Control of <i>Rhizoctonia solani</i> on Cotton Seedlings with <i>Pseudomonas fluorescens</i> and With an Antibiotic Produced by the Bacterium

A strain of <i>Pseudomonas fluorescens</i> antagonistic to <i>Rhizoctonia solani</i> was isolated from the rhizosphere of cotton seedlings. An antibiotic strongly inhibitory to <i>R. solani</i> was isolated from <i>P. fluorescens</i> cultures and identified as pyrrolnitrin (3-chloro-4-[2'-nitro-3'-chlorophenyl]-pyrrole). The antibiotic also inhibited growth of other fungi associated with the cotton seedling disease complex including: <i>Thielaviopsis basicola, Alternaria</i> sp., and <i>Verticillium dahliae.</i> A <i>Fusarium</i> sp. was only partially inhibited and <i>Pythium ultimum</i> was unaffected. Treating cottonseed with <i>P. fluorescens</i> or pyrrolnitrin at time of planting in <i>R. solani</i>-infested soil increased seedling survival from 30 to 79% and from 13 to 70%, respectively. Pyrrolnitrin persisted for up to 30 days in moist nonsterile soil with no measurable loss in activity.