Phytosanitary assessment of black and grey alder stands in Latvia

Black alder Alnus glutinosa (L.) Gaertn. and grey alder Alnus incana (L.) Moench, cover up to 11% of the forest area in Latvia. As alder dieback caused mainly by Phytophthora has been reported in several countries, we aimed to assess the distribution of Phytophthora-like symptomatic alders in Latvia and surveyed 208 alder-dominated forest stands. Symptomatic alders were found in 28% of stands, and in 8% of stands, the infected tree proportion comprised more than 10% of trees. Pocket Diagnostics® tests confirmed Phytophthora or other oomycete infection in 13% of stands, mainly in Central and Eastern Latvia. Trees that tested positive were mainly grey alder (1.0% of the overall number of surveyed grey alders), and fewer black alders tested positive (0.2% of the overall number of surveyed black alders). Grey alder had a significantly higher probability of having symptoms at both the stand level (probability 0.429) and the individual tree level (probability 0.017) in comparison to black alder (probabilities 0.185 and 0.004, respectively). In conclusion, alders with Phytophthora-like symptoms were found throughout the territory of Latvia, however, infection risk for the economically important tree species black alder is low.