Sweet cherry (primarily)
Bacterial canker can affect most stone fruits, but in the Intermountain West, sweet cherry is the primary host. The bacteria, Pseudomonas syringae, survive on most plant surfaces as an epiphyte. They enter the plant through wounds sustained from winter injury, or through buds in the fall. They may infect flowers when springs are cool and moist.
The bacteria grow in and kill the phloem tissue, resulting in cankers. The affected bark oozes a sticky, amber gum, and becomes sunken and darker and the inner tissue is orange to brown, with narrow, reddish streaks that extend into healthy tissue above and below the canker. Often the wood has a fermented odor. The affected limb will eventually die, and the leaves will remain attached.
If blossom infection occurs, cankers subsequently form on twigs and spurs, and the dead flowers remain attached on fruit spurs. Leaf and fruit infections are rare in the Intermountain West.
One to two weeks after bloom, watch sweet cherry trees for dead flower clusters and twigs. These will be associated with amber ooze.
Bacterial canker is a disease of cool, moist conditions. Some years will require more diligent action than others. Using a combination of sanitation, proper pruning, and fall copper sprays can help to mitigate the disease.
- Prune out affected twigs and branches.
- Rootstocks can affect susceptibility. Gisela 6 is most susceptible, followed by Krymsk 5, and Mazzard least susceptible.
- Other practices, such as weed control, maintaining a balanced soil pH, and pruning only in dry weather have all been shown to minimize disease.
- Apply a copper-based fungicide in spring at budbreak, or in fall during or soon after leaf drop.