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Late blight caused by Phytophthora infestans (Mont.) de Bary is one of the most serious potato diseases. Late blight epidemics can result in a complete yield loss. Therefore in, developed countries the disease is controlled by a large number of fungicides. The most effective way of reducing fungicides input is the Integrated Pest Management (IPM). The IPM enhances the role of non-chemical treatments including the use of resistant cultivars. Growing such cultivars can significantly reduce the amount of applied fungicides without losses in yield. But to ensure effective utilization of resistance as an element of IPM, the knowledge on type of resistance, real level of this resistance and stability of its expression in various environmental conditions is required.
The formation of phenotypic structure of P. infestans population in Poland was determined by analyzing 1603 isolates collected from 1987 to 2001. The race complexity, low at the beginning of experiment, has been increasing from year to year and reached in 2001 a high level 7.2 virulence factors per isolate. The single and less composed races dominating first1y in the population were replaced gradually by more composed races. The virulence factors 1, 2, 3, 4, 7 and 11 occurred most frequent1y, but factors 5 and 8 were noted sporadically. The A2 mating type was detected in 1988 at first and since that time its occurrence has been noted in Polish population each year. The oospores were formed in potato tissues. Race diversity, low at the beginning of the investigation, reached a peak in 1996-2001. During 1987-1990 weakly and middly aggressive phenotypes dominated in Polish population. In the later period very aggressive isolates were more frequent. Phenotypic race similarity of P. infestans populations in 1987 and 2001 was very low. The race structure of 1987 population was totally different from the race structure of populations of the next years. it was probably due to migration of new pathotypes. On the other hand the variation in complexity, diversity and similarity of races, as well as in aggressiveness observed in later years of investigation can be caused by the presence of both mating types and sexual recombination.
In Potato Integrated Pest Management system, the use of potato cultivars resistant to foliage and tubers infections caused by Phytophthora infestans is recommended. However, these both traits are not highly correlated. In presented here study, foliage and tuber resistance of examined potato cultivars and clones showed low correlation. The majority of tested cultivars were highly resistant to causal agent with regard to foliage, but tubers of these cultivars demonstrated moderate or high susceptibility. Among the examined breeding lines, five clones with concurrent resistance of both foliage and tubers were identified. Such clones are highly valuable as donors of tuber and foliage resistance in breeding programs.
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