Characterization of Polymyxa graminis involved in the transmission of peanut clump virus in tropical areas

The plasmodiophoraceous fungus, Polymyxa graminis Ledingham, is involved in the transmission of Peanut clump virus (PCV), a soil borne virus widely distributed in India and in West Africa. This fungus is known in temperate aras to be the vector of soil-borne viruses on barley, wheat and oat. Ecological requirements of P. graminis isolates from various origins are possibly different. Indian and African isolated indeed grow easily at 25-30°C°, whereas those from temperate areas are favoured by temperatures between 15 and 20°C. Host range of tropical isolates is apparently wider and less specific than that of isolates from temperature areas. Indian P. graminis isolates grow on monocotyledonous plants as well as on dicotyledonous plants, whereas in temperature areas, two fungal species are listed as separate species according to the host specificity; P. graminis growing only on graminaceous plants and Polymyxa betae Keskin on Chenopodiaceae. In order to clarify the taxonomic position and precise the requirements of P. graminis associated with PCV transmission, pure strains obtained from a single cystosorus are prepared from isolates of various origins. Host range, ecological requirements, such as temperature optimum and pH, are studied. First results confirm differences in the temperature optimum, the Indian strain growing best on sorghum between 20 and 30°C compared to less than 20°C for the strains from northern origins on barley.