Groundnut

It is a serious pest as a vector of virus diseases, such as the rosette virus disease, a major constraint to groundnut production, particularly in the dry season. The groundnut aphid is black or dark brown in colour, variable in size (1.5 to 2.0 mm long) with two black cornicles (horns at the rear of the body), and a black tail.

The fungus causes both seed and seedling rot and drastically reduces plant stand. In moist soil, seeds may be attacked and killed due to rotting. Seeds removed from soil show black sooty cover. The infected areas of seedlings are covered with black fungal spores. Mature plants are also attacked. Symptoms include permanent wilt of branches, and or wilting of entire plant. The dead and dried branches are easily detached from the collar region. Infected pods reveal patches of black sooty spores. Related species (Aspergillus flavus) causes deterioration of seeds.

It also produces the toxin (aflatoxin) in infected seeds that can cause death or other symptoms of toxicity when eaten by animals or humans.

A. flavus as a mould contaminant and toxin producer is much less serious during growth of the crop than during subsequent storage of kernels. Minimising moisture stress during growth can reduce invasion and toxin production by A. flavus.

It can cause serious losses, if a crop is infected early. Infected plants show water stress symptoms and may wilt suddenly without yellowing of the foliage, particularly, when temperatures are high.

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.

Aspergillus niger

The fungus causes both seed and seedling rot and drastically reduces plant stand. In moist soil, seeds may be attacked and killed due to rotting. Seeds removed from soil show black sooty cover. The infected areas of seedlings are covered with black fungal spores. Mature plants are also attacked. Symptoms include wilt of branches permanently, and or wilting of entire plant. The dead and dried branches are easily detached from the collar region. Infected pods reveal patches of black sooty spores. Related species (Aspergillus flavus) causes deterioration of seeds.

It also produces the toxin (aflatoxin) in infected seeds that can cause death or other symptoms of toxicity when eaten by animals or humans.

A. flavus as a mould contaminant and toxin producer is much less serious during growth of the crop than during subsequent storage of kernels. Minimising moisture stress during growth can reduce invasion and toxin production by A. flavus.

It is about 5mm in length, brown or green in colour with white marks and strips on the wings. The nymphs resemble the adults but without fully developed wings. These insects live in clusters or colonies, and are attended by ants that feed the honeydew excreted by the hoppers. These sucking insects attack the plants at the base of the stem, usually below ground level. The toxic saliva injected while feeding causes the plant to wither, turn yellow and die. The extent of damage can be important when the insect occurs in large numbers. The first sign of infestation is the presence of black ants.

It consists of three types namely groundnut chlorotic rosette, groundnut green rosette and groundnut mosaic.

The disease is caused by a complex of different strains of groundnut rosette umbravirus. Symptoms vary depending on strain(s) present. They include yellowing, mottling and mosaic symptoms on leaves and stunting and distortion of the shoots. Older leaves are dark green, reduced in size, and show downward rolling of leaflet margins. If the plants are infected when they are young, they may not produce nuts.

The virus is transmitted by aphids (Aphis craccivora and A. gossypii), which feed on the undersides of the leaves.

Symptoms of early leaf spot (Mycosphaerella arachidis, Cercospora arachidicola) consist of sub-circular dark brown spots produced on the upper leaflet surface. The spots are of lighter shade of brown on the lower side of the leaflets. Yellow halo is seen around the brown spots. Oval to elongate spots are also seen on stems, petioles, and pegs. Late leaf spot can be distinguished from those of early leaf spot.

Late leaf spots (M. berkeleyi, Cercosporidium personatum) are darker with no or light yellow halo. The late leaf spots on the lower leaflet surface are rough in appearance. They exhibit circular rings of fungus fruiting structures on the lower leaflet surface. Severe disease attack leads to shedding of leaflets resulting in premature ageing of the crop. Oval to elongate spots similar to early leaf spot are also formed on stems and pegs. Late leaf spot attack is usually seen along with rust disease.

It is a common pest of groundnuts in South and South-East Asia and a major pest in India, and it has recently invaded Africa. It was first found in Uganda in 1998 and is now recorded in Mozambique, Malawi, Democratic Republic of Congo and South Africa. In all African countries where this leafminer has been found, the pest has reached epidemic densities and severe yield losses have been observed on groundnut (The New Vision. 2004; Kenis and Cugala, 2006). The adult is a mottled moth, with a full wing span of up to 18mm. The moth lays eggs on the underside of the groundnut leaf and petioles. Yellowish green caterpillars hatch, tunnel into the leaves and feed between the upper and lower epidermis of the leaf. Mined leaves become distorted within a few days. Caterpillars are grey-green with a shiny black head. There are five larval instars. The first instar has an average length of 0.56 mm. At pupation, they rarely exceed 8 mm in length.

