Supression of Fusarium root and stem rot of greenhouse cucumber using composts containing fluorescent pseudomonads

Three composts (Ball, dairy, and greenhouse) were tested for their ability to suppress development of fusarium root and stem rot of greenhouse cucumber, caused by Fusarium oxysporum f. sp. radicis-cucumeriunum (Forc). Dairy and greenhouse compost significantly reduced disease (P≤0.05), Ball compost had no effect. Microbial assessment of the composts showed a correlation between disease suppression and pseudomonad levels. Pseudomonas species were isolated using dilution plating. Strains showing the greatest in vitro antagonism were identified as P. aeruginosa. Growth room trials were conducted to test P. aeruginosa P23, the non-antagonistic P. maculicola and two strains of P. fluorescens for Forc suppression. Cucumbers grown in compost with P23 had reduced disease severity compared to controls. The locus for biosynthesis of the antibiotic 2,4-diacetylphloroglucinol (DAPG) was detected by Southern blot and confirmed by PCR; DAPG production by P. aeruginosa in liquid culture was detected by thin layer chromatography