Biological Control of Dutch Elm Disease by Exploiting Resistance in the Host

Abstract

Biological control of Dutch elm disease, caused by the ascomycete Ophiostoma ulmi (Buisman) Nannf., has been extensively studied in the Netherlands using Pseudomonas spp., non-aggressive isolates of the Dutch elm disease fungus or isolates of selected fungi that are non-pathogenic for elms (Scheffer 1983; Scheffer 1984; Scheffer 1989; Scheffer 1990; Scheffer et al. 1989; Scheffer et al. 1980; Scheffer et al. 1989). This article reviews the results of the Dutch projects till the recent approval by Dutch authorities of a commercial product. Using Pseudomonas bacteria in attempts to control Dutch elm disease it was concluded that only preventive treatments had a prophylactic effect against Dutch elm disease, but that the methods employed, and the elm clone or species distinctly influenced the ultimate effect of the treatment. In Commelin elms which are resistant against the non-aggressive isolate of the pathogen, a good control was achieved, but in field elm (U. carpinifolia) no suppression of Dutch elm disease symptoms was obtained (Scheffer 1983; Scheffer 1984). The bacterial isolates used varied from a potent producer of antimycotic substances in vitro to a low producer, in this respect without differences in the capacity of suppressing the development of Dutch elm disease symptoms. This observation and the clone depending effect suggest that the major mechanism explaining this biological control is most probably related to induced resistance in the host tree rather than to a direct antibiosis between bacterium and pathogen.