Arbuscular mycorrhiza protect an annual grass from root pathogenic fungi in the field

1 Seedlings of the annual grass Vulpia ciliata ssp. ambigua were inoculated in the laboratory with a factorial combination of the cosmopolitan root pathogen Fusarium oxysporum and an arbuscular mycorrhizal (AM) fungus (a Glomus sp.) before being planted out into a natural population of V. ciliata at Mildenhall, UK, from which both fungi had been isolated. 2 At both 62 and 90 days after transplantation, inoculation with Glomus sp. had not increased plant P concentrations, but had protected the plants from the deleterious effects off. oxysporum infection on shoot and root growth, apparently by suppressing pathogen development in roots. The effects of Glomus sp. on plant performance were negligible in the absence of F. oxysporum. 3 After transplantation, comparisons made of the root-infecting mycofloras of uninoculated plants and plants inoculated only with Glomus sp. showed that the latter developed fewer naturally occurring infections of F. oxysporum and Embellisia chlamydospora, two species of fungi which are correlated with reductions in fecundity in natural populations of V. ciliata. 4 These results confirm conclusions from previous experiments that the main benefit supplied by AM fungi to V. ciliata is in protection from pathogenic fungi, rather than improved P uptake, and indicate that AM colonization significantly alters the root-infecting mycoflora of V. ciliata. We propose that AM fungi may confer similar benefits in other plant species, which may account for the difficulty in demonstrating a benefit of AM fungi to the P nutrition of host plant species under natural conditions.