Biocontrol traits of arbuscular mycorrhizal fungi (AMF) against root pathogens are well known, but information on biocontrol mode of action and biocontrol efficiency of AMF species are limited. In this study, we investigated the influence of three AMF on root rot in Cucumis sativus caused by Pythium ultimum. Plants were grown in symbiosis with the AMF Funelliformis mosseae, Rhizophagus irregularis, Claroideoglomus claroideum or left uninoculated, and 14 days after sowing half of the plants were inoculated with P. ultimum. Plants were harvested 14, 21 and 28 days after sowing and analyzed for dry weights, AMF colonization and P. ultimum infection. The results showed three scenarios of biological control by the AM symbioses: AM with F. mosseae fully counteracted the plant growth depression caused by the pathogen; R. irregularis AM resulted in plant growth depression, but counteracted plant growth depression by the pathogen; AM symbiosis with C. claroideum caused plant growth depression and had no biological control effect against P. ultimum. Overall, the study showed biocontrol features of AMF in terms of tolerance induction though depending on species of AMF. Such differential traits of AMF should be considered when developing biocontrol strategies against root rot in cucumber.
