New Agriculturist: Focus on . . .

Focus on . . .
Crops of the Semi-arid Tropics.

The Green Revolution of the 1960s and 70s was based on the dramatic yield increases of maize, rice and wheat made possible by combining outstanding plant breeding developments in Japan, Mexico (CIMMYT), the Philippines (IRRI) and the US with use of agronomic inputs. Other cereals and starchy tubers and legumes were not then deemed such a priority. Climatic changes in recent decades and especially the greater propensity for erratic rainfall and drought in the tropics, have helped focus attention on crops somewhat overlooked in the past: sorghum, the millets, pigeonpea, chickpea and groundnut.

Research on these and certain other crops has been the mandate of two of the CGIAR centres - ICARDA based at Aleppo, Syria and ICRISAT based near Hyderabad, India. ICRISAT Research, including that of its African centres at Niamey, Niger and Bulawayo, Zimbabwe are featured this month in Focus on . . .

CGIAR - Consultative Group on International Agricultural Research
CIMMYT - International Maize and Wheat Improvement Center
ICARDA - International Centre for Agricultural Research in the Dry Areas
ICRISAT - International Crops Research Insitute for the Semi-Arid Tropics
IRRI - International Rice Research Institute

Sorghum - better breeds for better yields

Sorghum breeding efforts at ICRISAT are concentrated primarily on resistance to the various stresses that sorghum encounters throughout the world: insect resistance, disease resistance and, of course, Striga resistance which is perhaps the major constraint to sorghum production globally and certainly throughout Africa. Sources of host plant resistance to Striga in sorghum have been identified and some progress has been made in incorporating that resistance into improved cultivars. But to-date there are no cultivars that will stand up to high levels of Striga.

As a parasitic weed, Striga germinates in the soil in response to an exudation of a chemical from the roots of the sorghum. So one of the mechanisms ICRISAT breeders are targeting are cultivars which show low levels of this germination stimulant: those cultivars are attacked by fewer Striga plants. There are also other resistance mechanisms, for example some strains have mechanical barriers to the attachment of the Striga to the sorghum root. There are also hypersensitive reactions where the sorghum root dies off when the Striga plant attaches so that, in effect, some sorghum plants are able to let die, and sacrifice, that bit of the root that has been infected. This therefore isolates and kills the Striga.

The breeding programme has now achieved moderate levels of resistance in partially improved sorghum lines. The focus of resistance strategies is now on developing molecular markers for the various resistance mechanisms so that breeders can begin to 'pyramid' resistance mechanisms and improve the levels of resistance that can be achieved in improved cultivars.

ICRISAT's sorghum breeding programme has worked very extensively for over 20 years on insect pest resistance and the major success has been in the area of sorghum midge resistance. Midge is an insect that attacks the developing grain on sorghum, laying its eggs at flowering time. The larvae then eats the developing grain. ICRISAT has managed to identify good sources of resistance to this pest and to incorporate that resistance into high yielding agronomically desirable cultivars.

In diseases, the main focus of work has been on resistance to grey mould and to anthracnose. Grey mould has not been a problem in the past when sorghum was always harvested in the dry season but it has become an acute problem in areas where improved sorghum cultivars have been adopted by farmers. These are short duration cultivars that have been developed to avoid drought stress at the end of the growing season in order to improve yield. But this exposes the developing and maturing grain to wet and humid conditions during the ripening phase, and hence the development of grey mould.

Finally, the leaf disease anthracnose, which is very common in much of West Africa and Latin America, is a difficult problem to deal with because the pathogen is variable and keeps changing. It therefore presents a particular problem for plant breeders who must attempt to keep one step ahead of the pathogen with appropriate resistant cultivars. However, the ICRISAT breeding programme is persisting in efforts to 'pin down' this constantly moving target of anthracnose.

