Indexed on: 01 Nov '89Published on: 01 Nov '89Published in: European Journal of Plant Pathology
The prophylactic effect in elm of one treatment with aPseudomonas isolate was monitored in two types of field trials. In one type only natural Dutch elm disease infections were monitored and hence large numbers of trees were necessary due to the low incidence of natural occurring infections. In the other type trees were artificially infected.The large-scale field trials in which only natural infections were monitored, were based on expected annual losses due to Dutch elm disease of approximately 2%. As a result of the Dutch sanitation program, which was based on the prompt removal of every weakened or diseased elm, the actual losses were generally threefold lower. Dutch elm disease incidence was 22–45% lower in the trees treated with aPseudomonas isolate in the year of treatment and the year after. The results of the biocontrol treatment were negatively influenced because on several locations trees were felled that showed initial signs of Dutch elm disease, which probably would have disappeared during the season.The advantage of artificial infections withOphiostoma ulmi was a reproducable development of symptoms and the possibility to maintain diseased trees, at least till the first signs of elm bark beetle breeding. For ‘Commelin’ elms an increase in symptoms was observed with increasingO. ulmi dose till 3000 conidia per tree; the standard 500 000 conidia used for most experiments was well above this critical value. No decrease in effectiveness of the bacterial pre-treatment was observed with increasingO. ulmi inoculum. Different bacterial treatments suggested that injections at a smaller interval (i.e. more injections per tree) may result in a better prophylactic effect, but the significance of the correlation remained doubtful. A comparison of several elm species and clones showed the importance of the host tree. Prophylaxis as a result of one bacterial treatment was shown repeatedly in ‘Commelin’ elms; the numbers of trees showing symptoms by the end of the second year were 10 to 85% lower in the bacteria-treated groups in comparison with the controls. Also in one experiment with ‘Belgica’ elms prophylaxis was observed, resulting in a 84% decrease in the number of trees showing symptoms by the end of the second year after the prophylactic treatment followed by inoculation withO. ulmi. In ‘Vegeta’ symptom development was only less severe and in field elms (Ulmus carpinifolia) some prophylactic effect was observed in one experiment, but no effect in two others.