Maize

The African armyworm is a very damaging pest, capable of destroying entire crops in a matter of weeks. Although they are regarded as occasional pests, in an outbreak large number of caterpillars will appear destroying the whole plant to ground level.

Caterpillars of the African bollworm also known as the corn worm or earworm attack mainly the developing cobs, although they may occasionally feed in the leaf whorl or on tender tassels. Eggs are laid on the silks. Caterpillars invade the cobs and feed on developing grain. Development of secondary infections is common. Local outbreaks of this pest are sometimes severe.

Stemborers are the most important insect pests of maize in sub-Saharan Africa. Yield losses vary between 10-70%. Several species have been reported. The importance of a species varies between regions, within a country or even the same eco-region of neighbouring countries. At least four species attack maize in eastern and southern Africa, with yield losses reported to vary from 20 to 40%, depending on agroecological conditions, crop cultivars, agronomic practices and intensity of infestation.

The most important are the African maize stalkborer (Busseola fusca) and the spotted stemborer (Chilo partellus) (see also below).

The pink stalkborer (Sesamia calamistis) and the sugarcane stalkborer (Eldana saccharina) are of minor importance in maize.

Early warning signs: Young plants have pinholes in straight lines across the newest leaves. This is the time to treat - before the caterpillars move into the stem.

The larvae of the Angoumois grain moth penetrate and feed inside maize grain. This insect may also infest the crop in the field prior to harvest. The moths are small (nearly 1 cm long) yellowish or straw-coloured, a have a fringe along the posterior margins of the wings. They can be observed flying around infested stores.

Female moths lay eggs at night. Eggs are laid singly or in clumps on the outside of cereal grains, in cracks, grooves or holes made by other insects. Eggs are initially white turning red near hatching. The larvae are whitish. The larvae prepare a round exit hole for the moth, leaving the outer seed wall only partially cut as a flap over the hole, resembling a trap door. The adult pushes its way out through this "window" leaving the trap door hinged to the grain. Infested grains can be recognised by the presence of these small windows. The adult lifespan may be up to 15 days, and 1 female can lay over 100 eggs.

It is dark green to bluish-green in colour with black cornicles. Particularly during dry/periods the colonies appear on the inflorescences and young leaves. Feeding by this aphid causes yellow mottling, but this damage is seldom of economic importance. Their role as vector of the sugarcane virus, maize dwarf mosaic virus and maize leaf-fleck virus makes them a pest of considerable importance. This aphid usually attacks maize plants at the end of the mid-whorl stage. Aphid colonies may completely cover emerging tassels, and the surrounding leaves, preventing pollen release. In severe outbreaks the ear shoot is also infested, and seed set may be affected.

It is recognised by the appearance of circular to elongate pustules scattered over both surfaces of the leaf. Pustules are powdery and cinnamon-brown in colour. They contain masses of spores (uredospores). Pustules can appear on any above-ground part of the plant, but they are most abundant on the leaves. With time the pustules split exposing the spores, which are spread by wind and initiate new infection. As maize matures, colour of spores in pustules change from reddish to black due to formation of teliospores (resting spores). The disease is spread by air transport.

Characteristic symptom of common smut is formation of galls or tumuors on above-ground parts of maize plant. Galls frequently are from one to several centimetres in diameter. The galls are at first covered by a shining, whitish-green membrane. As the gall enlarges, the membrane ruptures, exposing a powdery black mass of spores.

It is a spreading perennial grass with vigorous mat-forming stolons. It reproduces and spreads mostly by means of rhizomes but also propagates by seed. This grass is considered as one of the most important weeds in the world. It is present in virtually every tropical and subtropical country and in virtually every crop in those countries. Couch grass and other species of Cynodon are common in East Africa, and some species are occasionally troublesome as a weed of arable land and perennial crops.

Couch grass is reported in Ghana as a problem in crops such as eggplant, okra, onion, peppers and tomato.

Cutworms cut maize seedlings at or a little below ground level, make small holes along the initial leaves, or remove sections from the leaf margins.

