A presumably exotic Phytophthora species causing dieback of Buxus sempervirens in Germany and Romania

Abstract

Roots, collars and rhizosphere soils from diseased boxwood saplings (Buxus sempervirens) from German and Romanian nurseries were shown to be colonized by a Phytophthora species (P. taxon Buxus) closely resembling the recently described species P. himalsilva. After comparison of both taxa using morphological, phylogenetic and pathogenicity data the high similarity between P. taxon Buxus and P. himalsilva was confirmed. No differentiation was found regarding growth patterns, morphological characters and breeding behaviour. In pathogenicity tests with boxwood twigs, leaves and root collars, both the isolates from boxwood and P. himalsilva proved to be highly aggressive towards this host. Although some of the phylogenetic markers used showed some degree of differentiation, others did not provide evidence for a radiation of two taxa. While based on ITS, β-tubulin and ypt gene sequences P. taxon Buxus could not be clearly distinguished from P. himalsilva, the cox1 gene sequences did separate the strains from P. himalsilva and placed them more closely to P. citrophthora. Since P. himalsilva was reported to be a highly variable species, a definite decision on the taxonomic status of the isolates described here cannot be made yet. More importantly, our findings highlight the increasing importance of exotic pathogens causing plant diseases in commercially important plants, especially in nurseries. As the phylogenetic positioning of the box isolates indicated that they may belong to a group of species potentially endemic to Asia, the Phytophthora taxa involved in this disease might represent another important threat to European boxwoods with an exotic origin.