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Augmentation of Soil with the Nematophagous Fungi Hirsutella rhossiliensis and Arthrobotrys haptotyla

In previous studies, growth of Hirsutella rhossiliensis from pelletized assimilative hyphae was reduced by other soil organisms. In the current study, sensitivity to this biotic inhibition was compared when the fungus was added to soil as pelletized hyphae or as fungus-parasitized nematodes. The hypothesis was that the natural inoculum, the parasitized nematode, would be less sensitive than the artificial inoculum, pelletized hyphae. The soil was heated to 60°C for 2 h to remove putative antagonists or was not heated. The soil was packed into vials (17 cm³) and kept at 20°C in the laboratory or packed into cages (PVC pipe sealed at the ends with 480-μm pore mesh, 80 cm³) and buried 22 cm deep in a vineyard. After 2 or 4 weeks, assay nematodes were added to the vials or cages (recovered from the vineyard), respectively. The assay nematodes were extracted from soil after 2.0 or 2.5 days and examined for adhesive conidia of H. rhossiliensis. Consistent with the hypothesis, H. rhossiliensis was quite sensitive to biotic inhibition when formulated as pelletized hyphae but was insensitive to biotic inhibition when formulated as parasitized nematodes. These data suggest that the activity of the H. rhossiliensis pellet could be increased if the pellet better mimicked the natural inoculum. Similar experiments with the nematode-trapping fungus Arthrobotrys haptotyla, however, exhibited an opposite trend: A. haptotyla was more sensitive to biotic inhibition when added to soil as fungus-parasitized nematodes than as pelletized hyphae. Results from laboratory and field experiments were similar.