Sorghum

It is an occasional but seriously destructive pest. It causes serious damage to young plants in years of armyworm outbreaks. The leaves are eaten away often leaving only the base of the stem.

It causes damage to sorghum by eating the seeds. Damage is particular serious on compact headed sorghums. The caterpillars appear on the ear heads when the grains are in the milk ripe stage. These caterpillars also feed on whorl leaves. A heavy infestation can cause an almost complete loss of yield.

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 anthracnose fungus damages foliage and stems of grain sorghum. On susceptible hybrids, the stem holding the head (peduncle) becomes infected and a brown sunken area with distinct margins develops. When infected stems are cut lengthwise with a knife, one can see that the fungus has penetrated the soft pith tissue and caused brick-red discolourations. This peduncle infection inhibits the flow of water and nutrients to the grain causing poor grain development.

The fungus also invades individual grains and the small branches of the panicle. Rapid and severe yield loss can result from panicle and peduncle infections. Leaf lesions are small, elliptical to circular, usually less than 0.9 cm in diameter. These spots develop small, circular, straw-coloured centres with wide margins that may vary in colour from reddish to tan to blackish purple. The spots may coalesce to form larger areas of infected tissue.

These are common on sorghum. The sorghum aphid is light yellow in colour, and the maize aphid is dark green to bluish-green in colour. These aphids are often found sucking on ear heads or on the underside of leaves. They produce large quantities of honeydew, which enable black sooty moulds to grow. Attacked plants sometimes are stunted, leaves dry up and yield is reduced. Young plants suffering from drought stress may be killed. Aphids can be a problem during dry periods. Heavy aphid infestations on sorghum at the booting and heading stages seriously reduce both grain quality and yield. The maize aphid transmits the maize dwarf mosaic virus to sorghum.

Adults are small, 1-4 mm long, soft-bodied insects.

Birds are one of the most important pests of sorghum. They are capable of causing heavy losses. In Africa the most notorious species is Quelea quelea and is found in the Sahel region, from Senegal in West Africa through to Sudan, Uganda, Kenya, Southern Tanzania, Malawi, Zambia and South Africa.

Grain sorghum plants affected by the charcoal rot fungus fail to fill grain properly and may lodge in the latter part of the season. Infected stalks show an internal shredding at and above the ground line. This can be observed by splitting the stalk and noting the deteriorated soft pith tissue leaving the tougher vascular strands. Fungal structures (sclerotia) can be observed in the affected tissue, which appears as though it has been dusted with black pepper. Another type of stalk rot (Pythium sp. and Fusarium sp.) may show the shredded condition but the black specks (sclerotia) will be lacking.

Conditions under which charcoal rot is favoured include stressful hot soil temperatures and low soil moisture during the post-flowering period. Host plants are usually in the early-milk to late-dough stage when infection occurs. The fungus is common and widely distributed in nature.

The disease destroys all kernels in a head and replaces them with a cone-shaped gall or may affect only portions of a panicle. At harvest time, these galls are broken and spores contaminate the outer surface of other kernels.

This fungal disease can be troublesome in low lying areas that become flooded. Infected plants have thick, stiff, twisted, pale green leaves with bumpy surfaces. The leaves often turn downward, and the plants produce many shoots or suckers giving a bunchy appearance. Infected plants do not produce heads or produce a proliferation of leafy tissue in place of the head. Wild and cultivated grasses can serve as sources of infection.

Several species damage sorghum. They may cut off young plants at or slightly below the soil surface. Some feed on above-ground plant parts, and others feed on the roots. Plants with severed stems die, leaf feeding by cutworms causes ragged leaves and feeding on roots may kill young plants or stunt older plants.

Chaffer grubs (Schizonycha spp.) also feed on roots and may kill very young seedlings. Stand loss can occur within 10 days after plants emerge in severely infested fields.

Damping-off diseases are caused by various fungi. Seed may be attacked by one or more seed-borne or soil-borne pathogens prior to either germination or emergence. This usually occurs when conditions are not optimum for plant development, such as in poorly-drained, cold, wet soils, or in very dry, crusted soils. Sorghum seedlings are very delicate during the emergence period and are slow to establish a permanent root system. As a result, sorghum depends upon its primary, temporary root system for a period longer than many other crops. Under less than optimal growing conditions, this primary root system is extremely vulnerable to soil-borne pathogens. This is when damping-off in a field is most visible and damaging. Affected seedlings are unthrifty and may show yellowing, wilting and death of leaves. Roots of diseased plants may be discoloured (whitish grey to pinkish brown) and rotted.

