Summary• The ability of the nematode-trapping fungus Arthrobotrys oligospora and the nematode egg parasite Verticillium chlamydosporium to colonize barley ( Hordeum vulgare ) and tomato ( Lycopersicum esculentum ) roots was examined, together with capability of the fungi to induce cell wall modifications in root cells.• Chemotropism was studied using an agar plate technique. Root colonization was investigated with light microscopy and scanning electron microscopy, while compounds involved in fungus-plant interactions were studied histochemically.• Only A. oligospora responded chemotropically to roots. Colonization of barley and tomato by both fungi involved appressoria to facilitate epidermis penetration. V. chlamydosporium colonized tomato root epidermis and produced chlamydospores. Papillae, appositions and lignitubers ensheathing hyphae on tomato were also found. Phenolics (including lignin), protein deposits and callose were present in papillae in both hosts. Both fungi were still present in epidermal cells 3months after inoculation.• Nematophagous fungi colonized endophytically monocotyledon and dicotyledon plant roots. Arthrobotrys oligospora seemed to be more aggressive than V. chlamydosporium on barley roots. Both fungi induced cell wall modifications, but these did not prevent growth. The response of root cells to colonization by nematophagous fungi may have profound implications in the performance of these organisms as biocontrol agents of plant parasitic nematodes.

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.

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In a field experiment the rhizosphere effect of barley, pea and white mustard on the nematode‐trapping fungi were investigated throughout a growing season. The densities of nematode‐trapping fungi were slightly increased in the rhizospheres compared to the root‐free soil. Pea rhizosphere had the greatest numbers of species of nematophagous fungi with an average of 2.4 species recovered from 0.1 g material, and in white mustard and barley rhizospheres and root‐free soil less than 1.7 species were recovered from 0.1 g. Arthrobotrys oligospora was the most common species in both soil and rhizosphere. In a pot experiment the rhizosphere effect of pea and barley on nematophagous fungi was investigated in 5 different agricultural soils. Pea rhizosphere increased the densities of nematode‐trapping fungi, and up to 780 propagules of nematode‐trapping fungi g−1 rhizosphere soil were found, which was around 19 times higher than in the root‐free soil. The number of nematodes were 6–290 times higher in the pea rhizosphere than in the root‐free soil. Barley rhizosphere had little effect on the densities of nematode‐trapping fungi, while the number of nematodes increased by 3–13 times. The most commonly detected species were A. oligospora, Stylopage sp. and network‐forming Monacrosporium species, but there were no big differences in the species composition between the rhizosphere and the root‐free soil.

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