Root-colonizing fungi from oilseed rape and their inhibition of Verticillium dahliae

Root-colonizing fungi obtained from thoroughly washed root segments of symptomless young oilseed rape plants growing in field soil were examined for their inhibitory effect on the wilt fungus, Verticillium dahliae Kleb. The isolated fungi that were possible to identify belonged to the different genera;Trichoderma, Gliocladium, Mortierella, Fusarium and Alternaria. Oilseed rape seedlings from seeds exposed to both sporulating and nonsporulating fungal isolates were shown to be colonized by their hyphae in gnotobiotic conditions and the emergence/root growth was significantly affected in their presence. Inoculation with a few isolates not only covered the root surface but also seemed to extensively colonize the inner root cortical tissues partly by producing characteristic structures in the roots. All of these suppressed the pathogen in in vitro assays but the mode of suppression in most cases did not seem to be due to production of antifungal substances. In nonsterile field soil, several of the isolates conferred protection against V. dahliae by delaying symptom development. None of the isolates was shown to increase wilt symptoms. In addition to protection, two isolates, yet to be identified, also showed increased shoot dry weight compared with that of healthy controls. A majority of the isolates showed proteolytic activity whereas only three produced chitinolytic enzymes. Results for the isolates obtained by the method selected for this study could be considered as a starting point for evaluating antagonists capable of both competing for root colonization and preventing root infection, with the final aim being to achieve successful biological control of the wilt fungus.