Occurrence of Phytophthora species in the rhizosphere of dying black alder Alnus glutinosa in central and northeastern Poland

For many years, a progressive process in the decline of black alder has been observed in Poland. Symptoms of alder dieback include withering and death of branches, stunted growth, yellowing of leaves, and stagnation of the plant's development. The disease eventually leads to complete death of the alder. Previous observations and investigations indicate infections by pathogenic oomycetes of the genus Phytophthora. These organisms are responsible for damage to fine roots and necrosis of alder root collars. The objective of this study was to conduct an inventory on the occurrence of pathogens of the genus Phytophthora in the rhizosphere of dying alders. For this purpose, eight plots were selected in central and northeastern Poland from which soil was col- lected for analysis. Four of the plots were located in the Dobrocin Forest District (RDSF in Olsztyn), two in the Kutno Forest District (RDSF in Lodz) and two in the Radom Forest District (RDSF in Radom). Alder dieback was observed over a long period time on all plots together with thinning of crowns, discoloration of leaves and exudations at the base of stems. The research conducted, using the traditional method with young leaves of English oak Quercus robur and rhododendron Rhododendron sp. as plant traps, revealed the presence of four species of the genus Phytophthora: P. plurivora, P. polonica, P. gallica and P. pseudocryptogea. In addition, the presence of Globisporangium megasporum and Phytopythium citrinum was confirmed. All the aforementioned organisms have been reported to cause death of trees including black alder. In addition, two of these species P. gallica and P. pseudocryptogea were detected for the first time in the rhizosphere of dying alders in Poland. Information on the occurrence of these species is important because it increases knowledge about new pathogenic species that pose a direct threat to the persistence of alder stands in Poland.