Wheat

Symptoms include leaf discolouration from tip to base and from margin to centre. The discolouration takes on different colours depending on the plant. In barley, the leaf turns bright yellow; in oat, an orange, red or purple discolouration is seen and in wheat, rye and triticale, the infected leaves are generally yellow and sometimes red. Plants are usually stunted, with a decrease in tiller number and biomass and a weak root system. Suppressed heading, sterility and failure of grains to fill occur in the most severe cases. In the field, symptoms appear usually as yellow or red patches of stunted plants. The disease is most damaging in terms of yield reduction, if it infects a crop at an early stage of growth. The virus is spread by cereal aphids (e.g. Rhopalosiphum padi, R. maidis, Sitobion avenae, etc.). It is neither seed-borne nor mechanically transmitted. It also attacks maize, rice and several grasses.

The red-billed quelea is a small weaver bird native to sub-Saharan Africa and renowned for its attacks on small-grain crops within Africa. It is the most numerous bird species in the world. The red-billed quelea is mainly granivorous, except when feeding its chicks insects or when eating insects prior to migration or breeding, and it relies on a supply of grass seeds to survive. When unable to find grass seeds or when opportunities arise, quelea will attack crops such as wheat, millet, oats, sorghum, teff, rice.

It is a major pest throughout much of sub-Saharan Africa and can cause significant economical losses.

The bird is inherently nomadic and this nomadism accounts for its invasions into areas where it was previously absent.

The lesions are brown at first and are most easily distinguished from those of stem rust by their size and shape: they are usually small and circular. They turn black as the crop matures. They occur on the leaf blades and the leaf sheaths and may appear at any stage of the crop's growth.

It can cause considerable damage in wet years, especially where wheat has been grown for several years in succession. Symptoms consist of brown lesions on the glumes and around the nodes. At advanced stage of the disease black spots just like dots can been seen on the lesions. These are fungal spore bodies (pycnidia). The affected leaves become shrivelled with light brown patches on them. Glume blotch is spread by use of infected seeds, rain splash and infected crop residues.

White powdery growth appears on all above ground parts of plants. The white growth consists of fungal mycelium and spores. The growth later turns buff in colour. The disease is wind-borne.

Rodents, mainly the black rat (Bandicota bengalensis), also damage stored seeds.

Cereal aphids, being vectors of virus diseases, such as the Barley yellow dwarf virus, can be serious pests in wheat. The important cereal aphids that attack wheat in Kenya include Schizaphis graminum, Sitobion avenae, Rhopalosiphum padi, R. maidis, Metopolophium dirhodum and Diuraphis noxia (the Russian wheat aphid).

The Russian Wheat Aphid (Diuraphis noxia) is one of the most damaging pests of small grain cereals (e.g. wheat, barley, triticale, rye, and oats) in the world. This aphid is a relatively new pest of wheat in Kenya. It was first identified in farmers' fields in 1995. It then spread quickly to all the wheat growing areas of the country and it is nowadays the most important pest of wheat and barley. It is also a major pest in South Africa, but has maintained minor pest status in Egypt, Sudan and Ethiopia.

The Russian wheat aphid is pale to light green in colour with an elongated, spindle shaped body and grows to up to 2 mm long. It has short antennae with rounded very short, nearly invisible cornicles. The feature that easily distinguishes it from other cereal aphids is the presence of an appendage above the cauda, which gives the aphid the appearance of having 2 tails. They prefer to live in the leaf whorls or in tightly rolled leaves, and thus are partially protected from natural enemies and from contact insecticides. They are hardy and can survive extremely low temperatures. Dry weather favours rapid increase of the aphid.

Unlike many important cereal aphids, the Russian wheat aphid is not a known transmitter of diseases, but causes damage by injecting a toxin into the plants during feeding. This toxin prevents the production of chlorophyll and causes, in susceptible cultivars, leaf chlorosis, longitudinal leaf rolling and white/yellow (warm weather) or purple reddish (cold weather) streaking on the leaves. Extensive chlorosis leads to death of plants while leaf rolling retards plant development causing stunted growth. The tight rolling of flag leaves delays ear emergence, leading to floret sterile heads resulting in reduction of seed set. Aphid infestation also reduces the quality of the seeds produced, as shown by low kernel weight, increased rate of seed deterioration under accelerated ageing conditions, and reduced seedling vigour. The effect of infestation on seed quality is more pronounced under dry conditions. Infestation also may result in reduced seedling vigour.

In Kenya, the damage usually appears when crops have attained the tillering stage. Yield losses ranging from 25 to 90% have been reported.

It is characterised by pustules (a pimple-like or blister-like structure) that develop and break through the surface of the stems, leaves, sheaths, chaff and beards of the wheat plant. The kernels are badly shrivelled, many of them being so light and chaffy that are blown out with chaff in threshing. The remaining grains may be shrunken to one-half or two-thirds normal size. Myriads of brick-red spores escape from the pustules and are carried by the wind to other wheat plants. Wheat stem rust also attacks barley, occasionally rye and many wild grasses (Hordeum spp., Agropyron spp., Elymus spp., Hystrix spp. and some brome grasses). It does not attack oats.

It is a soil-borne fungus. It invades and blackens the roots, frequently killing them in the process. Affected stems are black and shiny just above the soil level. This symptom can only been seen by peeling away the leaf sheaths. The disease occurs in slowly widening patches, and in these areas plants with poorly filled or empty ears (whiteheads) may be present. The pathogen survives between crops on cereal roots and stubble. It also attacks barley, oats and rye.

The Western flower thrips (Frankliniella occidentalis). (Close-up) Inmmature thrips (left) and adults. Very much enlarged . Real size 0.9 to 1.1 mm.

Infected plants have reduced height. The smutted wheat heads are bluish green when theyt emerge from the boot. The healthy heads are yellowish green. The disease also induces excessive tillering. The spores are blown by wind to developing ears which they invade. Bunt infected flowers have green ovaries while healthy ones are white. The grain of wheat is replaced by a black mass of spores (spore ball) accompanied by a smell like of rotting fish.

The disease is also called stripe rust. Yellow or orange-yellow pustules develop on the glumes or chaff, on the leaves, and on the leaf sheaths. These lesions are arranged in parallel lines along the leaves. The disease may also attack the stems and the kernels. Infected leaves show distinct chlorosis. Damage to the disease is most serious, if plants are attack at milk stage or earlier. Under severe infection kernels may be shrivelled. Rapid disease spread is favoured by warm weather with frequent rainfall. Yellow rust also attacks barley, rye, and over 60 species of grasses.