"In India there is very good evidence of major improvements in sorghum yields through sorghum improvement and adoption of improved cultivars. Yield levels have more than doubled over the course of the last 20 years, particularly since the introduction of hybrid cultivars," says Dr John Stenhouse, leader of the sorghum breeding team at ICRISAT, "and we would hope that we can claim at least a part of the credit for that." Meanwhile, improved cultivars have not been widely adopted in Africa and, virtually throughout Africa, farmers continue to cultivate landrace materials. So, there is real scope for development and introduction of improved cultivars throughout the region.

Sorghum - ergot alert

The serious ear infection of cereal crops known as ergot is most commonly associated with wheat and rye but it is also a problem of sorghum. It is a disease that has been endemic in India and Africa for many years and has very recently made the transition to South America, Central America and now North America. It appeared in Australia for the first time two years ago and, everywhere, introduction has been followed by very rapid spread of the disease.

Ergot in sorghum Ergot is not only a disease that is damaging to the crop but it may be dangerous if ergot infested grain is consumed by humans or animals. There are toxins associated with the ergot fungus that are particularly unpleasant and can have very serious effects. The disease in sorghum is associated with seed production: it occurs particularly in hybrid seed production where male sterile lines are present and where, for some reason, there is a delay in pollination. That is the stage at which the fungus infects the plant, and it can lead to very high levels of infestation. The disease also has severe implications for the international seed trade if seed crops are affected.

Infested plants have the characteristic 'honeydew' that forms as an exudate from the developing grains. The result is a clubbed, very sticky head with practically no grain formation. The honeydew contains millions of spores. Seed that is produced from an infested field is likely to carry the infection and could start an epidemic elsewhere. In the field the disease is spread by both wind and by rain splashes. Spread by wind is rapid. As the honeydew drops on to lower leaves and on to the soil it infests the soil for subsequent seasons. In some of the new areas that have been infected, rates of spread of several hundred kilometres in a matter of weeks have been recorded. In the absence of sorghum, the fungus survives on wild sorghum plants and in soil.

The seed industry has responded rapidly by identifying chemical control methods and there are now effective chemicals that can be used against the disease which, when sprayed at flowering, or prior to flowering, can prevent the disease from becoming established. But, farmers should be always alert to the problem and take care when purchasing seed to ensure that is free from ergot.

Interesting results of pearl millet breeding

Breeding disease resistant millet at ICRISAT Pearl millet is one of a range of crops that has been subjected to an intensive breeding programme by ICRISAT. The objective has been to breed crops that are particularly adapted to suit the prevailing conditions in a given area where pearl millet is popular. For instance, for some areas of India where bird damage to pearl millet has been a persistent problem, a pearl millet with long bristled grain heads has been bred and is under test. The birds are discouraged from feeding on these millet grain heads by the big bristles that irritate their eyes and beaks. Farmer demand for such bristled millet varieties is expected to be tremendous.

In southern and eastern Africa there has been a surge in recent years in farmer adoption of improved pearl millet varieties. Germplasm of traditional cultivars from West Africa have been used as the basis for an improved, early maturing, drought tolerant, high yielding variety Okasahana 1, which was released in Namibia. It has proved very popular with the farmers in Namibia, which is the only country in southern and eastern Africa where pearl millet is the preferred cereal staple.

Another innovative step in the on-going battle to improve pearl millet has been in the area of molecular mapping. Downy mildew has been a particularly devastating disease of pearl millet, especially of single-cross hybrids where the disease can reduce yields by more than 40%. Together with two laboratories in the UK, The John Innes Centre in Norwich and pathologists at the University of Wales, Bangor, ICRISAT breeders have put together a molecular map for pearl millet and used it to tag genes that contribute to downy mildew resistance.

