Adapted from the article “Bacterial Canker and Blast in Prunes” by Bill Krueger UC Farm Advisor, Glenn County in the February 2010 Sacramento Valley Prune News
Bacterial canker is a serious disease of all Prunus species. The causal bacteria, Psuedomonas syringae is widespread in nature producing disease under predisposing conditions such as high ring nematode populations, poor drainage caused by clay or hardpan soils, sandy or gravelly soils, or inadequate nutrition, particularly, low nitrogen.
Young trees 2 to 8 years old are most often affected. Cankers on branches and twigs are usually elliptical producing substantial gumming. Cutting below the outer bark reveals reddish brown necrotic streaks or flecks in the phloem beyond the margin of the canker. Cankers may girdle and kill limbs or the entire tree. When this happens, a sour smelling liquid often exudes from the bark of the dying branch or tree. The root system is not affected and suckers usually grow from the roots.
Cankers begin with infection in the fall and winter and develop during dormancy and into early spring. Cankers are not perennial and usually die out in summer. Wet, freezing conditions during bloom and early leafing may result in the blast phase of the disease as flowers and newly opening leaf buds become infected, die, and turn brown. Dying buds and spurs become necrotic, turn dark brown and exude gum.
Avoid planting prunes on sites which are prone to develop this disease such as sandy or gravelly coarse textured soils, clay or hard pan soils with poor drainage, and sites known to have ring nematodes or with a history of bacterial canker.
Marianna 2624 is the most susceptible rootstock, but Myro 29C is only slightly less susceptible. Peach rootstocks are more resistant to bacterial canker but more susceptible to Phytopthora crown and root rot and should not be used if this disease is a concern. Peach rooted trees are prone to overcropping and may require management practices such as additional pruning and thinning.
When ring nematode is present, backhoeing and fumigating prior to planting can help get trees off to a good start and can reduce disease incidence. Be sure trees have adequate nitrogen but are not over-fertilized. Trees low in nitrogen are more susceptible to bacterial canker. Pruning in the late spring or early summer rather than in winter or early spring may help reduce the incidence of bacterial canker. Copper sprays during the fall and winter have not reduced bacterial canker under California conditions.