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. ududThe 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. ud
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机译：疫霉疫霉是迅速发展且高度适应的病原体。它是晚疫病的原因，晚疫病是马铃薯生产中最具破坏性的疾病之一。取决于是否存在两种交配类型，致病疫霉可经历无性繁殖和有性繁殖。在世界上大多数地区，生命周期的无性部分是占主导地位的生殖系统，导致占主导地位的克隆世系。但是，较早的报道表明，晚疫病病原体的性重组发生在北欧国家。本论文包括有关这将如何影响致病疫霉的种群生物学的研究。结果表明，北欧国家的致病疫霉的基因型变异很高。在田间观察到最高的变异，未发现主要的克隆谱系。在用人工接种的种子播种的田间试验中，在该季节中，源自受感染块茎的基因型对感染晚疫病菌的种群影响较小。移民基因型可能起源于被卵孢子感染的马铃薯作物，事实证明对这种疾病的流行病学更为重要。替代寄主的存在（多毛的茄属植物）被证明可以增加卵子的产生，并增加马铃薯晚疫病的侵袭性。从结果可以得出结论，在北欧国家，卵孢子在晚疫病病原体的种群生物学中起着重要作用。此外，在对P. infestans效应基因的变异进行的研究中，发现了选择压力以丢失完整的Avr4基因。在所有研究的分离株中都观察到这种移码突变，这意味着所有分离株都能够用R4抗性基因感染植物。 ud ud北欧国家晚疫病病原体的种群生物学很复杂，与世界上其他许多地方都不相同。控制这种疾病的困难很多，而且北欧疫霉菌种群的性质有可能进一步加剧这一问题。 ud
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