H-mutant bacteriophages as a potential biocontrol of bacterial blight of geranium

Bacteriophages specific to Xanthomonas campestris pv. pelargonii (Xcp), the causal agent of bacterial blight of geranium, Pelargonium xhortorum L.H. B ailey, were isolated from soil and sludge samples from Florida, California, Minnesota, and Utah. Sixteen phages were evaluated for their potential to lyse 21 Xcp strains collected from around the world. The Xcp strains varied in their susceptibility to the phage isolates with 4 to 14 phages producin g a lytic or highly virulent reaction. A mixture of five h-mutants was deve loped from phages that exhibited the broadest host-ranges and tested agains t the same Xcp strains, The h-mutant phage mixture lysed all 21 Xcp strains . Three experiments were designed to determine the efficacy of using a mixt ure of four h-mutant phages to control the spread of the bacterial blight p athogen on potted and seedling geraniums under greenhouse conditions, Plant s surrounding diseased inoculated plants were treated with a phage mixture at 5 x 10(8) pfu/mL daily, biweekly, or triweekly, or treated with Phyton-2 7(R), at 2.0 mL.L-1 every 10 or 14 days. In potted geraniums, daily foliar sprays of the phage mixture had reduced disease incidence and severity by 5 0% and 75%, respectively, relative to control plants after 6 weeks. In two plug experiments, the phage mixture applied daily also had reduced disease incidence and severity by 69% and 86%, and 85% and 92%, respectively, when compared with controls after 5 weeks. In all three experiments, disease inc idence and severity were less for plants treated daily with phages than for those treated less frequently with phages or with Phyton-27(R), Chemical n ame used: copper sulfate pentahydrate (Phyton-27(R)).