Barrier Zone Formation as a Resistance Mechanism of Elms to Dutch Elm Disease

Abstract

Host responses of elms susceptible and resistant to Dutch elm disease were histologically examined. In a time course study the susceptible elm clone Ulmus × hollandica 'Belgica' and U. × hollandica '390', a clone which shows a high degree of resistance to non-aggressive isolates and a moderate degree of resistance to aggressive isolates of Ophiostoma ulmi , were inoculated in twig or trunk with either an aggressive or a non-aggressive isolate of O . ulmi . For purposes of comparison, the susceptible elm U . americana and the more resistant clones U . × hollandica 'Groeneveld', U . 'Lobel' and U . 'Sapporo Autumn Gold' were included. Depending on clone-isolate compatibility, infected twigs reacted by a walling off process, by barrier zone formation, or failed to resist the infection and died. Trees inoculated into the trunk reacted comparably but in the case of a compatible combination they always formed a barrier zone and the cambium never died in the year of inoculation.