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

A strain of Pseudomonas fluorescens antagonistic to Rhizoctonia solani was isolated from the rhizosphere of cotton seedlings. An antibiotic strongly inhibitory to R. solani was isolated from P. fluorescens 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: Thielaviopsis basicola, Alternaria sp., and Verticillium dahliae. A Fusarium sp. was only partially inhibited and Pythium ultimum was unaffected. Treating cottonseed with P. fluorescens or pyrrolnitrin at time of planting in R. solani-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.