Protecting fragile soils with pearl millet

Pearl millet is unusual in that it is very well adapted to acid soils, or soils having low nutrient content or low water holding capacity. It is also more tolerant to higher temperatures than probably any other cultivated cereal. These useful characteristics mean that it is finding a new niche in some unexpected places. For example, in Brazil, which is not a traditional pearl millet growing area, there are now over a million hectares of pearl millet. Farmers in the Cerrados regions, the upland hilly tracts to the south and east of the Amazon basin, are growing pearl millet as a component of a soyabean production system. They are using the crop indirectly as a means of protecting soil from deterioration and therefore the need for further encroachment on the remaining rain forest.

Pearl millet brings nutrients up from deep within the soil and also produces a large quantity of biomass in a very short period of time. Brazilian farmers grow pearl millet for this biomass which they chop and spread over the ground to protect the soil surface from wind or rain erosion. The soyabean seed is planted through this protective mulch, which also suppresses weeds and therefore reduces the need for herbicides. Fertilizers applied to the highly weathered soils typical of the region are usually leached down too deeply within the soil and out of reach of soyabean roots. But pearl millet biomass provides structure to the soil and binds fertilizer nutrients in the surface layers where they remain accessible to the soya. Finally, the pearl millet mulch protects soil moisture from evaporation losses, leaving more for use by the soya.

The potential and problems of groundnuts in Africa

As far as groundnuts (peanuts) are concerned, Africa and Asia present contrasting pictures. The groundnut situation in Africa is very discouraging. The productivity of the crop, as well as total production, has been in decline despite endeavours by agricultural scientists and others to improve production. In contrast, in Asia, particularly in China, Vietnam and, to a lesser extent, India, productivity and area planted have all been increasing at a rapid rate.

The reason appears to be that in Asia there has been an internal demand for the crop as well as an export market. Strong government support in the form of policy and infrastructure has further encouraged production of groundnuts and, because income from sales has been reaching farmers, farmers in turn have responded by producing more groundnuts.

In Africa the marketing and infrastructure have not been good. Additionally, the disease known as groundnut rosette virus, which is carried by aphids, has ravaged crops. The disease is always present and at normal incidence of 5-6% is acceptable but, when there are epidemics, the whole crop is devastated. This has happened so many times in Africa in the past 10-15 years that farmers have lost confidence in the crop. Further problems include a very significant scarcity of good seed material. Added to that, there is the problem that when groundnuts are grown under uncertain environmental conditions, and particularly if the crop suffers drought in the latter part of growth while the pods are maturing, it becomes contaminated with aflatoxin. This has been another factor which has discouraged international trade.

Work has been continuing since the sixties to breed varieties of groundnut that are resistant to the rosette virus disease and many resistant varieties have been developed. However, seed of these varieties has proved inaccessible to farmers for a variety of reasons. If groundnut production is to be increased in Africa there is an urgent need to improve price structure and to have good quality seed available to farmers at prices they can afford.

There is, however, good potential for groundnut production since demand for vegetable oils is growing rapidly as a result of urbanization in both Africa and Asia. Groundnuts grow well in the same soil types and agro-climatic conditions as tobacco and some agronomists recognize the opportunity for groundnut to replace tobacco, especially if health campaigns reduce demand for tobacco. Moreover, groundnut, with its capacity to fix nitrogen, helps maintain soil fertility whereas tobacco is a soil depleting crop.

Go IPM for groundnuts

There is growing concern that excessive and often unnecessary use of pesticides in groundnuts is endangering the health of humans and livestock. Application of pesticides just before harvesting is particularly hazardous because the agrochemicals accumulate in the pods and can be transferred to the extracted cooking oil. Also, groundnut stover (haulm or hay) is fed to animals and the pesticide residues can contaminate their milk.

At ICRISAT, Dr G V Ranga Rao is examining Integrated Pest Management systems which would satisfactorily protect crops with a maximum of two sprays instead of the more usual four or five. The ultimate objective is to reduce pesticide use to zero.