Characteristic symptoms include pink to brick-red colour on ears, husks and kernels. The fungi often gain entrance to the ears through channels made by earworms and borers. Bird damage to the ears also facilitates disease infection.

Symptoms

Roots: dry rot.

Seedlings: blight and subsequent death of the seedling.

Leaves: leaves become a dull green colour when rots and stalks are infected early.

Stalks: lesions are a dark brown to black colour in which black perithecia may be produced near the lower nodes. The pith is shredded and is pink to red in colouration.

Ears: the fungus infects the ear via the silk channel and causes a red rot of the kernels from the tip of the ear. This may spread over the whole ear.

The fall army worm is larvae stage of fall army worm moth.  It is a migratory pest native to America and was first reported in Africa in 2016. The army worm causes damage by feeding on leaves and stems of maize, rice, sorghum, sugarcane, vegetable and cotton.

Fall army worm damages maize in all stages. In maize seedlings, it causes damage by feeding within the whorl. Larger larvae can cut the base of the plant. Mature plants suffer attack on reproductive structures. Symptoms of army worm damage in maize are a mass of holes on leaves and the whorl (funnel), ragged edges and larval frass. Young larvae skeletonize the leaf lamina. Severe feeding damage to young plants can kill the growing point; causing 'dead heart' in maize. Maize cobs are attacked by larvae boring through the kernels. At high densities, large larvae may act as armyworms and disperse in swarms, but they often remain in the locality on wild grasses, if available.

Several species of grasshoppers and locust feed on maize. The edible, a long horned grasshopper Homorocoryphus nitidulus vicinus (Ruspolia differens) has been reported to occasionally attack maize in Tanzania (Bohlen, 1973). This grasshopper attacks maize in the silking stage, arresting pollination. Other grasshoppers and locust attack maize from the mid-whorl stage to maturity, and may consume every part of the plants. Attacks vary in severity from location to location.

Symptoms are similar to Southern leaf blight but the spots are much narrower. They are initially light brownish in colour, and with age they bleach to ashen grey surrounded by narrow light-brownish border. When wet, spore mass is formed on the spots with a light shade. This disease is favoured by prolonged periods of high relative humidity. It can cause yield losses of 30 to over 50%.

The first symptoms become evident when tassels and cobs (ears) appear. These parts may be completely or partly converted into smut galls. Smut galls are initially covered by a delicate membrane that breaks open and exposes a mass of reddish-brown to black spores and strands of vascular tissue. The strands or fibres in the galls distinguish this disease from common smut. Head smut is seed-borne.

They attack stored maize grains. Both the adults and the larvae (grubs) of these beetles feed in the grains. Adults come from infested cobs in the field or from an infested maize store and lay eggs in the grains. They attack maize both in the field and after harvest. Attacked maize grains lose all their contents and are not fit to eat. These pests become a serious problem in short time if no control measures are applied. The larger grain borer also attacks dried cassava roots and even the wooden structures of the stores.

The adult is oval in shape, about 6 mm in length and reddish brown in colour with black spots on the wing covers. The body is covered with short, light coloured hairs. The larvae are 7-9 mm in length, soft and covered with dark coloured spines. They pupate on leaves. Both larvae and adults of the maize ladybird beetle feed on leaves, scrapping them, usually on the underside, leaving the upper epidermis intact. This beetle will cause much damage only when present in large numbers. It also attacks cereals such as wheat and sorghum.

The maize ladybird beetle rarely causes serious defoliation and therefore control is usually not necessary.

The adults are about 3 mm long, slender and cream to pale yellow green in colour. These leafhoppers have two small black spots between the eyes and brown marks behind the eyes extending along the body. They have brown lines along the wings. They usually hop away when disturbed. The direct damage cause by maize leafhoppers by sucking plants is insignificant, but the indirect damage is high because they transmit the maize streak virus, a major disease of maize. Cicadulina mbila is the most important vector.

