Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
2020 was a bad year for bacterial blossom blast in stone fruits. Blossom blast (and bacterial canker) is most common on Prunus species including cherries, apricots, peaches, nectarines, and plums. The disease is caused by Pseudomonas syringae pv. syringae and Pseudomonas syringae pv. morsprunorum. These are common bacteria in stone fruits. This bacterium can colonize bark and flower buds in stone fruits and occasionally cause bacterial blossom blast in pome fruits (apples). These bacteria also cause limb and trunk cankers, “dead bud”, and leaf spotting. Cankers serve as a reservoir for the disease and can girdle and kill entire limbs.
Bacterial blossom blast is commonly found after a frost event. Some strains of P. syringae have ice nucleation activity, allowing ice to form around them in bud, flower, and stem tissue. This will cause more severe damage during frost or near-frost events in spring. This then predisposes the bud or flower tissue to infection by the same bacteria. Frequent rainfall, high humidity and cool temperatures in late winter and early spring also favor growth of this pathogen.
Blossom blast can be confused with blossom blight, caused by Monilinia fructicola (brown rot pathogen). If you see fungal sporulation it is Monilinia, if you there is no sporulation then It is likely Pseudomonas bacterial blast.
Control of bacterial canker and blossom blight is difficult. Cankers should be pruned from trees when feasible. Some benefit has been achieved from copper applications made when most of the leaves have dropped in the fall and just before bud swell in the spring.
Bacterial Blossom Blast in cherry