It can be caused by two distinct pathovars of Pseudomonas syringae - pv. morsprunorum (Psm) and pv. syringae (Pss). Psm is host-specific to Prunus spp., whereas Pss has a much wider host range.
Products containing copper have traditionally been relied on for control. AHDB Horticulture-funded Project TF 217 set out to identify whether there is any evidence of copper resistance in UK populations of the above two pathovars and to develop new control guidelines.
No evidence of resistance appeared in the Psm isolates tested, but some was apparent in those of Pss. However, these were not completely resistant to copper and growth was still inhibited at higher copper concentrations, but it does highlight the need to understand which pathogen is responsible for causing disease in any particular orchard.
Spray trials to identify the most effective control products showed that the copper product Cuprokylt + wetter (Activator 90) is still the most effective chemical control option available for bacterial canker caused by Psm. Copper sprays may be less effective against Pss due to the presence of resistance strains, so growers should seek accurate diagnosis of their orchard strain.
In another AHDB-funded project on ornamental cherries, the highest levels of Psm were seen in the spring and summer, so the current label recommendations for three sprays in late summer may be starting too late to have a significant impact and growers may wish to consider earlier spray applications.
The full grower summary for Project TF 217 is available on the AHDB Horticulture website - horticulture.ahdb.org.uk.