Tackling Asia's Pigeonpea Plague
Pigeonpea (Cajanus cajan) has been adopted as a drought-tolerant grain legume by millions of smallholder farmers in the subtropical Indian subcontinent, and in some parts of Eastern Africa and the Caribbean. It is also grown to a minor extent in the Americas and Australia. Pigeonpea grain provides the major source of dietary protein for an estimated 1.1 billion people, most living in subcontinental Asia. The crop also has diverse uses beyond human food, and every part of the plant provides revenue to growers.
Production has been prejudiced by sterility mosaic disease (SMD), first reported in 1931 from India and Myanmar, but now a serious threat in all pigeonpea growing countries in Asia. SMD is so called because of the resulting lack of flower production and the mosaic symptoms that appear on leaves. The pathogen stimulates excessive vegetative growth, though infected plants appear to have a normal growth pattern. The leaf symptoms are often not conspicuous under field conditions and farmers are often unaware of the problem until the flowering stage, when the crop fails to produce flowers. Unaware of the nature of the disease the hapless farmers may resort to application of chemical sprays with the hope of inducing flowering, but this is a futile exercise and adds to their losses. In cases of partial infection, plants do produce some flowers but the seed formed is shrivelled and poor in quality.
SMD is spread rapidly by a tiny arthropod mite, Aceria cajani. Since affected plants remain green, the disease is also known as 'green plague'. Pigeonpea losses worth over US$300 million per annum are attributed to SMD in India and Nepal alone.
Advances towards sustainable disease management
The cause of SMD remained a mystery despite many decades of research, preventing the development of diagnostic tools essential for selection of resistant varieties. Additionally, various geographic variants of SMD occur, and host-plant resistance to these isolates has been hard to find. But recently, ICRISAT-led research, with a consortium of partners from the national and international centres, funded through the Crop Protection Programme (CPP) of the United Kingdom Department for International Development (DFID), has made vital breakthroughs in the identification, detection and transmission of the SMD causal agent, now confirmed as pigeonpea sterility mosaic virus (PPSMV). This, in turn, has enabled the development of procedures for precise identification of the virus and some of its isolates, and also in the selection of broad-based SMD-resistant pigeonpea cultivars that resist 'sterility' in the endemic areas.
Several resistant cultivars have now been identified to possess durable resistance, and six of them (ICP 7035, ICPL 87051, 96053, 96058, 99050, 96061) are being evaluated on-station and on-farm at several locations in India. Of these, ICP 7035 is a medium-duration (160-175 days maturity) landrace, with broad-based SMD resistance that is suitable for cultivation in diverse agroecological regions; it can be used for vegetable and seed purposes and also as a broad-based source of resistance in breeding programmes. The remaining five varieties are resistant to the Patancheru (P) isolate, prevalent in central India, and also to wilt, caused by the fungus Fusarium udum.
Efforts to tap SMD resistance in wild pigeonpea germplasm resulted in identification of seven accessions (ICP 15615, 15626, 15926, 15700, 15701, 15725, 15734) from two wild species, Cajanus albicans and C. scarabaeoides, which have broad-based SMD resistance and also possess resistance to wilt and insect pests. These can be used as potential parents in disease resistance breeding programmes. Meanwhile, efforts are continuing to identify more resistant lines for testing and adoption by farmers in diverse agroecological regions.
Additionally, village-level training programmes and field days have been organised to increase awareness among farmers about SMD and its control, including promoting use of locally available natural ingredients as biopesticides. Resistant cultivars have also been provided. And, in major pigeonpea growing areas, 'seed-village' programmes have been initiated to sustain the seed production of elite varieties at local level to overcome the heterogeneity of seed.
This holistic approach to containing SMD is paying dividends in the form of increased revenue to farmers, resulting from reduced input costs and increased production. Consequently, through cultivation of varieties that retain 'fertility' in endemic areas, and produce good harvests, farmers can now see a possible end to the 'sterility' in pigeonpea crops, which has devastated pigeonpea yield over many decades.
Article submitted by Dr P Lava Kumar. For further information e-mail: email@example.com