In pot tests, primary roots of Douglas-fir seedlings germinating in soil were inoculated by soil injection with suspensions of Laccaria laccata and (or) Trichoderma sp., or Pseudomonas cepacia , or were treated with benomyl fungicide applied as soil drenches. The root pathogen Fusarium oxysporum was introduced 7-10 days after these treatments. All three organisms and the fungicide suppressed disease. L. laccata protected as well as the fungicidal drenches and better than the nonmycorrhizal biocontrol agents. The protected seedlings, in comparison to those exposed only to the pathogen, had fewer root lesions, yielded fewer colonies of F. oxysporum in isolations from roots, had longer roots at 8 wk and greater heights and weights at 14-20 wk. Only L. laccata prevented needle curling, a nonspecific symptom of stress in Douglas-fir seedlings. In the absence of F. oxysporum , L. laccata caused dry weight increase up to 55 percent greater than that of nonmycorrhizal plants after 12 wk. Where the pathogen was present, L. laccata caused weight increase 24 percent greater than that of benomyl treated plants and 102 percent greater than untreated plants after 14 wk. Top/root ratios indicated selective stimulation of top growth by the mycorrhizal fungus concomitant with the formation of ectomycorrhizae. Forest Sci. 28:191-201.
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Sinclair et al. (1982) studied this question.