A canker from the bacteria Pseudomonas syringae has developed in this young cherry tree.
Photo by Ken Johnson, OSU
With copper bactericides failing in some areas to control bacterial canker (Pseudomonas syringae) in cherries, growers will have to adopt an integrated approach, says an Oregon State University plant pathologist.
Bacterial canker is the most important disease of sweet cherries and the number-one killer of young sweet cherry trees in the Pacific Northwest. In higher rainfall districts, such as Oregon’s Willamette Valley and Hood River area, tree losses as high as 75 percent have been reported in young orchards. In drier conditions, tree losses can range between 10 and 20 percent.
“In many areas, copper has failed,” said OSU’s Dr. Ken Johnson, adding that in the last three trials with copper bactericides in the Willamette Valley, the copper fungicide Kocide made the disease worse than an unsprayed treatment.
“We’re telling people that we don’t think copper does much good for bacterial canker in many places,” Johnson said. “It’s not everywhere that copper doesn’t work because there are some places where it’s still effective. But there are also places where it’s not.”
What do growers use if copper is no longer working? That’s the problem, he said, noting that none of the other products have a long enough residual life to kill the bacterial colonies living on the plant.
Kasumin (kasugamycin), an antibiotic that may soon be registered for fireblight control on pears and apples, has shown efficacy in controlling bacterial canker, but the registrant, Arysta Life Sciences, is not interested in pursuing a cherry registration because of resistance concerns, according to Johnson.
The bacteria have learned to live intimately with the host plant, in the nooks and crannies, on the leaf surface, and such, Johnson explained to growers attending an OSU cherry research symposium at The Dalles, Oregon, last winter. “The bacteria are hunkered down and live on the inside and outside of the plant surface. If you apply a surface treatment, you kill some of them—maybe 80 percent—but not 100 percent because some are hunkered too tightly into the plant.”
Regardless of the material used, the results are the same, he said. The few bacteria that are left behind emerge during the next warm day and replace the population that was just knocked out.
“That’s why we had such a benefit from copper—the residue lasted a few months and got those that remained when they poked their heads up,” Johnson explained. “And, that’s why we struggle to find good materials. The timing has to be just perfect.”
The bacteria lives everywhere, including on the surfaces of weeds and grasses commonly found in orchards. Some type of stress—drought, cold temperatures, pruning cuts, heading wounds, root injuries—usually brings on the infection in young trees.
Symptoms on young trees are cankers that appear slightly sunken and darker than other areas. The inner canker tissue is orange to brown with brown streaks extending into healthy tissue above and below the canker. There may be a clear to amber ooze associated with the canker. Cankers can girdle limbs or trunks. Limbs or entire trees may die during hot weather. Pseudomonas population levels are high during bloom, low in summer, and high again during cooler fall temperatures and rain.
Scientists have recently discovered a rootstock connection to bacterial canker susceptibility.
OSU’s Dr. Robert Spotts found significantly more dead Gisela 6 trees due to canker after one year in a rootstock trial comparing trees on Mazzard, Maxima, Colt, and Krymsk rootstocks.
In the Netherlands, Johnson said that growers are experiencing a bacterial canker problem similar to that in the Pacific Northwest in high-density plum orchards. Within two years of planting, growers are losing plum trees to rapid decline from girdling cankers. Trees appeared healthy at planting. Researchers are experimenting with interstems to see if the problem is associated with rootstock and scion combinations.