ROOT PATHOGENIC AND ARBUSCULAR MYCORRHIZAL FUNGI DETERMINE FECUNDITY OF ASYMPTOMATIC PLANTS IN THE FIELD

1 We applied the fungicides benomyl and prochloraz to natural populations of the winter annual grass Vulpia ciliata ssp. ambigua at three sites in East Anglia, UK, in an attempt to assess the relative losses and benefits to the plant caused by root pathogenic and arbuscular mycorrhizal fungi in the field, and to explore the possibility that the two groups of fungi interact to determine plant fitness. 2 Prochloraz did not affect fungal colonization of roots or plant performance, but benomyl reduced arbuscular mycorrhizal colonization in the roots of V. ciliata. However, total plant biomass, shoot and root biomass and phosphorus inflows were unaffected in benomyl treated plants. The only direct effect of benomyl on the plants was to increase fecundity (seed number) at one site. 3 These null or positive effects of benomyl on plant performance may be explained by the deleterious effects of the fungicide on the abundance of other root-inhabiting fungi such as Fusarium oxysporum or Embellisia chlamydospora isolated from the roots of V. ciliata. 4 Whilst direct comparisons of plant performance with the abundance of root-inhabiting fungi showed that arbuscular mycorrhizal fungi appeared to play a relatively insignificant role in the ecology of the plant, the abundance of root pathogenic fungi such as F. oxysporum, E. chlamydospora and a species of Phoma was found to be negatively correlated with plant fecundity, even though these fungi produced asymptomatic infections. 5 The poor relationship between plant fecundity and benomyl application contrasted markedly with the clear effects of benomyl on root pathogenic and arbuscular mycorrhizal fungi, and with the clear impact of the root pathogenic fungi on plant fecundity. The most likely explanation for this apparent paradox was that the two groups of fungi were in some way interactive, and that when both groups were reduced in abundance, the resultant effects on the plants were neutral. 6 A generalized linear model applied to the fecundity data appeared to indicate that the arbuscular mycorrhizal fungi interacted directly with the root pathogenic fungi, and improved fecundity by interfering with the negative effects of the pathogens. We concluded that asymptomatic root pathogenic fungi were important determinants of fitness in V. ciliata and that the main benefit supplied by arbuscular mycorrhizal fungi to the plant was apparently in protection from pathogenic attack, not in phosphorus uptake. The implications of the results for plant population dynamics are discussed.