Control of the maize leafhoppers is difficult since they are very active, remain infectious for a long time and are very quick in transmitting the virus.

A serious new disease of maize appeared in the farmers' fields in eastern Africa in 2011. Called maize lethal necrosis (MLN; or corn lethal necrosis, CLN), it can devastate maize crops. Infection rates and damage can be very high, seriously affecting yields and sometimes causing complete loss of the crop.

The disease is difficult to control for two reasons:

1.It is caused by a combination of two viruses that are difficult to differentiate individually based on visual symptoms.

2.The insects that transmit the disease-causing viruses may be carried by wind over long distances.

MLND is mainly spread by a vector, transmitting the disease from plant to plant and field to field. The most common vectors are maize thrips, rootworms, leaf beetles and aphids. Hot spots appear to be places where maize is being grown continuously.

In Africa, the disease was first reported in Kenya, (South Rift Valley - Bomet and Naivasha districts) in September 2011 although its extent at that point suggested that the disease has been present for some time. According to the Kenyan Ministry of Agriculture, two percent of the maize harvest was affected in 2012. MLND has also spread rapidly into Tanzania, Uganda and South Sudan in the meantime.

National and global research and extension organizations, laboratories, and seed companies are working together to control the spread of the disease and to develop and deploy disease-resistant maize varieties for the farmers as soon as possible.

For more information click here Cimmyt website

It is 4-5 m long and greyish in colour. The transparent wings are about twice as long as the body and show marked dark-brown veins. It is commonly found in groups in the funnel of the plants, the whorl, leaf sheath or underside of leaves. This insect produces large quantities of honeydew. As a result, sooty mould is often evident near the sites of aggregation. Nymphs and adults are in close association with ants, which feed on the honeydew produced by this plant hopper.

This plant hopper transmits the maize mosaic nucleorhabdovirus (MMV), maize stripe tenuivirus (MSpV), and maize line virus that can become a limiting factor in maize production.

The virus causes a white to yellowish streaking on the leaves. The streaks are very narrow, more or less broken and run parallel along the leaves.

The virus is transmitted by leafhoppers (Cicadulina mbila and C. bipunctella zeae). Maize streak virus is a serious constraint to maize production in sub-Saharan Africa. The reduction in yields depends on the time of infection. Plants infected at early stage usually do not produce any cobs. Yield losses in East Africa vary between 33 and 55% under natural infection conditions. In Nigeria, 75-100% of maize plants can be infected at the end of the growing season. However, resistant varieties in these areas appear to withstand these epidemics (Anon., 1983). Sugarcane, sorghum, millet, wheat, barley, oats, rye and wild grasses can also be severely affected.

Small oval (egg-shaped) spots first appear as water-soaked areas. They are dark and greyish-green in colour, turning greenish tan. With age they get bigger and become cigar-shaped. After rains or heavy dews, spores develop abundantly on both surfaces of the spots, particularly at the centres, giving a dark-green, velvety look to the spots. The spots may join and form large areas, which may kill entire leaves. Heavily infected leaves appear dry as if affected by drought. The disease is favoured by heavy and frequent rains, high relative humidity (above 90%) and relatively low temperatures (20-25&degC). Warm dry conditions check disease development.

The parasitic weeds Striga spp. known as witchweeds, are important pests of maize, particularly in drier areas. The weeds grow on the roots of maize affecting development of maize plants. The young weeds tap the roots of maize plant and draw water and nutrients. A single weed plant produces many thousands of tiny seeds that survive in the soil for long periods. A heavy infestation can cause complete yield loss.

Striga weeds infest 40% of the arable land in the savannah region, causing annual crop losses of 7 to 13 billion dollars. Around the Lake Victoria basin infestation by Striga hermonthica causes 30 to 100% loss in maize yield. Striga infestation is associated with increased cropping intensity and declining soil fertility. Witchweed infestation has resulted in the abandonment of much arable land by farmers in Africa. The problem is more serious in areas with low soil fertility and rainfall.

