Bacterial canker

A classic bacterial canker symptom showing cankers and gummosis of woody tissue.
PHOTO COURTESY OF MICHIGAN STATE UNIVERSITY

12 STEPS TO MANAGE BACTERIAL CANKER

In Oregon, bacterial canker is the leading killer of young sweet cherry trees. It is particularly destructive in higher rainfall areas. Spring frosts and cold winters promote bacterial canker, as do any kind of wounds.

A publication authored by Robert Spotts, Jeff Olsen, Lynn Long, and Jay Pscheidt with Oregon State University offers these 12 steps for the management of bacterial canker.

 1.    Do not interplant new trees with old trees that are major sources of the bacteria.
2.    Keep irrigation water off the aboveground tree parts as much as possible for the first two or three years after planting.
3.    Avoid all types of injury, including mechanical, insect, and frost injuries. Paint trunks with white latex paint to prevent winter injury.
4.    Use summer pruning. Prune only during dry weather.
5.    Remove and destroy diseased branches and trees.
6.    Choose resistant cultivar/rootstock combinations.
7.    Locate orchards in areas less likely to be affected by frost and slow -drying conditions.
8.    Provide optimal soil conditions for growth of sweet cherries, paying attention to pH and nutrition. Excessive nitrogen, especially applied late in the growing season, may promote growth that is susceptible to low-temperature injury in early winter, followed by bacterial infection.
9.    Control weeds, especially grasses. They often support large populations of Pseudomonas syringae. Clover and vetch groundcovers support lower populations. Consider clean cultivation of row middles for the first three years.
10.    Strains of P. syringae that are resistant to copper-based bactericides are widespread in the Mid-Columbia area. Copper sprays may result in more bacterial canker and should not be used.
11.    Test for and control plant pathogenic nematodes before planting. High populations of ring nematode have been associated with more bacterial canker.
12.    In high infection areas, plant trees later in spring to avoid cool, wet ­conditions. In Michigan, where bacterial canker damage was widespread, growers are less sure about using copper as a bactericide, as it appears that Pseudomonas syringae pv. Syringae (the main causal agent of bacterial canker in sweet cherry) is showing resistance. Two strains of pseudomonads attack both sweet and tart ­cherries, although each strain tends to favor one or the other.

Tiffany Lillrose, a doctoral student at Michigan State University, found research conditions highly favorable for her work. “High spur mortality will affect yields for years to come,” she said, “and if infections become systemic, they may kill whole trees.”

Canker

In New York, Dr. Juliet Carroll has been working for some years on canker, along with Drs. Thomas Burr, Terence Robinson, Stephen Hoying, and Kerik Cox. She and Lillrose made presentations during the Great Lakes Expo in Grand Rapids, Michigan, in December, and Lillrose also spoke at the Northwest Michigan Orchard and ­Vineyard Show.

Current management practices focus on reducing populations of bacteria and maintaining optimal growing conditions for the trees, Carroll said. Because growth of the bacteria is favored by cool, moist conditions, applications of copper are applied in fall and spring. Best timing of these sprays has been found to be at 20 percent and 80 percent leaf drop in the fall and during the late dormant and prebloom period, not only because this is when weather is cool and moist, but, in fall, bacteria gain entry into buds and spurs via leaf scars.

“It is critical to maintain good soil fertility, plant trees on berms, provide irrigation, and to test for and eliminate nematodes prior to planting,” she advised. “Cankers and dead wood should be pruned when weather is dry.”

Predisposing factors associated with the disease include freeze injury, wounds through which the bacteria gain entrance, nematode damage, sandy and poorly drained soils, drought, and coinfection with canker fungi, such as perennial canker, she said.

She and her Cornell University associates carried out several experiments starting in 2006 to determine what management practices were effective against canker. They came to recommend that pruning be done using stub cuts—not flush cuts at the trunk— cutting out branches six inches or more away from the trunk. When canker infected the stubs, it was not able to run back to the trunk and damage the tree.

They tried treating the stubs with copper materials or phosphorous acid, but these failed to significantly reduce any infection following ­pruning.

“Pruning stubs lowered the risk of direct scaffold and trunk invasion by bacterial canker, based on observations of 315 trees, none of which developed trunk cankers from inoculated stubs,” she said. “Pruning stubs may serve to protect trees against bacterial canker by distancing the main trunk and scaffolds from bacterial invasion.”

She added that pruning stubs may be at greatest risk of canker infection during the late dormant pruning time. Pruning after harvest significantly reduced the risk of bacterial canker infection into pruning wounds. This may be related to either the physiological condition of the trees at that time or the typically hot, dry weather in late July and early August, which is unfavorable for development of the disease.

Work at MSU

Interest in bacterial canker in Michigan has been spurred by the desire to plant smaller cherry trees and support them on trellis wires, much as is done in training tall-spindle apples. But limb rub where wires contact the tree has resulted in bacterial canker infection.

As part of her previous work, Lillrose tested different kinds of wire. She found that steel wire resulted in 40 to 50 percent infection in inoculated trials. Using a plastic wire called Duraline reduced that to 10 percent, and there was no infection when a coated steel wire called PolyPlus was used.

In the humid climate of the eastern United States, bacterial canker is a constant threat, Lillrose said. It can enter trees through stomata on the leaves and leaf scars in fall, through pruning wounds, from points of winter injury, through cracks caused by herbicide damage, and through bark occlusions at narrow crotches.