ISOLATION AND CHARACTERIZATION OF ACTINOMYCETE ANTAGONISTS OF A FUNGAL ROOT PATHOGEN

By use of selective media, 267 actinomycete strains were isolated from four rhizosphere-associated and four non-rhizosphere-associated Briti sh soils. Organic media with low nutrient concentrations were found to be best for isolating diverse actinomycetes while avoiding contaminat ion and overgrowth of isolation media by eubacteria and fungi. While a ll isolates grew well at pHs 6.5 to 8.0, a few were unable to grow at pH 6.0 and a significant number failed to grow at pH 5.5. Eighty-two s elected isolates were screened for in vitro antagonism towards Pythium ultimum by use of a Difco cornmeal agar assay procedure. Five isolate s were very strong antagonists of the fungus, four were strong antagon ists, and ten others were weakly antagonistic. The remaining isolates showed no antagonism by this assay. Additional studies showed that sev eral of the P. ultimum antagonists also strongly inhibited growth of o ther root-pathogenic fungi. Twelve isolates showing antifungal activit y in the in vitro assay were also tested for their effects on the germ ination and short-term growth of lettuce plants in glasshouse pot stud ies in the absence of pathogens. None of the actinomycetes prevented s eed germination, although half of the isolates retarded seed germinati on and outgrowth of the plants by 1 to 3 days. During 18-day growth ex periments, biomass yields of some actinomycete-inoculated plants were reduced in comparison with untreated control plants, although all plan ts appeared healthy and well rooted. None of the actinomycetes signifi cantly enhanced plant growth over these short-term experiments. For so me, but not all, actinomycetes, some correlations between delayed seed germination and reduced 18-day plant biomass yields were seen. For ot hers, plant biomass yields were not reduced despite an actinomycete-as sociated delay in seed germination and plant outgrowth. Preliminary gl asshouse experiments indicated that some of the actinomycetes protect germinating lettuce seeds against damping-off caused by P. ultimum.