1Central Potato Research Institute, Shimla, 171 001, (Himachal Pradesh).
2Central Potato Research Institute Campus, Modipuram, Meerut, 250 110, (Uttar Pradesh).
3Department of Botany, Meerut College, Meerut, 250 001, (Uttar Pradesh).
*Part of the Ph.D. thesis (2002) of Sr. author, approved by CCS University, Meerut, 250 005, (Uttar Pradesh).
Studies on the effect of host resistance on P. infestans aggressiveness revealed that mostly A2 mating type was more aggressive than A1 mating type. However, in some cases aggressiveness varied with host background. The aggressiveness of A2 was more on seedlings of 10 crosses, whereas that of A1 was more on seedlings of 6 crosses. The performance of mixture of A1 and A2 mating types also varied with host background. The aggressiveness of the mixture was, in general, higher than A1 or A2 alone on potato cultivars but its effect on seedlings of different crosses was not similar. No statistical differences were observed in aggressiveness of the two mating types when tested on germplasm accessions using detached leaves and tuber slice tests. It is concluded that aggressiveness of the two mating types varies with host background and the germplasm accessions need to be evaluated using both the mating types to select parents with durable host resistance for use in resistance breeding programme.
