Selection of biological agents from composts for control of damping-off of cucumber caused by Pythium ultimum

Abstract

The microflora of three composts were studied to isolate and test microorganisms for biological control of Pythium ultimum on greenhouse-grown cucumber. A more diverse bacterial population was observed in compost from paper mill sludge (170 groups) than in composts from plant waste and from manure (75 and 88 groups, respectively). In compost from paper mill sludge, 12 fungal species were isolated, compared with 22 and 26 in composts from plant waste and from manure, respectively. Selected bacterial and fungal isolates were evaluated in assays on agar plates against P. ultimum. A total of 10 bacterial isolates and 4 fungal isolates significantly (P ≤ 0.0001) reduced P. ultimum growth rate. These isolates were evaluated in the greenhouse for control of damping-off of cucumber. Disease incidence and severity and foliage and root masses were measured 3 weeks after soil inoculation with the microbial agents and P. ultimum. Under greenhouse conditions, Zygorrhinchus moelleri and Bacillus marinus were the most effective microorganisms against damping-off, followed by Penicillium thomii, Pseudomonas fluorescens, Pseudomonas aeruginosa, and Graphium putredinis. These isolates were tested in a second greenhouse trial and the best control was achieved with Z. moelleri and Penicillium thomii.