Castor, a trap crop for pests in groundnut - Helicoverpa and Spodoptera Since the advent of agrochemicals, farmers have grown accustomed to resorting to sprays as soon as insects or insect damage are apparent in their crop. Other farmers have developed a routine of regular spraying, on a calendar basis, whether pests are seen to be present or not. Such misuse of pesticides is both expensive and counter-productive since beneficial insects are often equally and, sometimes, even more susceptible than the pest. The most resistant pests usually survive in sufficient numbers to rapidly build up new populations, unhindered by the predators that would previously have kept them in check, and even more resistant to subsequent applications of pesticide.

Studies have shown that a crop of nearly mature groundnuts can tolerate up to 50% defoliation by insects without yield loss. There is therefore, little point in spraying at this stage. The crop is most susceptible to insect damage during the first 20 to 30 days following germination and application of pesticides should be restricted to this period of growth, and only if pests are present in sufficient numbers to constitute a real threat.

The larvae of the two main defoliators of groundnut (Helicoverpa and Spodoptera litura) actually prefer to feed on castor and sunflower. Furthermore, when eggs of Spodoptera laid on castor or sunflower hatch the larvae remain on the same leaf without dispersal for ten days. If eggs are laid on groundnut, the larvae disperse in the crop immediately after hatching. This difference in behaviour offers farmers a way to easily monitor a pest presence and control it. A mass of 200-300 larvae on a castor or sunflower leaf are easily visible to farmers walking through their crops and the leaf can be taken from the sunflower for disposal by crushing or burning. Therefore farmers should plant castor or sunflower among their groundnut as a 'trap' crop for these two pests.

Despite trap cropping perhaps one-third of the adult pests present will lay eggs on the groundnut crop and farmers should be alert to collect egg masses on the leaves. Many will undoubtedly be missed but the caterpillars fall prey to insect-eating birds. These can be encouraged by providing perching posts within the crop. Relying solely on trees and fences on field boundaries to provide perches will be insufficient for good control because birds will only feed within a limited distance of a perch.

There is the further option of using bio-pesticides in place of chemicals. Neem extracts are used successfully in cotton, tobacco and vegetables, and can equally well be used to protect groundnuts. Sprays of naturally occurring viruses such as nuclear polyhdyrosis virus for Spodoptera and Helicoverpa are also options and are specific to the pest and do not harm beneficial organisms, nor are they hazardous to human or animal health.

Vanquishing the virus

Peanut clump virus is carried by an obligate parasitic fungus from the soil that transmits the disease by infecting roots of such plants as groundnut, pigeonpea, and wheat. In these three crops the disease causes a severe growth reduction with a yellow mosaic on leaves. Early infected groundnut and pigeonpea plants show very severe stunting and produce very few pods. The virus is also carried in seed from infected groundnut, maize, millets and wheat plants and can thus spread over large distances through seed exchange.

A 5-year collaborative project between Université Catholique de Louvain, Louvain-la-Neuve, Belgium and ICRISAT has been investigating a rotational approach to reducing or eliminating the impact of this pervasive disease.

The fungus that transmits the virus infects, but does not multiply on groundnut and other dicotyledonous crops. These crops can thus beneficially be used in the rotation to reduce the inoculum in the soil. Grassy monocotyledonous plants can also induce infection by the fungus and its multiplication and a management method has been developed using pearl millet as a 'trap' crop for peanut clump virus. Because millet seeds are small and relatively inexpensive they can be sown at high rates in infested ground and allowed to germinate. The seedlings will stimulate the fungus to develop and infect the millet plants. If the millet seedlings are then killed by hoeing, discing, or ploughing before the fungus completes its life cycle it will die and the inoculum in the soil will be reduced. Therefore the disease in groundnut crops sown immediately after ploughing the millet seedlings will be also reduced. However, it is essential to kill the millet seedlings two to three weeks after germination because, if left longer, the fungal vector of the virus begins to multiply and the soil is further infested instead of the inoculum being reduced. Trials conducted in farmers' fields in Andhra Pradesh and Rajasthan in India, and in Niger in West Africa gave very promising results; in some trials the disease was reduced from 60% to 15%.