Sorghum Crop

SORGHUM

1. Introduction:

Ø  Benefits of sorghum

Ø  Ecological requirements

Sorghum is adapted to a wide range of environmental conditions and will produce significant yields under conditions that are unfavorable for most other cereals. Sorghum is particularly adapted to drought. Sorghum also tolerates waterlogging and can be grown in areas of high rainfall. It is, however, primarily a plant of hot, semi-arid tropical environments with rainfall from 250 mm that are too dry for maize but performs best with more than 900 mm annually. It is also grown widely in temperate regions and at altitudes of up to 2500 m in the tropics. Sorghum tolerates a wide range of temperatures. Sterility can occur when night temperatures fall below 12-15 deg C during the flowering period. Sorghum is killed by frost. Sorghum can be grown successfully on a wide range of soil types. It is well suited to heavy clay soils (vertisols) found commonly in the tropics, where its tolerance to waterlogging is often required but is equally suited to light sandy soils. It tolerates a range of soil pH from 5.0-8.5 and is more tolerant to salinity than maize. It is adapted to poor soils and can produce grain on soils where many other crops would fail.

2. Crop management practices

Ø  Planting-Recommended spacing;

Sorghum is normally grown from seed. A fine seed-bed is preferable but is often not achieved. The seed is usually sown directly into a furrow after ploughing, but can also be broadcast and harrowed into the soil. Optimum plant spacing depends on soil type and availability of moisture.

For favourable conditions, row spacing of 45-60 cm and plant-to-plant spacing of 12-20 cm, giving populations of about 120 000 plants per ha, is normal. For drier or less fertile conditions, wider spacing and lower plant populations are usually optimal. The seed rate varies from 3 kg/ha in very dry areas to 10-15 kg/ha under irrigation. Occasionally, seedlings are grown in a nursery and transplanted into the field early in the dry season, e.g. on the flood plains around Lake Chad in Africa.

Seed rate is 7-10 kg/ ha or 3-4 kg/acre. Dry planting is highly recommended. Thus plant before or at the onset of rains by either drilling in the furrows made by tractor or oxen plough. When dry planted, planting depth should be 5 cm but when planting in moist soil use planting depth of 2.5-4 cm. In semi-arid areas where the ox plough yoke is fixed at 90 cm, the recommended spacing between plants is 15 cm. 

Ø  Fertilizers/Manure rate;

Subsistence farmers rarely apply fertilizer, as responses depend on moisture availability, which is usually very uncertain. Under more favorable conditions, farmyard manure is used with advantage, but even so, the quantities used are usually below optimum. Optimally sorghum needs the availability of about 20 kg N/ha and 20 kg P/ha at planting time, which can be supplied by alternate cropping with legumes and application of compost or manure. Also intercropping with legumes is recommended with grain legumes such as beans, cowpeas, pigeon peas, and green gram. Manure and compost improve organic matter content of the soil, soil moisture retention ability, and soil structure. Manure can be broadcast in the field or applied in planting furrows and mixed with soil before seeds are planted. The standard farm wheelbarrow when full holds approximately 25 kg of dry manure/compost.

Ø  Weed Control

Purple witchweed (Striga hermonthica)

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.

Control:

Striga can be controlled manually by vigorous uprooting before it produces seeds and/or by intercropping sorghum with fast-spreading legumes which deprives the weed of sunlight and exude chemical substances the reduce striga growth. Desmodium for the instance has been shown to depress striga almost completely. Considerable efforts have been dedicated to developing resistant varieties.

The crop is usually weeded by a combination of inter-row cultivation with animal-drawn implements and hand weeding within rows. Thinning is carried out at the same time as hand weeding, or at intervals during the crop cycle, particularly where thinning are used to feed livestock. Gapping by transplanting thinning is encouraged when thinning is done within 2 weeks after emergence and when the soil is moist.

Ø  Insects and their Control;

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.

1.      African armyworm (Spodoptera exempt)

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.

Control

ü  Monitor regularly field margins, low areas where plants have lodged, beneath plant debris around the base of plants, on the ground, and underneath the plant leaves.

ü  Check daily young crops if conditions are known to be favourable to the past outbreaks.

ü  Spray Bt or botanicals such as neem and pyrethrum extracts.

ü  Spray when caterpillars are small. Once caterpillars are mature (about 3 to 3.5 cm long) they may have caused serious damage and it may no longer be economical to treat the crop.

ü, Conserve and encourage natural enemies.

ü  Practice field sanitation.

  

African armyworm (Spodoptera exempt). Mature caterpillars measure up to 4 cm.

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2        African bollworm (Helicoverpa armigera)

ü  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.

Control

ü  Inspect field once or twice a week after sorghum begins to bloom. Check for presence of caterpillars by shaking heads over a bucket or sweep net.

ü  If necessary spray with Bt or neem extracts. Good spray coverage is very important, particularly in partially opened heads or in varieties that have tight heads where young caterpillars are well protected

ü  Handpick and destroy the caterpillars. This helps when their numbers are low and in small fields.

