Epsilon Open Archive

Abstract

Phytophthora infestans is a rapidly evolving and highly adaptable pathogen. It is the cause of late blight, one of the most devastating diseases in potato production. Depending on whether both mating types are present or not, P. infestans can undergo both asexual and sexual reproduction. In most parts of the world the asexual part of the life cycle is the dominant reproduction system resulting in dominant clonal lineages. However, earlier reports indicate that sexual recombination of the late blight pathogen occurs in the Nordic countries. This thesis includes studies on how this will affect the population biology of P. infestans. The results show that the genotypic variation of P. infestans in the Nordic countries is high. The highest variation was observed within fields, and no dominating clonal lineages were found. In a field trial planted with artificially inoculated seed, the genotypes originating from the infected tubers had a minor impact on the population of P. infestans during the season. Immigrating genotypes, which probably originated from potato crops infected by oospores, proved to be more important for the epidemiology of the disease. The presence of the alternative host (hairy nightshade) was shown to result in an increased oospore production and a higher aggressiveness of late blight on potato. From the results it can be concluded that oospores play a major role in the population biology of the late blight pathogen in the Nordic countries. Furthermore, in a study of the variation in effector genes of P. infestans, indication of selection pressure towards losing intact Avr4 genes was found. In all studied isolates this frame shift mutation was observed which means that all isolates would be able to infect plants with the R4 resistance gene.

The population biology of the late blight pathogen in the Nordic countries is complex and differs from that in many other parts of the world. The difficulties to control this disease are numerous and the nature of the Nordic population of P. infestans threatens to further add to this problem.