Light microscope observations of histological changes induced by Ophiostoma ulmi in various nonhost trees and shrubs

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Ophiostoma ulmi (the Dutch elm disease pathogen) was inoculated into 13 nonhost tree and shrub species. Four classes were recognized based on their susceptibility to this pathogen. In Prunus pensylvanica (class I), bubbles formed in vessel members within 3 days after inoculation, and gels formed 5 days later. At day 5, the vascular cambium covering the colonized zone became altered. A barrier zone was formed in 60% of the inoculated twigs. When twigs wilted, the vessels around the twig were occluded, and barrier zone formation was partial. In Populus balsamifera (class II), tyloses formed in vessels of invaded xylem within 8 days after inoculation followed by accumulation of compounds suspected to be phenolics in parenchyma cells. Barrier zone formation occurred in 66% of the inoculated twigs. In Sorbus americana (class III), a pronounced dark discoloration developed rapidly in the invaded xylem. Sparse gels and O. ulmi cells were present in vessel members. Gel formation was limited to vessels that were adjacent to parenchyma cells. Pit membranes of bordered and half-bordered pit pairs became thicker and more darkly stained than in controls. These observations suggest that the first steps of pathogenesis are induced by the action of harmful metabolites of O. ulmi. Few noticeable changes occurred in the species of class IV.