Since 2000 an increasing decline and dieback of alders (Alnus glutinosa) has been observed in Poland. Woodlots were monitored in 2004 and 2005, and sixty-eight isolates of Phytophthora were obtained from diseased alder trunks and from the surrounding rhizosphere. Ten Phytophthora species were identified: seven isolates P. alni subsp. alni, five P. cactorum, one P. cambivora, one P. cinnamomi, eight P. plurivora, four P. gonapodyides, seven P. megasperma, seven P. pseudosyringae, seven P. syringae and twenty-one P. lacustris. P. alni subsp. alni and P. cambivora were only found in samples of diseased alder trunks, whereas P. cactorum, P. plurivora, P. megasperma, P. pseudosyringae, and P. syringae was isolated from both tree and soil samples, and P. gonapodyides, P. cinnamomi and P. lacustris only from the soil samples. All mentioned above species colonized leaves and young shoots of alder but with different levels of aggressiveness. P. alni subsp. alni was the most aggressive in its colonization of shoots, and P. plurivora from alder trunks caused the largest necrosis on leaves. The isolates of P. plurivora from alder trunk tissues colonised leaves more rapidly than the isolates from the rhizosphere. P. gonapodyides, P. megasperma, P. pseudosyringae and P. syringae were the weakest colonisers of excised leaves and shoot segments of A. glutinosa.

Key words: Phytophthora, pathogen; identification, decline and dieback alder; pathogenicity