Alder Decline in Austria : Disease/Environment Interactions in Forest Decline. Proceedings of a Workshop of the Working Party Disease/Environment Interactions in Forest Decline IUFRO 7.02.06, Vienna, Austria, March 16-21, 1998
According to differential diagnoses recorded at the FBVA between 1951 and 1997, lethal damage of Alnus glutinosa, Alnus incana and Alnus viridis in Austria were predominantly caused by insects mining in the stem, especially by Cryptorrhynchus lapathi, furthermore by root rot (Armillaria sp.) and dieback of the crown caused by various microfungi (Ophiovalsa suffusa and others). Decline as a consequence of phloem destruction at the stem base, which is one characteristic feature of the Phytophthora decline of Alder in Great Britain, was first observed in 1987 in Lower Austria on Alnus glutinosa. In 1996, Phytophthora sp. could be isolated from several diseased common alders of three Upper Austrian stands. Investigation plots established in these stands revealed a percentage of diseased trees between 31 and 50% and a percentage of dead trees between 10 and 17% in October 1997. Between June and October 1997 the percentage of newly infested trees varied between 3 and 44 among the stands. In most cases, infested trees seem to die after one year, although a few trees showing no symptoms in spring were dead 4 months later. The main symptom was simultaneous crown decline, prededed by small and sparse leaves. With the exception of basal phloem necrosis, none of the symptoms, that are characteristic for Phytophthora decline was constantly present on declining trees: cracks at the stem base followed by tarry spots were the most frequent symptoms; abnormal fruiting and sprouts on the stem were observed more rarely. There was obviously no connection between bending of trees due to bank erosion and the frequency of diseased trees, although this was assumed to be a phenomenon favouring infection by Phytophthora. Probable predispositions, e.g. drought periods followed by heavy rainfall and severe winter frost, are discussed.
