Fig. 1.—

Disease cycle of DED. DED is caused by the exotic ascomycete fungi O. ulmi, O. novo-ulmi, and O. himal-ulmi which are vectored by elm bark beetles. In the absence of DED fungi, elm bark beetles complete their life cycle on elms without inducing any significant damage. Acquisition of O. ulmi and O. novo-ulmi by elm bark beetles, however, has had a catastrophic impact on elm species native to Europe and North America. When young elm bark beetles carrying spores of DED fungi (A) feed on healthy trees (B), they provide the pathogen with access to water-conducting vessels within the xylem. Invasion of the vascular system by DED fungi (C) rapidly induces wilting and eventually death (D). Trees that have been killed or weakened by DED attract virgin female bark beetles looking for suitable breeding sites. These females then emit aggregation pheromones that attract individuals of both sexes. Many of the beetles carry spores of DED fungi (E) and will thus allow the latter to colonize galleries in which females have oviposited after mating (F). There, the DED fungi produce abundant mycelium and reproductive structures (G) including elongated asexual synnemata and sexual perithecia with a globose base. Spores produced by both structures are embedded in a drop of sticky mucilage and will easily attach to the exoskeleton of young elm bark beetle adults (A) emerging from the galleries.