Biological Control of Bacterial Blight of Geranium with H-Mutant Bacteriophages

Abstract

Bacterial blight, caused by Xanthomonas campestris pv. pelargonii (XCP), is considered the most serious disease of geraniums (Pelargonium × hortorum). A novel approach that uses bacteriophages (phages, viruses that kill bacteria) for the biological control of geranium blight will be presented. Phages were once abandoned as biological control agents due to the emergence of bacterial mutants resistant to the phages employed. However, our approach utilizes a mixture of three to eight different phages including host-range mutants (H-mutants). H-mutants are spontaneously derived from their wild-type parent phages and lyse not only parent wild-type bacteria, but also phage-resistant mutants originating from parent bacteria. Two phages specific for XCP initially were isolated from soil samples from Florida and California. These phages produced virulent reactions in six of 30 XCP strains, and lysogenic reactions in 22 strains. After selection of these phages for increased virulence and additional phages were isolated from MN and UT, 17 phages were evaluated for sensitivity to 21 XCP strains from around the world. Four to 14 phages produced virulent reactions in the 21 XCP strains. Five phages produced virulent reactions in at least 17 XCP strains. A mixture of five phages tested against the 21 XCP strains produced virulent reactions for all 21 XCP strains. Geraniums in 10-cm pots were inoculated with XCP and placed on a greenhouse bench in the middle of 5 non-inoculated plants. After 2 weeks of daily spraying plants with a phage solution (109 pfu phage/ml) or water, there was a 71% reduction in the number of bacterial lesions on phage-treated plants.