Mechanisms Involved in Biological Control of Dutch Elm Disease

An effective suppression of Dutch elm disease symptom development was observed in elms resistant to the non-aggressive strain of Ophiostoma ulmi, but not to the aggressive strain, after a preventive treatment by this non-aggressive strain. Anequally effective isolate of Verticillium dabliae suppressed symptom development even in the very susceptible field elm (Ulmus carpinifolia). Induction of resistance is proposed for the major mechanism explaining these and earlier experiments. The primary stimuli initiating a resistance reaction are unknown, but the intermediates of the univalent reduction of oxygen may play a role. In an experiment the activities of two groups of enzymes involved in the catalytical scavenging of these radicals, superoxide dismutase and peroxidase, were monitored, but no correlation with induction of resistance was observed. Resistant elms showed the highest activities, probably reflecting a more effective defence against radicals than susceptible ones.