Three or four mines per groundnut leaflet can cause so much distortion that a leaf exposes as little as 30% of the potential photosynthetic area to the sun. Later, when the caterpillar becomes too large to occupy the mine, they emerge to the leaf surface and either fold over a single leaf and hold it down with silk, or web together two or more leaflets. They live and feed in the shelter they have constructed. Pupation takes place inside the webbed leaflets. Damaged leaves become brownish, rolled and dry, which results in early defoliation and affects the growth and yield of the plants.

Millipedes are among the economically important soil pest of groundnuts. They are brown to blackish in colour and curl when disturbed. They attack groundnut seedlings, between planting and approximately 20 days after planting, feeding on the emerging cotyledons and moving to the root system at the collar region. The cortex is often damaged serving as an entry point for secondary infection by microorganisms. The development of plants surviving the attack is often retarded.

Millipedes also attack maturing groundnut during pod formation, i.e. when the pods are still soft. Immature pods from severed pegs are often perforated and thus suffer secondary infection or invasion by rot-causing organisms such as Aspergillus flavus. Millipedes may also damage flowers. Birds are main predators of millipedes.

They are widespread but their seriousness in yield reduction is unknown. They cause plant stunting, gall or knot formation on roots, and in severe cases wilting of affected plants

Pustules (spots or blisters) can form on all aerial plant parts except flowers. Orange coloured pustules first appear on the lower surface of leaflets. Later, pustules may appear on the upper surface of the leaflets. The pustules on the stem are elongate and elevated. The pustules when mature rupture to release masses of reddish-brown spores, which blown by wind, spread the disease from plant to plant and far away to other groundnut fields.

Spider mites may become serious pests of eggplant during the dry season. Attacked leaves show a stippled appearance (white specks), and their surface covered with a fine web. Continuous infestation causes the leaves to dry-up. The plants are stunted and yields reduced.

Stored groundnuts are attacked by moths (Ephestia cautella, Plodia interpunctella, Cadra cautella), and beetles (Caryedon serratus, Tribolium castaneum, Trogoderma granarium).

The larvae of moths and the grubs and adult beetles bore into and damage seeds. Moths cause extensive webbing. The bruchid beetle Caryedon serratus is the major pest of groundnut in pod shell in West Africa. A good post harvest pest management programme based on good storage practices is very important.

Termites are serious groundnut pests throughout the southern African region and West Africa. Species of Microtermes and Odontotermes are the most damaging, while Macrotermes cause occasional damage. The small-sized Microtermes spp., in particular, attack and invade growing groundnut plants through the roots and stem near ground level, hollowing them out and causing the plants to wilt and die with a consequent reduction in crop stand.

Roots damaged by other soil pests, such as white grubs, are also prone to attack by termites. Some termite species (Macrotermes spp., Hodotermes mossambicus) cut off stem bases, and may cause 25-100% of plant losses. As the crop ripens the outer layers of the pods are scarified (removal of soft corky tissue between the veins of the pod) by termites allowing contamination of the seed with soil fungi, such as Aspergillus flavus, which produce lethal "aflatoxins".

Scarification of pods is by far the most common type of termite damage at plant maturity, a factor often aggravated by late harvest. Scarification as high as 30% has been reported. Infested plants are not obviously diseased and are frequently harvested and contaminate the rest of the crop. Species such as Microtermes spp. also penetrate the pod to feed off the soft inner lining, filling the pod with soil. This form of attack leads to additional loss through premature germination of kernels. Stacks of plants left drying in the fields are also frequently attacked by species such as Odontotermes spp. with farmers losing between 30-40% of their crop at this stage.

Termite damage is generally most serious towards the end of the growing season just prior to harvesting, and it is particularly serious during periods of drought (ARC/LRN. 2007).

Several species of thrips attack groundnuts. They have been reported as important pests of groundnuts in Uganda. The flower thrips (Frankliniella schultzei and Megalurothrips sjostedti) infest mainly buds and flowers. Attacked flowers are discoloured and scarred; terminal leaf buds are blackened and distorted after unfolding. Other species of thrips (e.g. Scirtothrips dorsalis and Caliothrips indicus) infest foliage.

Thrips feeding causes yellowish-green patches on the upper leaf surface and brown necrotic areas and silvery sheen on the lower surface of the leaf; leaves become thickened and some curling occurs. In severe infestations, young leaves are severely deformed, plants are stunted and leaves are blighted.

Whitegrubs are the larvae of scarab "chafer" beetles. They are white, C-shaped with a brown head and three pairs of legs. Many species of white grubs are associated with groundnut damage in parts of sub-Saharan Africa. The most important are Schyzonycha species.

White grubs attack plants at all stages of growth. They eat roots and damage pods of groundnuts. White grubs feed mainly on the taproots and/or peripheral roots leading to stunting or death. They inflict cuts in the crown region of taproots; these lesions are often invaded by rot-causing fungi. White grubs also cut out pods from the base of groundnut pegs and destroy larger, soft pods. Plants are often attacked in a row. White grubs seem to prefer soils with sandy or loamy sand textures and are seldom observed in clay soils.