Recent and future developments in potato fungal pathology

An overview is given on the world-wide occurrence and frequency of mating type A2 ofPhytophthora 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 ofP. 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 ofP. 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 ofP. 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 modern 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 ofP. 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 andErwinia spp. Integrated control of dry rot and gangrene seems to have a better future than the use of systemic fungicides.

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