Combating pigeonpea mosaic disease

VIROLOGISTS AT the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, Andhra Pradesh in collaboration with the scientists at the Scottish Crop Research Institute, Scotland, U.K., have made a breakthrough in the identification of the causal organism for the deadly pigeonpea sterility mosaic disease.

What was baffling the plant pathologists and virologists for well over seven decades, since the disease was first reported in the erstwhile Madras Presidency, is now confirmed to be a crippling disease caused by a virus, which is named as pigeonpea sterility mosaic disease virus (PPSMV). This crucial finding was possible with the application of most advanced biotechnological tools. The information generated has led to the development of efficient monitoring and screening technologies, which will be highly useful in developing highly stable resistant-varieties of pigeonpea. It will signal a major step towards an integrated approach to manage sterility mosaic virus, wilt and podborer problems of pigeonpea. These efforts will contribute to sustainable and environmentally sound methods of pigeonpea production and enhance income of the poor farmers in the semi- arid tropics, say Dr. D.V.R. Reddy and Dr. P. Lava Kumar, Virology Unit at ICRISAT.

Pigeonpea sterility mosaic disease was first reported in 1931, and this scourge caused severe economic loss to the growers. Infected plants will not flower and thus there will no pod formation. Some leaf symptoms will be manifested. An estimated US $ 100 million worth of pigeonpea was lost annually due to this disease. Almost all varieties of pigeonpea have been found to be susceptible to this disease. Until recently, scientists could not find out the causative agent, but identified its transmitting agent as a mite. The only method of managing the disease was through host-plant resistance. The resistant-varieties from one region have been found to be susceptible in other pigeonpea belts.

ICRISAT scientists are working on this disease for over a decade now, and have closely examined the biodiversity of the transmitting agent of the disease, a mite, and several resistant varieties of pigeonpea including its wild relatives. We collected the mites from different pigeonpea growing regions such as Myanmar and Nepal, and found that were no molecular difference between them. Then we found out that it has to be the pathogen that is contributing to the biodiversity. Between 1996 and 1998 tremendous progress has been made in mite-transmitted virus diseases, and in 1999 with the help of new tools of biotechnology, the unique virus that was causing this devastating disease was isolated, explains Dr. Reddy and Dr. Lava Kumar.

Normally viruses do not cause sterility in plants. This is the only virus, which causes sterility in plants. This extremely unstable virus has an interesting morphology, and it interferes with the flowering mechanism in plants. This virus exists as different biotypes in different regions. A similar virus, which attacks wheat in the US was identified in 1996, but it does not cause any sterility. It leads to only leaf symptoms, according to Dr. Reddy.

The Department For International Development (DFID), UK, has given a grant of 200, 000 Pounds for a three-year research project on sterility mosaic disease of pigeonpea starting from 1999. Sri Venkateshwara University, Tirupathi, will be another collaborator in this research programme. The project will help in full characterization of the virus and also help in developing simple diagnostic kits for detecting and differentiating the biotypes of the virus. Under the project, broad-based and stable mosaic resistant pigeonpea varieties will be identified, and technology transfer to national agricultural research system and the non- governmental organizations will be initiated. It will also help in supplying promising, disease-resistant pigeonpea seeds to the farmers, according to Dr. Reddy.

The pigeonpea sterility mosaic disease, is one of the most serious scourges of this important pulse crop. (Inset) The symptoms of the crippling virus disease manifested in the leaves.