ARBUSCULAR MYCORRHIZA PROTECT AN ANNUAL GRASS FROM ROOT PATHOGENIC FUNGI IN THE FIELD

1 Seedlings of the annual grass Vulpia ciliata ssp. ambigua were inocu lated in the laboratory with a factorial combination of the cosmopolit an root pathogen Fusarium oxysporum and an arbuscular mycorrhizal (AM) fungus (a Glomus sp.) before being planted out into a natural populat ion of V. ciliata at Mildenhall, UK, from which both fungi had been is olated. 2 At both 62 and 90 days after transplantation, inoculation wi th Glomus sp. had not increased plant P concentrations, but had protec ted the plants from the deleterious effects off. oxysporum infection o n shoot and root growth, apparently by suppressing pathogen developmen t in roots. The effects of Glomus sp. on plant performance were neglig ible in the absence of F. oxysporum. 3 After transplantation, comparis ons made of the root-infecting mycofloras of uninoculated plants and p lants inoculated only with Glomus sp. showed that the latter developed fewer naturally occurring infections of F. oxysporum and Embellisia c hlamydospora, two species of fungi which are correlated with reduction s in fecundity in natural populations of V. ciliata. 4 These results c onfirm conclusions from previous experiments that the main benefit sup plied by AM fungi to V. ciliata is in protection from pathogenic fungi , rather than improved P uptake, and indicate that AM colonization sig nificantly alters the root-infecting mycoflora of V. ciliata. We propo se that AM fungi may confer similar benefits in other plant species, w hich may account for the difficulty in demonstrating a benefit of AM f ungi to the P nutrition of host plant species under natural conditions .