Interactions of Gliocladium virens with Rhizoctonia solani and Pythium ultimum in Non-sterile Potting Medium

Interactions of Gliocladium virens with Pythium ultimum and Rhizoctonia solani under simulated in vivo conditions were observed microscopically. Different types of propagules of the three fungi were paired on nitrocellulose membranes and incubated at 25°C in non-sterile potting medium in Petri dishes for 1-5 days. Alginate-wheat bran prill were used as carriers for G. virens. Prill inoculated with G. virens and pre-incubated in potting medium for 3-5 days before placement on membranes did not inhibit the germination of Pythium sporangia, but subsequent Pythium growth was markedly stunted and distorted, with some hyphal collapse and cytoplasmic leakage. G. virens had no visible effect on older Pythium mycelium. Two to 5 days' growth of G. virens caused cytoplasmic leakage of Rhizoctonia mycelium, prevented secondary branching of hyphae and occasionally coiled around Rhizoctonia hyphae. Prill that were newly colonized by G. virens, but not prill pre-incubated for 3 or 5 days, stimulated the growth of Pythium mycelium and sporangia, Rhizoctonia mycelium and unprimed monilioid cells, probably by supplying nutrients. The timing of the interactions and their specificity for the different pathogen propagules were consistent with the production of gliotoxin by G. virens. This view was supported by in vitro experiments, in which pathogen propagules were incubated in a range of concentrations of gliotoxin in potato dextrose broth. Pythium sporangia and mycelium were inhibited by 1 or 2 μmg ml^-1, but Rhizoctonia monilioid cells and mycelium required 3-5 μmg ml^-1 for inhibition. At the lowest effective concentrations the inhibition was sometimes reversible, but propagules were killed at high concentrations of gliotoxin.