2.5.4S
VIRUS DISEASES AFFECTED BY ENVIRONMENT: INDIAN PEANUT CLUMP VIRUS, A CASE STUDY

P DELFOSSE1, AS REDDY1, A LEGREVE2, KT DEVI1 and DVR Reddy 1

1Virology Unit, International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru 502 324, AP, India; 2 Unite de Phytopathologie, Faculte des Sciences Agronomiques, Université Catholique de Louvain (UCL), B-1348 Louvain-la-Neuve, Belgium

Background and objectives
Polymyxa sp., an obligate root endoparasitic fungus, transmits Indian peanut clump virus (IPCV) to groundnut and to various cereal hosts in the Indian subcontinent. The vector infects and multiplies well in such cereal hosts as sorghum, pearl millet and maize. One Polymyxa isolate from Pakistan was also found to multiply in wheat. Although the fungus can transmit the virus to dicotyledonous plants, it does not multiply in them. Peanut clump disease epidemiology was studied for four consecutive seasons in India in order to develop suitable cultural practices for the management of the disease.

Results and conclusion
The disease incidence was found to be influenced mainly by distribution and quantity of rainfall, soil temperatures and date of sowing. High rainfall at the time of sowing in the rainy season contributed to high disease incidence. Conversely, groundnut crops sown early and maintained with minimal irrigation prior to the onset of monsoon rains showed low disease incidence. Dry and hot conditions prevailing during the summer season (April to June) are conducive for breaking the dormancy of resting spores of Polymyxa sp. Therefore, high virus incidence occurred even in such a fortuitous host as groundnut sown in the rainy season. Groundnut crops grown in post-rainy season, when temperatures are lower than the rainy season, showed negligible disease incidence. Nevertheless, wheat crops grown at this time can show very high disease incidence. This is attributed to low temperatures (below 23°C) which are not conducive for the multiplication of Polymyxa sp. Indeed, the fungus has a very narrow temperature range (23-30°C) for its development. In addition conditions that prevail during or prior to the post-rainy season (December to February) are not conducive for breaking the dormancy of Polymyxa sp. It is suspected that groundnut may not be able to induce the germination of dormant spores of Polymyxa sp., whereas root exudates of wheat plants can trigger the germination of resting spores even though the dormancy was not broken by favourable conditions. The implications of these findings for the management of peanut clump disease will be discussed.