Protection of black spruce seedlings against Cylindrocladium root rot withectomycorrhizal fungi

Cylindrocladium floridanum Sobers and Seymour, principal causal agent of ro ot rot of conifers in forest nurseries, was grown in vitro with four specie s of ectomycorrhizal fungi, Paxillus involutus, Hebeloma cylindrosporum, La ccaria bicolor, and Tricholoma sp. The effect of inoculating black spruce s eedlings (Picea mariana (Mill.) B.S.P.) with P. involutus was also investig ated. Tricholoma sp. and especially P. involutus and H. cylindrosporum inhi bited growth of C. floridanum in Petri dishes, while L. bicolor was inhibit ed and completely covered by C. floridanum. Hyphae of the pathogen in the c ontact zone were deteriorated in the presence of P. involutus and H. cylind rosporum. The inoculation of P. involutus reduced the number of infected bl ack spruce seedlings by approximately 50%. However, the simultaneous inocul ation of two different strains of P. involutus did not significantly reduce disease development. Linear regression demonstrated that the percentage of infected plants was negatively correlated with mycorrhiza formation. The r elation between these two variables also showed that even if colonization b y P. involutus is poor or null, it has some inhibitory effect against Cylin drocladium root rot.