Investigations into the declining health of alder (Alnus glutinosa) along the river Lagan in Belfast, including the first report ofPhytophthora lacustriscausing disease ofAlnusin Northern Ireland

Abstract

Additional key words: Plant health, Forest pathology, riparian, root and collar rot. AbstractCommon alder (Alnus glutinosa) is an important tree species, especially in riparian and wet habitats. Alder is very common across Ireland and Northern Ireland, and provides a wide range of ecosystem services. Surveys along the river Lagan in Belfast, Northern Ireland led to the detection of several diseased Alnus trees. As it is known that Alnus suffers from a Phytophthora induced decline, this research set out to identify the presence and scale of the risk to Alnus health from Phytophthora and other closely related oomycetes. Sampling and a combination of morphological and molecular testing of symptomatic plant material and river baits identified the presence of several Phytophthora species, including Phytophthora lacustris. A survey of the tree vegetation along an 8.5 km stretch of the river revealed that of the 166 Alnus trees counted, 28 were severely defoliated/diseased and 9 were dead. Inoculation studies using potted Alnus saplings demonstrate that P. lacustris was able to cause disease, and Koch's postulates for this pathogen-host combination were completed, which suggests a future risk to Alnus health from P. lacustris in Northern Ireland.