Cream to pink sticky droplets "honeydew" ooze out of infected florets on panicles. The droplets dry and harden, and dark brown to black sclerotia (fungal fruiting bodies) develop in place of seeds on the panicle. Sclerotia are larger than seed and irregularly shaped, and generally get mixed with the grain during threshing. Conditions favouring the disease are relative humidity greater than 80%, and 20-30°C .

The sclerotia falling on the soil or planted with the seed germinate when the plants are flowering. They produce spores that are wind-borne to the flowers, where they invade the young kernels and replace the kernels with fungal growth. The fungal growth bears millions of tiny spores in a sticky, sweet, honeydew mass. These spores are carried by insects or splashed by rain to infect other kernels.

Nymphs and adults of the head bug (Calocoris angustatus) and the African head bug (Eurystylus oldi) feed on developing kernels as panicles emerge from the boot. Head bugs are small (3 to 5 mm long, and about 1 mm wide) and variable in colour from yellowish green (C. angustatus), or pale brownish-yellow to dark brown with red markings (E. oldi). Females insert long, cigar-shaped eggs between the glumes or anthers of sorghum florets. Eggs usually hatch in less than a week. Nymphs and adults suck juice from developing kernels as panicles emerge from the boot. Kernels attacked early in development are shrivelled, small, and off-coloured, resulting in yield loss. Bug-damaged kernels become infected by secondary pathogens that further deteriorate grain quality. Feeding punctures are visible on older kernels. The life cycle is completed in about three weeks. At least two generations feed on the same crop when panicles in the field do not mature at the same time.

This disease is characterised by the large, dark-brown smut galls that emerge in place of the panicle. The gall is first covered with a whitish membrane, which soon breaks and allows spores to be scattered by the wind. Plants become infected while in the seedling stage but evidence of infection is not apparent until heading time. The smut gall produces thousands of spores, which become soil-borne and initiate systemic infection of seedlings in subsequent years. Different races of the fungus exist which may result in a sorghum hybrid being resistant in one area but not another.

Leaf blight (Helminthosporium turcicum) Attacks sorghum, Sudan-grass and maize. The causal fungus is carried on the seed and also lives in the soil on dead or decaying plant material. It may cause seed rot and seedling blight, especially in cool and excessively moist soil. Seedlings then can become infected readily and may either die or develop into stunted plants. Small reddish-purple or yellowish-brown spots usually develop on the leaves of infected seedlings.

The spots may join to kill large parts of the leaves, which then dry to the extent that severely affected plants look as if they have been burnt. A greenish, mould-like growth of spores develops in the centre of the leaf spots during warm, humid weather. The spores are spread by wind or rain and infect other leaves and plants. Under warm, humid conditions the disease may cause serious damage by killing all leaves before plants have matured.

It attacks all groups of sorghum including Sudan-grass and Johnson grass. Galls formed by loose kernel smut are long and pointed. The thin membrane covering the galls usually breaks soon after galls reach full size. The dark-brown spores contained in the galls are wind-blown away leaving a long, dark pointed, curved structure (called columella), in the central part of the gall. As in covered kernel smut, the spores of the fungus are carried on the seed and germinate soon after the seed is planted and invades the young sorghum plant. It continues to grow unobserved inside the plant until heading, when the long pointed smut galls appear in the heads in place of normal kernels. Unlike covered kernel smut, this disease stunts the infected plants and often induces abundant side branches.

Maize dwarf mosaic is a virus disease that occurs over all the sorghum producing areas. Its ability to cause damage is dependent on the presence of an over-seasoning virus host (mainly Johnson grass), aphid populations to facilitate virus transmission and the susceptibility of the varieties being grown.

Affected plants have mottled (light green blotches) terminal leaves. These alternate light- and darker-green areas in the leaf can be more easily seen when held between the viewer and a light source. Observers should always look at the newest leaves for the most severe symptoms. Highly susceptible hybrids are stunted with chlorotic symptoms in the upper leaves and suffer significant yield losses. Some hybrids produce a red leaf symptom when plants are infected and when night temperatures are below 13°C .

