long by 4-8 mm broad, and strikingly coloured, with red or orange underneath and the upper surface bright metallic blue or green with black spots. They suck the seeds of unopened bolls causing staining of the cotton lint in a similar way as stainer bugs; as a result the development ceases and the boll aborts.

Blue bugs seldom breed on cotton. They usually invade cotton when the bolls are well formed, from other host plants, for instance sorghum, sunflower, castor and many wild hosts such as crotalaria and hibiscus.

Cotton lygus (Taylorilygus vosseleri)

The adult bugs are up to 5 mm long and pale green to light brown in colour. The nymphs are green. Adults and nymphs prefer to suck young leaf buds, flower buds and bolls. The toxic saliva causes the flower buds to turn yellow and drop off. Leaves, which have been attacked in the bud stage, later show a ragged appearance with many irregular holes. The cotton lygus is an early season pest.

Cotton seed bug (Oxycarenus hyalinipennis)

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. It is a late season pest, which mainly appears when the bolls have opened. Nymphs and adults suck the seeds. Feeding by large numbers of bugs reduces considerably the germination rate, seed weight and oil quality. The lint is not affected.

Several species of root-knot nematodes attack cotton roots. Infested plants are often stunted and leaves yellow. Galls or knots are formed on the roots. Infestation can be particularly serious in light soils. Nematode infestation predisposes plants to Fusarium wilt infection.

Spider mites feed on the lower surface of leaves. Adults are about 0.6 mm long. As a result the leaf surface appears tan or yellow and the upper surface has a speckle or mottled appearance. A close inspection of the lower leaf surface shows the mites as tiny moving greenish or reddish speckles. Heavily infested leaves turn yellow, curl up, dry and are shed. Mites produce a large amount of webbing. In heavy infestations a fine web may cover the leaves. Plants may die when infestation is severe, particularly in hot dry conditions.

Whiteflies (Bemisia tabaci) suck the sap of leaves. Feeding weakens plants and may lead to yellowing and wilting, premature leaf shed and reduced plant growth, affecting the quality and quantity of yield. Severe attack may kill young plants. Honeydew excreted by whiteflies and sooty mould that develops subsequently can affect the fibre quality considerably and cause problems in processing the lint.

The whitefly is a vector of important virus diseases on various crops including cotton. It transmits the cotton leaf curl virus and the African cotton mosaic disease. Attacks are common during the dry season, and usually recede with the onset of rains.