Seasonal Variation in Pathogenicity of Phytophthora infestans

During a four year study it was shown that the mean lesion area (M.L.A.) developed on leaflets of different potato cultivars by isolates of P. infestans after artificial inoculation varied periodically during the year. Disease symptoms observed changed from pinpoint necroses to regular sporulating lesions depending on the season and the cultivar. Analysis of variance revealed that differences in lesion size between experiments performed in different seasons were significant. The largest M.L.A. was found in late spring or in autumn but autumnal increase of M.L.A. was usually significantly lower than that in spring. Periodicity of M.L.A. was accompanied by a similar phenomenon in sporulation density of P. infestans isolates cultured in vitro. However, the peaks of sporulation, in comparison with peaks of M.L.A. appeared in different seasons. The highest disease level coincided with a decline in sporulation density of P. infestans cultured in vitro and the decrease of M.L.A. corresponded with an increase in sporulation density per cm² of medium. Variation in M.L.A. found in this study seemed not to be related to variability in plant reaction. In this connection a hypothesis has been proposed that the changes of M.L.A. were due to variation in pathogenicity of the fungus.