SECONDARY METABOLITE-DEPENDENT AND ENDOCHITINASE-DEPENDENT ANTAGONISMTOWARD PLANT-PATHOGENIC MICROFUNGI OF PSEUDOMONAS-FLUORESCENS ISOLATES FROM SUGAR-BEET RHIZOSPHERE

Forty-seven isolates representing all biovars of Pseudomonas fluoresce ns (biovars I to VI) were collected from the rhizosphere of field-grow n sugar beet plants to select candidate strains for biological control of preemergence damping-off disease. The isolates were tested for in vitro antagonism toward the plant-pathogenic microfungi Pythium ultimu m and Rhizoctonia solani in three different plate test media. Mechanis ms of fungal inhibition were elucidated by tracing secondary-metabolit e production and cell wall-degrading enzyme activity in the same media . Most biovars expressed a specific mechanism of antagonism, as repres ented by a unique antibiotic or enzyme production in the media. A lipo peptide antibiotic, viscosinamide, was produced independently of mediu m composition by P. fluorescens by. I, whereas the antibiotic 2,4-diac etylphloroglucinol was observed only in glucose-rich medium and only i n P. fluorescens by. II/IV. Both pathogens were inhibited by the two a ntibiotics. Finally, in low-glucose medium, a cell wall-degrading endo chitinase activity in P. fluorescens by. I, III, and VI was the appare nt mechanism of antagonism toward R. solani. The viscosinamide-produci ng DR54 isolate (bv. I) was shown to be an effective candid:lte for bi ological control, as tested in a pot experiment with sugar beet seedli ngs infested with Pythium ultimum. The assignment of different pattern s of fungal antagonism to the biovars of P. fluorescens is discussed i n relation to an improved selection protocol for candidate strains to be used in biological control.