None of these methods described will, alone, provide complete control and without complete control there is the certainty that surviving plants will mature and replenish the soil seed bank. Therefore, integration of one or more methods is essential for any substantial reduction of the problem. Furthermore, such integrated treatments will almost certainly need to be repeated over a number of years for long-term control.

In south-western Nigeria, satintail is a major weed reducing maize yields. The rhizomes of this weed often reduce the efficacy of farmers' weed control practice (slashing followed by 2-4 times of additional weeding) and contribute to high yield losses.

Symptoms first appear as small yellow dots that become elongated between veins. They later become brownish to creamy white in colour with reddish to purplish brown borders. Light brown leaf spots with a brown margin, at first elliptical, becoming rectangular, up to 25 mm long and 2 - 6 mm wide. The spots are at first restricted by the leaf veins, but later they may merge. Leaves dry out and die prematurely.

Silks, portions of the husks and cobs may turn black. A black mould may develop on cobs. Disease development is promoted by prolonged wetness on foliage, extended dew, RH (97-100%) and relatively warm temperatures (24-35°C).

Spread is by airborne spores; and the fungus is also seed-borne. Survival in soil occurs for up to 12 months.

Symptoms resemble those of common rust, particularly in the uredial stage (urediospores). The cinnamon-brown pustules tend to be smaller and more circular in outline than those of common rust. Pustules of telial stage (teliospores) are chocolate brown to black and circular to elongate. They are distinguished from common rust by retention of the epidermis of the leaf over the pustule for a long time. No alternate host has been reported for Southern rust.

They can damage maize from the seedling stage to maturity. The presence of small, faint yellow blotches on the lower leaves is an indication of spider mite injury. As the colonies of mites increase in size they cause the lower leaves to become dry. The mites then migrate to the upper leaves. In Africa several species of spider mites have been reported on maize (mainly Tetranychus spp. and Olygonichus spp.). In Kenya, they are occasionally found on maize, but usually they are not of economic importance.

Stemborers are the most important insect pests of maize in sub-Saharan Africa. Yield losses vary between 10-70%. Several species have been reported. The importance of a species varies between regions, within a country or even the same eco-region of neighbouring countries. At least four species attack maize in eastern and southern Africa, with yield losses reported to vary from 20 to 40%, depending on agro-ecological conditions, crop cultivars, agronomic practices and intensity of infestation.

The most important are the African maize stalkborer (Busseola fusca) (see above) and the spotted stemborer (Chilo partellus). The pink stalkborer (Sesamia calamistis) and the sugarcane stalkborer (Eldana saccharina) are of minor importance in maize.

Early warning signs: Young plants have pinholes in straight lines across the newest leaves. This is the time to treat - before the caterpillars move into the stem.

Often referred to as "white ants", they occasionally cause partial or total defoliation of maize seedlings, but are mainly damaging to older maize plants. Severely damaged plants may lodge and be completely destroyed by termites. The longer a field has been cultivated, the greater will be the yield losses caused by termites. Their feeding inside the stems causes the plant to wither and sometimes die. Termites begin to attack the roots and stems about 3 months after planting, and eventually cover them with tunnels built of soil. As plants mature the amount of damage increases rapidly. Infestation is particularly serious in dry season. It has been established that termites can damage up to 25% of maize crops in Malawi (WISARD Project Information, 2001).

White grubs are the larvae of scarab "chafer" beetles. They are white, C-shaped with a brown head and 3 pair of legs. Some species of whitegrubs (e.g. Phyllophaga spp, Heteronychus spp.) feed on roots of maize plants. Root damage is manifested by wilting seedlings, poor stands, and patches of tilted or lodged plants showing uneven growth. Injured plants can easily be pulled out of the ground.

Feeding of adults on maize leaves is usually not of economic importance. However, adults of the black maize beetles (Heteronychus spp.) are reported as major pests of cereals in many parts of Africa. They eat the stems of young shoots just below the ground. One adult beetle may destroy several seedlings in a row.