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Phytophthora infestans (Mont.) de Bary, the causal agent of potato late blight, was first discovered in Europe in 1845. Until 1980, only Al mating type isolates were known to occur in Europe. The absence of A2 mating type isolates restrained the fungus from sexual reproduction. In the early 1980s, A2 mating type isolates were discovered in Europe. Presumably, a new introduction of P.infestans isolates originating from Mexico had taken place. In this paper, the significance of the presence of A 1 and A2 mating type iso[ates in the Netherlands is reviewed. Now that both mating types ate present, sexual reproduction can occur and its consequences for the control of potato late blight ate discussed.

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An overview is given on the world-wide occurrence and frequency of mating type A2 of Phytophthora infestans, on ploidy levels, increased virulence and fitness of the pathogen population in Germany and the Netherlands. Experiments on the survival of oospores in the open showed that these may survive in Europe until the next growing season. The resistance reaction of the differential set of R-genes used to determine virulence of races of P. infestans is found to vary by differential clone and test method. Work has been started to come to a more homogenous differential set. The impact of oospores and the new population of P. infestans on epidemiology and hence on disease forecasting is unknown.During the past 40 years, considerable research efforts have been made to reveal the biochemical pathways of pathogenesis, compatibility and incompatibility of the host-pathogen reaction of P. infestans. However, the questions about host-pathogen recognition, the genes involved and which chemical compounds cause hypersensitivity or durable resistance, have not been answered yet. It is expected that the modem molecular biological approaches will more rapidly lead to such answers. One of such approaches, RFLP fingerprinting, indicated a total replacement of the old European population of P. infestans by a new one, and revealed the origin of Mendelian aberrations found to be due to selfing when crossing isolates.The world-wide occurrence ofPhoma exigua var.foveata, the causal organism of gangrene, is discussed and a clue on its possible origin and time of introduction in Europe is given. Soil infestation is a minor but persistent source of infection and impedes elimination of the pathogen. Green-cropharvesting, a new vine killing method, considerably reduced the level of black scurf caused byRhizoctonia solani and of gangrene as compared to other vine killing methods. Promising results have been obtained combining green-crop-harvesting and the application of antagonistic organisms in controlling black scurf and Erwinia spp. Integrated control of dry rot and gangrene seems to have a better future than the use of systemic fungicides.