The aim of this study was to determine whether the weeds accompanying potato crops can be a source of Alternaria spp. causing Alternaria leaf blight and to determine the genetic similarities of Alternaria alternata isolates infecting selected weeds: Chenopodium album, Cirsium arvense and tested potato cultivar. Three-year field experiment was conducted on the potato cultivar ‘Vineta N’. The isolates were classified into different species on the basis of macro- and microscopic features. In each year of the study, A. alternata dominated among the isolated fungi colonizing the leaves of potato plants and the selected weeds. The genetic similarities of A. alternata isolates was determined by the RAPD-PCR method. Tested genetic forms of A. alternata were closely related; only small differences in the pattern of the separated amplification products was evidenced. The dominance of A. alternata on the weeds accompanying potato crops suggests that if weed infestation is extensive, the pathogen is very likely to spread and its population to increase.

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