ANTIBIOSIS AND ANTAGONISM OF SCLEROTINIA-HOMOEOCARPA AND DRECHSLERA-POAE BY PSEUDOMONAS-FLUORESCENS PF-5 IN-VITRO AND IN PLANTA

Pseudomonas fluorescens strain Pf-5, which produces several antifungal metabolites, including the antibiotics pyoluteorin, pyrrolnitrin, and 2,4-diacetylphloroglucinol, was res ted for its ability to inhibit Sc lerotinia homoeocarpa (causal agent of dollar spot) and Drechslera poa e (causal agent of 'melting-out') in vitro and in turfgrass; Tn5 mutan ts with altered antibiotic production also were tested. Inhibition in vitro differed with the medium used, but both fungi generally were inh ibited by Pf-5. In most cases, a mutant deficient in pyoluteorin but n ot pyrrolnitrin or 2,4-diacetylphloroglucinol was as inhibitory as Pf- 5, whereas a pyrrolnitrin deficient mutant was less inhibitory than Pf -5 in most fungus/medium combinations. High-performance liquid chromat ography analysis of culture extracts showed that bacterial genotype an d nutrition have an interactive effect on antibiotic production, such that conditions causing an increase in one antibiotic may increase or decrease another. The purported deficiencies for the pyrrolnitrin- and pyoluteorin-deficient mutants were confirmed. In S. homoeocarpa-infes ted grass clippings incubated in a moist chamber, Pf-5 reduced mycelia l growth, whereas the pyrrolnitrin-deficient mutant did not and the py oluteorin-deficient mutant was intermediate. In greenhouse experiments , Pf-5 reduced dollar spot disease incidence in bentgrass and bluegras s when sprayed over inoculated turf. In grass clippings infested with D. poae and incubated in a moist chamber under favorable conditions fo r spore production, Pf-5 did not reduce significantly the number of sp ores produced compared with the nontreated control. However, Pf-5 redu ced melting-out disease incidence and severity in bluegrass inoculated with spores of D. poae under greenhouse conditions.