External symptoms and histopathological changes following inoculation of elms putatively resistant to Dutch elm disease with genetically close strains of Ophiostoma

To better characterize the host-pathogen interaction leading to Dutch elm disease, pathogenicity tests were carried out under controlled conditions. Putative resistant hybrid clones 2213 and 2245 from the same Ulmus parvifolia Jacq. x Ulmus americana L. cross and putative resistant U. americana clone 503, as well as saplings of U. americana grown from seeds, were inoculated with strains of Ophiostoma ulmi (Buism.) Nannf. or Ophiostoma novo-ulmi Brasier, including strains H327 and AST27, which carry different alleles at the Pat1 pathogenicity locus and display different levels of aggressiveness. The occurrence of wilted leaves and xylem streaks in inoculated elms indicated that the three clones tested were in fact susceptible to Dutch elm disease, although clones 2213 and 2245 were less susceptible than other elm material tested. In addition to the usual histopathological changes induced during the development of Dutch elm disease on clones 2213 and 2245, such as the formation of alveolar structures, tyloses, gels, and barrier zones, microscopic observations also revealed the presence of cells exhibiting a yellow autofluorescence under blue illumination around xylem vessels invaded by the pathogen. This may represent a new defence reaction against Dutch elm disease. The more aggressive H327 strain induced different levels of xylem responses than the less aggressive AST27 strain.