African bollworm (Helicoverpa armigera) on beans. Caterpillars are 3 to 4 cm in length

3        Stemborers: Spotted stemborer (Chilo partellus)

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.

Control:

ü  Early planting to ensure maximum pest escape.

ü  Use resistant varieties.

ü  Habitat management. Intercropping sorghum with pulses (cowpeas, groundnuts) in alternate rows, may reduce stemborer incidence by 20-30%.

ü  Sanitation (destruction of crop residues, volunteer plants and alternative hosts). Crop residues (stalks and other residues) should be destroyed after harvest through burning the stalks, or fed to livestock. However, burning of crop residues may not be practical in communities where soil fertility is low and no fertilisers are used since crop residue is the only source of organic matter.

ü  Biological control. Recent research work on stemborers has been focussing on the introduction exotic parasitoids in countries where Chilo partellus is wide spread.

Spotted stemborer (Chilo partellus)

4        Bugs

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.

Control:

ü  Use neem-based pesticides. Reportedly they reduce feeding by green shield bugs

ü  Check for bugs by beating or shaking panicles over a sweep net or bucket.

Green stink bug (nymphs and adults). Adults are about 1.2cm long. (Host: Pearl Millet)

The sorghum aphid (Melanaphis saccari) and the maize aphid (Rhopalosiphum maidis)

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.

Control:

·         Conserve natural enemies. Parasitic wasps and predatory insects, including lady bird beetles, damsel bugs, lacewings, and hover fly larvae are important in natural control of aphids.

The maize aphid Rhopalosiphum maidis. Colony on leaf of maize Image

5        Sorghum midge (Stenodiplosis sorghicola)

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. The adults are small (3 mm long), deep-red midges, with transparent wings. Eggs are laid in the flowering heads.

Control:

ü  Synchronised planting. Epidemics of sorghum midge damage are common within an area, when sorghum is not planted at the same time, or different cultivars are planted that do not mature at the same time. Sorghum that is planted and flowers later than normal is exposed to sorghum midge for a longer period and can suffer severe damage.

ü  Planting density and thinning. Midge damage is reported to be higher in crops with low plant density (CABI, 2000).

ü  Selective removal of alternative hosts. Wild species of sorghum (for example, S. halepense and S. sudanense) act as alternative hosts for sorghum midge.  Removing these alternative hosts from the vicinity of the sorghum crop can reduce the rate of multiplication of sorghum midge populations. Field sanitation. Crop residues should be collected and destroyed to reduce the carryover of larvae in the chaffy spikelets from one season to another.

ü  Fallowing and close season. Fallowing reduces the carryover and build-up of midge populations from one season to the next. However, this is not a feasible practice for smallholders due to the shortage of land.

ü  Crop rotations. Sorghum is generally rotated with cotton, groundnuts, sunflowers or sugarcane. This may reduce the damage caused by the sorghum midge.

ü  Mixed cropping. Damage by the sorghum midge is reduced when sorghum is intercropped with leguminous crops (CABI, 2000)

Sorghum midge damage

Ø  Diseases and their Control:

1.      Anthracnose (Colletotrichum graminicola)

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.

Control:

ü  Use resistant hybrids.

ü  Rotate with non-cereals preferably with pulses.

ü  Good management of crop residues.

Covered kernel smut (Sporisorium sorghi)

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.

Control:

ü  Use of certified disease-free seed.

ü  Plant resistant hybrids.

Loose kernel smut (Sphacelotheca cruenta)

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.

Control:

ü  Certified disease-free seeds.

ü  Plant resistant varieties.

ü  Control weeds.

ü  Rotation with non-cereals.

ü  Practice good field sanitation

Head smut (Sporisorium reilianum)

This disease is characterized 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.

Control:

ü  Plant resistant hybrids to avoid losses.

ü  Use certified disease-free seed.

ü  Rotate with non-cereals.

ü  Plough deep.

Charcoal rot (Macrophomina phaseolina)

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.

Control:

ü  Avoid moisture stress.

ü  Manage properly crop residue.

ü  Rotate crop with non-cereals. Legumes are also susceptible to the disease.

ü  Avoid excessive plant populations.

ü  Balance nitrogen and potassium fertility levels.

ü  Grow drought-tolerant, lodging-resistant hybrids.

Rust (Puccinia purpurea)

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.

Control:

ü  Use resistant varieties.

ü  Rotate with non-cereals.

ü  Control weeds.

Ergot (Claviceps sorghi)

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 20-30degC .

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.

Control:

ü  Plant resistant varieties, where available.

ü  Remove affected panicles.

ü  Avoid planting seeds from infected panicles.

ü  Plough deep.

ü  Rotate with non-cereals preferably with pulses.

ü  Practice good field sanitation

Leaf blight (Helminthosporium turcicum)

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.

Control

ü  Plant resistant varieties.

ü  Use certified disease-free seeds.

By Abdulkaremathey.