Differential Selection by Nematodes on an Introduced Biocontrol Fungus vs. Indigenous Fungi in Nonsterile Soil.

Trophic interactions of introduced biocontrol fungi with soil animals can bea key determinant in the fungal proliferation and activity.This study investigated trophic interaction of an introduced biocontrol fungus with soil nematodes. The biocontrol fungusTrichoderma harzianum ThzID1-M3 and the fungivorous nematode Aphelenchoides sp. (10 per gram of soil) were added to nonsterile soil, and microbial populations were monitored for 40 days. Similar results were obtained when the experiment was duplicated. ThzID1-M3 stimulated the population growth of indigenous nematodes (p<0.05), regardless of whether Aphelenchoides sp.was added.Without ThzID1-M3, indigenous nematodes did not increase in number and the added Aphelenchoides sp. nematodes almost disappeared by day 10. With ThzID1-M3, population growth of nematodes was rapid between 5 and 10 days after treatment. ThzID1-M3 biomass peaked on day 5, dropped at day 10, and then almost disappeared at day 20, which was not influenced by the addition of nematodes.In contrast, a large quantity of ThzID1-M3 hyphae were present in a heat-treated soil in which nematodes were eliminated.Total fungal biomass in all treatments peaked on day 5 and subsequently decreased.Addition of nematodes increased the total fungal biomass (p<0.05), but ThzID1-M3 addition did not affect the fungal biomass.Hyphae oftotal fungi when homogenously distributed did not support the nematode population growth; however, hyphae of the introduced fungus when densely localized did.The results suggest that soil fungivorous nematodes are an important constraint onhyphal proliferation of fungal agents introduced into natural soils.