As part of a wider study into the role of soil fungi in the ecology of
the winter annual grass, Vulpia ciliata ssp. ambigua (Le Gall) Stace
& Auquier, we applied the fungicides benomyl and prochloraz to three n
atural populations of the grass growing in East Anglia, United Kingdom
. The rhizosphere and root-infecting fungi associated with the three p
opulations were analysed each month between February and May 1992 when
plants set seed. There were marked differences between the fungal flo
ras associated with each of the three populations of V. ciliata, despi
te the fact that associated plant species and soil nutrient status wer
e broadly similar between sites. This was attributed to wide differenc
es in soil pH between the three populations. Prochloraz did not affect
fungal abundance, but benomyl decreased the isolation frequencies of
Fusarium oxysporum from roots and the frequencies of Penicillium and T
richoderma spp. isolated from rhizosphere soil, and increased the freq
uency of isolation of Mucor hiemalis from the rhizosphere of V. ciliat
a. There were also significant increases in the isolation frequencies
of F. oxysporum from roots and M. hiemalis, Trichoderma spp. and Phoma
fimeti from the rhizosphere of V. ciliata as plants matured. The sign
ificance of these results for the design of ecological field experimen
ts are discussed in light of a previous study which has shown that asy
mptomatic root-infecting fungi can affect plant fecundity and hence ab
undance in natural populations of V. ciliata. We propose that differen
ces in microbial communities between sites, controlled in part by soil
chemistry, are a major factor determining plant performance under fie
ld conditions.