Bacteriophages and the control of Erwinia carotovora subsp carotovora

Fourteen bacteriophages of Erwinia carotovora subsp. carotovora, the causal agent of bacterial soft rot, were isolated from samples of fertilizer solutions taken from a greenhouse producing calla lily (Zantedeschia spp.). To avoid a single-host selection bias, a mixture of four bacterial hosts was used to enrich bacteriophages. Molecular characterization of the phages with restriction endonuclease digestions indicated that two distinct types had been isolated. Representatives from both molecular groups were studied by electron microscopy to determine their morphology. The phages possessed icosahedral heads with long, flexible tails. On the basis of morphotypes, the bacteriophages of E. carotovora subsp. carotovora were placed in the order Caudovirales. Limited host-range studies revealed that both phage groups would lyse only E. carotovora subsp. carotovora isolates taken from calla tubers infected with bacterial soft rot. When assayed for persistence in fertilizer solutions for calla lilies, phage titres were found to be stable in the solutions made with sterile reverse-osmosis water, but not with nonsterile tap water. Phages did not persist in preplant-treatment solutions for calla tubers. Biological control of E. carotovora subsp. carotovora by the bacteriophages was assessed in fertilizer solutions, on the surface of plugs of calla tuber tissue, and under greenhouse conditions. Phage isolates were able to significantly reduce bacterial populations in fertilizer solutions and on the surface of plugs of calla tuber tissue. The biocontrol ability of the bacteriophages was inhibited by the presence of Fe - ethylenediaminetetraacetic acid in fertilizer solutions. On plugs of tuber tissue and in greenhouse trials, phages were able to reduce the incidence of soft-rot symptoms by 50% or more.