STATUS OF MATING TYPES AND METALAXYL RESISTANCE IN INDIAN POPULATIONS OF PHYTOPHTHORA INFESTANS AND FORMATION OF OOSPORES IN HOST TISSUE
B SINGH, S ROY and GS SHEKHAWAT
Central Potato Research, Institute, Shimla, India
Background and objectives
Throughout the world, except for Mexico , only the A1 mating type of Phytophthora infestans was known to occur which reproduced asexually and survived as mycelium in seed tubers/refuge piles. The situation changed dramatically in early 1980s, when a hitherto unknown A2 mating type was detected in most of the European countries almost simultaneously. Later, it was recorded from almost all the continents. This new strain is more aggressive and is rapidly displacing the native population in at least some parts of the world. It has been experimentally demonstrated that when the two mating types are present in equal proportion, they reproduce sexually and produce oospores in the host tissue. Whether this also holds true under natural field conditions is not known. In India, the A2 mating type was detected in 1991 . It was also demonstrated that when both the mating types are paired on rye-agar media, they produced oospores that germinated readily when passed through the digestive tract of garden snails. Our studies were designed to investigate (i) the shift in P. infestans mating type population, (ii) the relationship between metalaxyl resistance and A2 mating type, (iii) the factors responsible for oospore production in host tissue, and (iv) the occurrence of oospores in nature.
Results and conclusions
A2 mating type (new strain) was found to occur in Himachal Pradesh, Meghalaya and Darjeeling hills. The entire Indo-Gangetic plain, which accounts for 80% of the potato production in India carried a native population of only P. infestans (the A1 mating type). In 1991, the A2 mating type constituted only 5% of the total P. infestans population in the Himachal hills. It increased to 92.6% in 1995, suggesting that the new strain is rapoidly displacing the native strain. A similar situation existed in the Meghalaya and Darjeeling hills, where the A2 mating type population increased to 75%. Formation of oospores in host tissue was dependent on ambient relative humidity (r.h.) and moisture content of host tissue (<80%). They were formed only when ambient r.h. fell below 60%. No oospores were detected in the field until 100% of the crop was damaged. During this period, ambient r.h. never fell below 70%. However, oospores were detected in peelings of the potato stem of resistant cultivars which were collected in late September when ambient r.h. ranged between 40 and 70%. The evidence tends to suggest that oospores would be formed in nature wherever ambient r.h. remains low during some part of the crop season. Analysis for metalaxyl sensitivity revealed that the A1 mating type isolates were comparatively more sensitive to metalaxyl than the A2 mating type.
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