Variation in Phytophthora infestans : sources and implications

The oomycete pseudofungus <em>Phytophthora infestans</em> (Mont.) de Bary, the causal organism of late blight, is considered to be one of the most devastating pathogens affecting potatoes and tomatoes worldwide. In Europe, the pathogen caused severe epidemics on potatoes after its introduction in 1845. Late blight management became much more troublesome after the introduction of another <em>P. infestans</em> population of Mexican origin. The present population of <em>P. infestans</em> in the Netherlands and an increasing number of other European countries consists of both A1 and A2 mating type isolates and sexual reproduction has been reported. The goals of this thesis were to study late blight epidemiology and population biology of the present <em>P. infestans</em> population. Therefore, sources and patterns of variation for pathogenicity in sexual <em>P. infestans</em> populations were studied and the impact of increased levels of aggressiveness on the stability of partial resistance to late blight <em></em> was investigated. <em></em> Results showed that considerable levels of variation for aggressiveness are maintained in regional populations of <em>P. infestans</em> in the Netherlands and that oospores are readily produced in field crops and volunteer potato plants. Oospore production and viability proved to be highly dependent on combining abilities of parental strains and there are indications that oospores are able to survive in soils for three to four years. Increased aggressiveness and cultivar-by-isolate interactions between partial resistant potato cultivars and <em>P. infestans</em> strains negatively affect the stability of resistance. Population genetic studies in its centre of origin and diversity revealed differentiation within the Toluca Valley implying host specificity. From the results presented in this thesis, it is clear that the introduction of a sexually reproducing population of <em>P. infestans</em> in the Netherlands has had a major impact on late blight epidemics and population biology of the late blight pathogen. Sexual reproduction has therefore led to a genetically more diverse population of <em>P. infestans</em> in the Netherlands that is marked by an increased adaptability to host and environment.