A number of bugs feed on the milkripe sorghum grains: shield bugs including stink bugs (Nezara viridula, Acrosternum spp), Mirperus jaculus, Riptortus dentipes, Lygus bugs, blue bug (Calidea degrii) among others. The bugs puncture the seeds and suck the contents. Feeding punctures remain as dark spots on the testa. The seed weight is reduced; the rate of germination may be depressed. Sorghum is most susceptible to bug damage during the milk and soft dough stage. Injury normally is not damaging from hard dough to maturity. The damage is only of economic importance when bugs are present in large numbers.

The parasitic weed Striga is a major pest of sorghum, particularly in Africa, where severe infestations can lead to land being abandoned. Striga attaches itself to sorghum roots depriving it of nutrients and preventing them from establishing and growing properly.

Rust appears on leaves as small raised pustules or blisters that rupture and release many reddish-brown spores. These pustules occur on both the upper and lower leaf surfaces. This disease usually appears when plants near maturity and infection is confined primarily to mature leaves. Grain yield losses are usually not serious and occurrence of the disease is sporadic. Forage sorghum yields may be affected most. The rust fungus also attacks Johnson grass and over-seasons on this host.

It is the most important pest of sorghum at the seedling stage. The adult is similar to the housefly, smaller in size (3-5 mm long), and greyish in colour, and abdomen yellow with brown patches. The larvae or maggots are yellowish or whitish in colour, up to 8 mm long. The fly lays eggs either at the base of young shoots near soil surfaces, or in older plants, on the leaves. The maggots crawl inside the sheath and bore into the heart of the young shoot killing the growing point and the youngest leaf, which turns brown and withers. This damage is known as "dead heart". When good growing conditions prevail the young plants are usually able to compensate the damage by producing new tillers, which may partly escape attack, but later the ripening of the ear heads will be unequal. In weak plant repeated infestation may cause serious losses. Sometimes the damage is so severe that many seedlings die and the field has to be replanted. Older plants (over 30 days after seedling emergence) are generally not damaged by the shoot fly. However, when shoot flies are abundant (during the rainy season under moderate temperatures and high humidity) older plants may be attacked, but they do not produce the dead-heart symptoms. Instead, the damaged leaf becomes thin and papery, and wraps around the other leaves. As a result, the plants may fail to grow normally. Late infestations may also damage the panicle in the formative stage, resulting in rotting or drying up of a portion of the panicle affected by shoot fly damage.

It is reported as one of the most important pests of sorghum in some countries, whilst in others (e.g. Ghana) is considered a sporadic pest. Nearly 30% of sorghum grain was damaged by sorghum midge in 1990 in western Kenya. In southern Africa, there are reports of 25% of sorghum grain damaged by sorghum midge (CABI, 2000). The adults are small (3 mm long), deep-red midges, with transparent wings. Eggs are laid in the flowering heads. The small orange larvae feed in the developing seed. Attacked seeds become shrunken and flat resulting in empty or "chaffy heads" as shrivelled grains fail to develop. The larva pupates inside the spikelet, and before adult emergence, the pupa wriggles its way to the tip of the spikelet. After adult emergence, the pupal case remains attached to the chaffy spikelet. Thus, damaged panicles have small, transparent midge pupal cases attached to the tip of the damaged spikelets.

Sorghum is attacked by several species of stemborers. The most important species include the spotted stalkborer (Chilo partellus), the pink stalkborer (Sesamia calamitis) and the maize stalkborer (Buseola fusca).

The feeding activity of the caterpillars inside the stems causes stunted plant growth, sterile or poorly developed ear heads. Plants may dry and die if the infestation is severe.

Sorghum is very susceptible to damage by storage pests, the main ones being greater grain weevils, in particular the rice weevil (Sitophilus oryzae), the flour beetle (Tribolium castaneum) and the grain moth (Sitotroga cerealella). Heavily attacked grain loses much of its content and become unfit for sale and consumption.

Termites, also known as white ants are common pests of upland rice in West Africa where they may cause serious damage during dry periods. They may also occur in lowland areas in light texture soils. They generally attack plants in their later growth stage by hollowing out their root system and filling it with soil resulting in the lodging of the rice plants. The attacked plants are then predisposed to further damage by ground-dwelling pests such as rodents, ants, and secondary infection by fungi and bacteria. Damaged plants can easily be pulled up by hand because the roots are severed.