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Bacterial Canker of Cherry

Although cherry growers had a productive year during the 2003 season, bacterial canker still caused reduced yields particularly in sweet cherries. There are two related causal organisms that cause bacterial canker in cherries, Pseudomonas syringe pv. syringae and P. syringae pv. morsprunorum. The later is the most prominent in Michigan and thought to be the one causing damage in Ontario.

These bacterial pathogens survive the winter in bark around small cankers, in infested buds, and the vascular system of infected trees. In the spring the bacteria multiply and are dispersed by rain to blossoms and young expanding leaves.

Under cool, wet conditions in the spring, the pathogens infect buds and leaves through wounds. Growers may confuse symptoms with frost damage to buds in the spring (Figure 1a). Actually, frost encourages infection in the spring and exacerbates the damage. Severely infected buds fail to open and eventually die. Small cankers eventually form at the base of the dead buds and often exude gum during the late spring or early summer (Figure 1b). Leaves at the end of the cankered limbs or branches often wilt and die during the summer or early autumn. The bacteria can also infect and cause necrotic spots on leaves and fruit, however this is very rare. They can also survive on the surfaces of leaves and blossoms without causing any damage throughout the growing season.

During the warmer and dryer summer months, the bacteria populations tend to be low. In the autumn particularly when conditions are wet and cool, the bacteria multiply and may enter the host trees through the leaf scar up to two or three days after leaf fall.

In Michigan, growers have been applying copper to trees at 75% leaf fall in the autumn to keep the over wintering bacteria populations low and reduce the potential of infection the following spring. Copper can be phytotoxic to trees and is not registered for use on sweet cherries in Canada. Michigan researchers and growers observe inconsistent bacteria population reduction the following spring in orchards sprayed with copper the previous autumn. The inconsistent reduction may be due to the non-systemic properties of copper. Bacteria populations in orchards with many cankers or that enter the vascular tissue are protected and would not come in contact with copper. Furthermore, there is no real scientific data showing that reducing bacteria populations in the fall affects the population the following spring. More research into fall and spring bacterial population thresholds capable of causing a disease out break is required. The timing of application to sufficiently reduce over-wintering bacterial populations that results in reduced infection and disease the following spring is also required.

Sweet cherry orchards growing beside sour cherry orchards often have more bacterial canker regardless of whether copper was sprayed the previous fall. Neighbouring sour cherry orchards, which are not as severely affected by this disease, harbour the pathogenic bacteria populations, which spread to the sweet cherry orchards resulting in economic losses.

Clearly either resistant cultivars or an effective product applied at the proper time that reduces bacterial population and infection in the spring without injuring trees is required. Until resistance or an effective product is discovered and registered, growers must manage this disease by keeping trees healthy and preventing stresses that may predispose them to infection including maintaining a balanced nutrient program, keeping soil pH around 6.0 – 6.5 and managing other disease and insects.

Figure 1a: Symptoms of bacterial canker infected buds in the spring

Figure 1b: Infected spur with a developing canker at the base