Inhibition of root extension in tobacco by the mycorrhizal fungus Glomus macrocarpum and its prevention by benomyl

Root weight and length were extremely reduced in tobacco seedlings inoculated with disinfested spores of Glomus macrocarpum or soil naturally infested with the tobacco stunt pathogen. Shoot weights were not consistently reduced, and when reduced, were not reduced to the same extent as root weights. The degree of inhibition of roots appeared to be related to the degree of colonization by G. macrocarpum or endogonaceous fungi present in naturally-infested soil. The effects of G. macrocarpum and natural inoculum on root development were prevented or reduced by a benomyl drench, and the effect of benomyl was accompanied or preceded by reductions in colonization of roots. These experiments support the evidence that G. macrocarpum causes tobacco stunt disease and suggest that the mechanism for the pathogenicity of G. macrocarpum to tobacco is inhibition of root development.