Suppression of gray leaf spot (blast) of perennial ryegrass turf by Pseudomonas aeruginosa from spent mushroom substrate

Bacteria isolated from spent mushroom substrate (SMS) were evaluated for the suppression of Pyricularia grisea, the causal agent of gray leaf spot of perennial ryegrass (Lolium perenne) turf. Thirty-two of 849 bacterial isolates (3.8%) from SMS significantly inhibited the mycelial growth of P. grisea in vitro. Six bacterial isolates that afforded maximum inhibition of P. grisea were also refractory to Rhizoctonia solani, Rhizoctonia cerealis, Sclerotinia homoeocarpa, and Fusarium culmorum. Each of the six isolates was identified as Pseudomonas aeruginosa by fatty acid profile analysis. The biocontrol activity of the bacterial isolates was not compromised by a prolonged exposure to UV radiation in vitro. In controlled-environment chamber experiments, all 32 bacterial isolates were tested for suppression of gray leaf spot on Pennfine perennial ryegrass when applied as seed treatment or foliar sprays. Foliar applications of the bacteria (10⁸ cfu/ml 0.1% carboxymethylcellulose), but not the seed treatment, significantly reduced disease severity and incidence. The three most efficient isolates from foliar application treatments, which were among the six bacterial isolates identified as P. aeruginosa, were further evaluated for suppression of gray leaf spot as a function of timing of application. The three isolates of P. aeruginosa suppressed gray leaf spot in perennial ryegrass in Cone-tainers when applied at 1, 3, and 7 days prior to inoculation with P. grisea both in controlled-environment chamber experiments, and in potted ryegrass plants maintained in the field. All application intervals, regardless of the bacterial isolate, provided significant reduction of gray leaf spot severity. Suppression of gray leaf spot by isolates of P. aeruginosa under controlled-environment chamber conditions was not different from that observed in potted ryegrass plants maintained in the field. In field experiments, an isolate of P. aeruginosa provided significant suppression of gray leaf spot when applied at 1, 7, and 14 days prior to inoculation with P. grisea. The bacterium proved effective against gray leaf spot of perennial ryegrass maintained as fairway and rough heights. These results indicate that P. aeruginosa may be a potential biocontrol agent for gray leaf spot of perennial ryegrass turf.