A combination drench -spray will best accomplish this. Check labels for legal uses on caladiums. They tend to form on leaf margins but when the bacterium infects a major leaf vein the entire leaf can collapse. Spots are frequently very black and surrounded by a bright yellow halo. Most other plants in the Aroid family such as Aglaonema, Anthurium, Dieffenbachia and Syngonium are also hosts of this pathogen.
Control -Eliminate all stock plants which have Xanthomonas leaf spot.
All bacterial diseases are very difficult to control unless plants are grown without overhead watering or exposure to rainfall. Bactericides such as copper containing compounds may be somewhat effective if used on a preventative and regular basis.