INTERACTIONS OF MYCOPHAGOUS COLLEMBOLA AND BIOLOGICAL-CONTROL FUNGI IN THE SUPPRESSION OF RHIZOCTONIA-SOLANI

A rhizosphere-inhabiting collembolan, Proisotoma minuta (Insecta: Isotomidae), and three known biocontrol fungi were studied in sterilized and non-sterilized soil for suppression of Rhizoctonia solani on cotton in a greenhouse environment. R. solani in dried oat culture was incorporated into soil at four inoculum densities ranging from 10 to 150 mg kg(-1). Trichoderma harzianum on wheat bran and Gliocladium virens as dried oatmeal culture were incorporated at 200 and 50 mg kg(-1) soil, respectively, and Laetisaria arvalis dried, micromilled mycelium was applied as a seed dressing. Each fungus was applied either alone or with a population of P. minuta at 1000 kg(-1) soil. Most effective biological control occurred in sterilized soil when the fungal biocontrol agents were integrated with the insect population; all combinations provided more effective disease suppression than the fungal agents used alone. In non-sterilized soil, having a natural competitive microflora, only P. minuta used alone and the L. arvalis + P. minuta treatment provided consistently significant disease reduction compared to R. solani-infested soil without added agents. Moderate disease control in non-sterilized soil was obtained with T. harzianum or G. virens when combined with the insect population. Plant-growth dry weight measurements did not consistently reflect the disease control benefit. The specific mechanisms promoting increased biocontrol capacity of insect + fungus combinations, though not clearly defined here, must lie within a complex of factors including preference of R. solani as a food source for P. minuta, aversion of the insect to the two sporulating Hyphomycetes used for biocontrol, and direct parasitism of R. solani by the fungal agents.