Phytophthora Decline of Alder (Alnus Spp.) In Europe
Thomas L. Cech
Abstract: In several European countries, alder (Alnus glutinosa and A. incana) growing along rivers is threatened by a decline that is consistently related to stem necrosis caused by Phytophthora. It was first thought to be a form of Phytophthora cambivora, but morphological differences as well as its homothallism indicate a new species. High pathogenicity on A. glutinosa and A. incana have been proven by experiments in Great Britain, the Netherlands, and Germany. At present the disease is widespread in Great Britain, whereas in Denmark, France, Germany, Ireland, the Netherlands, and Sweden only a few cases have been reported. In Austria, decline of A. glutinosa was observed in 1986 and 1996. Symptoms include simultaneous crown decline, preceded by abnormally small leaves, stem necrosis, and exudates from the bark resulting in tarry spots. Cracks at the stem base with strong callus formation suggest earlier attacks or mechanical injuries prior to infection. Several times Phytophthora was isolated, and it showed high similarity to British strains. In addition, other bark-attacking fungi were identified, among them Phomopsis alnea, Ophiovalsa suffusa, Pleomassaria holoschista, and Pezicula cinnamomea. Probable predispositions, such as drought periods followed by heavy rainfall, and severe winter frost are discussed along with practical control measures, such as fungicide treatment.
Keywords: Alnus spp.; decline; Phytophthora; predispositions.