Involvement of Phytophthora species in the decline of European beech in Europe and the USA

Abstract

European beech (Fagus sylvatica L.) is an important forest tree species which was always considered being nonproblematic with regard to its susceptibility to pathogens and insects. However during the past decade, in Europe and the north-eastern USA an increasing number of trees and stands were showing symptoms typical for Phytophthora diseases: increased crown transparency, abnormally small and often yellowish foliage, a dieback of the crown, tongue-shaped necroses of the inner bark and the cambium with tarry or rusty spots on the surface of the bark either extending up to 7m from the stem base (collar rot) or occurring isolated higher up the stem (aerial bleeding cankers), fine root destructions and necrotic lesions on suberized roots. Large-scale investigations on both continents using specific isolation methods and ELISA kits for detection, and morphological and physiological parameters as well as ITS-DNA sequence analysis for identification demonstrated that several Phytophthora species were regularly involved as inciting agents of the decline. The most important species were Phytophthora citricola, P. cambivora and P. cactorum in Europe, and P. inflata in the USA. Their pathogenicity to beech was shown by stem inoculation and soil infestation tests performed by various groups. A small-scale nursery survey in Germany revealed that beech fields are regularly infested with various Phytophthora species. Options for disease management and control are discussed, and an emphasis is put on the prevention of disease spread via infested nursery stock.