Genotypic and phenotypic variation of Phytopthora infestans on potato in the two Swedish regions Bjäre and Östergötland in 2015

Abstract: Potato late blight, caused by the oomycete Phytophthora infestans, is a very im-portant disease on potato, and one of the most devastating plant diseases in agri-culture worldwide. Despite the small area of potato in comparison with other cul-tivated crops, 40 % of the total use of fungicides in Sweden is used on potato. The aim of this master thesis was to compare populations of P. infestans in two geographically separate regions in Sweden; Bjäre and Östergötland. Bjäre is known for its intense potato production and early infections of P. infestans, whereas in Östergötland, infections of late blight occur much later in the growing season. It is believed that the population of P. infestans on Bjäre has an increased sexual reproduction as a result of the cultivation practises in the region. The popu-lation structure of P. infestans was determined by the phenotypic and genotypic characterisation of samples collected from potato fields in the two different areas. The phenotype was assessed through aggressiveness tests and the genotype was determined by using molecular markers (microsatellites). The results show that the early infections observed on Bjäre could have been de-rived from oospores or tuber infections, whereas in Östergötland the primary source probably was caused by asexual sporangia. The population of P. infestans in Östergötland showed a higher genotypic diversity than the population on Bjäre, which opposed the hypothesis. Weather conditions favouring an asexual spread of a few oospore driven genotypes on Bjäre this year and the sampling methodology could explain the little genotypic variation found on Bjäre. In Östergötland, the observed higher genotypic variation was probably due to several infection sources, where oospore infections might have been involved during the season as a result of both mating types being present in infested tubers. The isolates from Bjäre showed a higher aggressiveness compared to the iso-lates from Östergötland when referring to the latency period, but there were no significant differences in lesion growth rate and number of spores per lesion. As seen in this study, there are many factors that will affect the success of P. in-festans; establishment, development and spread of the pathogen can vary between regions and years. This study shows that a sexual reproduction can also be of im-portance in fields with good crop rotation practices in an area that is normally dominated by an asexual spread of the pathogen.