Well in this case the bacterium is (accounting for mutations which might happen) more or less the same, not some antibiotic-resistant "super-bug". But I guess most phage research has gone into phages that attack bacteria which cause disease in humans, not in trees, so some further research would be needed? And such research seems to be indeed going on: https://pmc.ncbi.nlm.nih.gov/articles/PMC3911242/ "We report the isolation and characterization of the first virulent phages for X. fastidiosa, siphophages Sano and Salvo and podophages Prado and Paz, with a host range that includes Xanthomonas spp."