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

1 We applied the fungicides benomyl and prochloraz to natural populati ons of the winter annual grass Vulpia ciliata ssp. ambigua at three si tes in East Anglia, UK, in an attempt to assess the relative losses an d benefits to the plant caused by root pathogenic and arbuscular mycor rhizal 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 ben omyl reduced arbuscular mycorrhizal colonization in the roots of V. ci liata. However, total plant biomass, shoot and root biomass and phosph orus inflows were unaffected in benomyl treated plants. The only direc t effect of benomyl on the plants was to increase fecundity (seed numb er) at one site. 3 These null or positive effects of benomyl on plant performance may be explained by the deleterious effects of the fungici de on the abundance of other root-inhabiting fungi such as Fusarium ox ysporum or Embellisia chlamydospora isolated from the roots of V. cili ata. 4 Whilst direct comparisons of plant performance with the abundan ce of root-inhabiting fungi showed that arbuscular mycorrhizal fungi a ppeared 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 corre lated with plant fecundity, even though these fungi produced asymptoma tic infections. 5 The poor relationship between plant fecundity and be nomyl application contrasted markedly with the clear effects of benomy l on root pathogenic and arbuscular mycorrhizal fungi, and with the cl ear impact of the root pathogenic fungi on plant fecundity. The most l ikely explanation for this apparent paradox was that the two groups of fungi were in some way interactive, and that when both groups were re duced 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 wi th the root pathogenic fungi, and improved fecundity by interfering wi th the negative effects of the pathogens. We concluded that asymptomat ic root pathogenic fungi were important determinants of fitness in V. ciliata and that the main benefit supplied by arbuscular mycorrhizal f ungi to the plant was apparently in protection from pathogenic attack, not in phosphorus uptake. The implications of the results for plant p opulation dynamics are discussed.