Bateriophages of Xanthomonas campestris pv. begoniae Their occurrence, survival and potential use as a biological control agent

Bacterial spot of begonia, caused by Xanthomonas campestris pv. begoniae (Xcb), can be a very serious disease during begonia production. In nature, bacterial populations commonly support populations of bacteriophages, which have sometimes been used to suppress susceptible bacterial populations. In this study, bacteriophages of Xcb were collected and isolated from foliage production and greenhouse environments in central Florida. Many of the phages isolated could infect several strains and, in some cases, multiple species in the genus Xanthomonas.From this collection, ten bacteriophages were selected based on their reaction to isolates of bacterial plant pathogens in the genus Xanthomonas. They were also evaluated based on phage morphology and genome characteristics. There was not much diversity among the phages studied in regard to morphology and genome characteristics. From these ten phages, four phages were used in biological control experiments in the greenhouse to control bacterial spot of begonia. Each of the four biological control bacteriophages studied had a linear dsDNA genome of approximately 12.5 kilobases and nine of the these ten phages appear to belong to the family Tectiviridae, based on morphology. Based on morphology, the other phage may be a Cystovirus or Levivirus.Five attempts at biological control using these bacteriophages were not successful because of environmental conditions in the greenhouse, which greatly favored the pathogen. In the first four biological control experiments, a mixture of the four phages was applied to begonias. In the fifth biocontrol experiment, the same phage mixture was mixed and applied with the irrigation water. No significant level of control was achieved in any of the experiments.Several negative environmental factors, which affected phage survival and persistence, were identified some were studied in detail. Bacteriophage survival on leaf surfaces in environments common to the foliage and nursery industries was examined. A variety of different filters and light sources were used to examine the effects on bacteriophage populations on both leaf surfaces and on inert surfaces. Two important negative factors found in this study were desiccation and inactivation from high energy light, such as UV. It was also shown that the four phages could not persist well on begonia leaves and were washed off during overhead irrigation during the first four biological control experiments. Bacteriophages also could not infect Xanthomonas campestris pv. begoniae on leaf surfaces under laboratory conditions at the same rate as in nutrient broth. Under greenhouse conditions, these phages could not infect Xanthomonas campestris pv. begoniae on leaf surfaces. This study identified other negative factors, such as ultraviolet light and desiccation that need to be addressed for successful biological control of bacterial spot in nursery and foliage production systems.