Copper for Suppression of Bacterial Canker in Sweet Cherry?

Bacterial canker of sweet cherry is caused by a couple of related organisms, Pseudomonas syringae pv. syringae and Pseudomonas syringae pv. morsprunorum. Various pathovars ("pv") of P. syringae cause numerous plant diseases including some that fruit growers are familiar with such as blossom blast of pear.

Bacterial canker infections occur through any kind of wound in a tree including through leaf scars as leaves fall in the autumn. P. syringae does well during cool, wet weather, so spring and fall are the main infection periods.

Since P. syringae is a bacteria, fungicides have no effect on it, but copper applications have been used in an attempt to manage it in some jurisdictions for many years. At least two copper formulations are registered in Canada for management of bacterial canker of sweet cherry. However, this approach does not eliminate the disease and at best, can only help suppress P. syringae that have not already infected plant tissues. Copper is not systemic, so those bacteria already within their hosts (such as in existing cankers) are protected from these applications.

Registered copper products suggest two applications per year; one in the fall when 75% of leaves have dropped and one in the spring before bud break. Spring applications are risky if buds are too far along since copper is quite phytotoxic to sweet cherry. Fall applications probably reduce surface populations of P. syringae, but at 75% leaf drop, there has been plenty of opportunity for the bacteria to invade and be protected in small leaf scars. Once again, in both spring and fall, bacteria that have already infected a host tree are unharmed by copper sprays.

There is also a relationship between cold injury and bacterial canker. P. syringae is an opportunistic pathogen so any tissue damaged by frost or cold temperatures can more readily be infected. In addition, the bacteria act as ice nucleation sites, so already infected tissue is more at risk from cold injury. It's a vicious cycle, compounded by the fact that it is difficult to tell the difference between cold injury and bacterial canker even if they did occur separately.

P. syringae also colonize sour cherry although symptoms of the disease are not as severe in sour cherry. Sweet cherry orchards adjacent to sour cherry may be more at risk from bacterial canker. Limiting the bacteria in the adjacent crops should in theory help reduce the local bacterial population but that may not be practical or always advisable.

So, should copper sprays be applied against bacterial canker or not? Dr. George Sundin from Michigan State University has worked with P. syringae for some time. His field studies indicated that the blossom infecting phase of P. syringae was relatively independent of the amount of bud colonization in the fall. In other words, fall copper applications had no effect on the populations of P. syringae infecting blossoms the following spring. Although there are some growers who believe that a fall copper application is useful against bacterial canker, there is no data to support that strategy at present.

In general, it probably is advisable to use copper on newly planted orchards where bacterial cankers are not already present, but for older, already infected orchards it may be pointless unless the goal is to reduce local surface populations of P. syringae to try to protect nearby young trees.

SinceP. syringae is opportunistic, avoid injury during susceptible periods when possible. Prune when trees are fully dormant or, if pruning must be done in the spring, wait for a few days of forecasted warm, dry weather so pruning cuts can heal more rapidly. That won't prevent all infections from occurring (nothing really seems to!), but it might help. Stressed trees are more likely to become infected, so all other good horticultural practices that keep trees healthy and productive should be followed.

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