RHIZOSPHERE AND ROOT-INFECTING FUNGI AND THE DESIGN OF ECOLOGICAL FIELD EXPERIMENTS

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.