Conidial trap formation of nematode-trapping fungi in soil and soil extracts

Abstract

Conidial traps (CT) in nematode-trapping fungi are trapping structures formed directly on germination of conidia, without an intermediate hyphal phase. Most of the nine fungi tested were able to form CT on water agar surfaces in the vicinity of soil, but the ability varied between species. Generally, the ability to form CT was greatest in Arthrobotrys dactyloides and Monacrosporium gephyropagum followed by Arthrobotrys superba and Arthrobotrys oligospora. The mechanism behind CT formation was further studied in soil extracts using a microwell plate assay. Preincubation of soil or soil extracts at room temperature before inoculation with conidia increased the incidence of CT. Reduction of the number of microorganisms in soil extracts by sterile filtration, dilution or heating decreased CT formation, as did the addition of nutrients. The results suggest that a certain level of competition for nutrients by microorganisms is necessary for CT formation. In contrast, we were unable to induce CT when conidia were allowed to germinate in pure culture on hyphal trap-inducing or non-inducing agar media with or without cell wall affecting compounds.