Sept. 11, 1997
A team of Simon Fraser University biologists has made some important discoveries about anthracnose canker, the most serious disease affecting apple trees in coastal B.C.
"Cankers are diseases that attack the bark of trees. Anthracnose canker can kill young apple trees and wipe out entire orchards in four to six years," reports Jim Rahe, team leader. "We've found that fungicides currently recommended and extensively used don't control the disease."
Even the fungicides which are most toxic under lab conditions weren't effective in field trials on the Burnaby campus. As a result, Rahe says that emphasis should switch to cultural control. He is trying to get the word out to the $1.5 million coastal industry.
The SFU team discovered that the fungus which causes anthracnose canker produces two kinds of spores which are important in the epidemiology of the disease: one is produced in the fall and spread by rain; the other, produced in spring and summer, can be spread by wind.
It's the windblown spore that is largely responsible for spreading the disease in an orchard and initiating the disease in previously healthy trees. The waterborne spore serves mainly to increase the number of cankers in a tree once it has been infected.
Based on SFU research into the life cycle of the fungus, it's clear that winter pruning must include removal of all cankers. Although this may be too late to prevent fall infections in a tree that is already infected, it will greatly restrict the potential of the disease to spread into new trees.
Rahe recommends that new trees be planted as far away from old trees as possible. The new trees should be inspected carefully and frequently during the first few years. Any cankers that develop should be removed immediately and trees that develop numerous or large cankers should also be removed. This means that the orchardist or homeowner must be able to recognize the cankers at all stages of development.
As well, trees shouldn't be replaced until all cankers are removed from remaining trees. And orchard replanting should be in blocks and never within rows of older, infected trees.
"Success will depend on the number and size of infected trees, how widely the disease is dispersed and the amount of effort committed to cultural control," says Rahe, who hopes to extend his research into bull's-eye rot, a costly disease in the $200 million Okanagan apple industry.
"The fungus responsible for canker can also attack fruit in the field and cause rot to develop in apples during storage in packing houses after the crops have been picked," he explains. "It was a very serious problem in the Okanagan fruit industry last year. "In order to control bull's-eye rot we need to learn more about when the spores that infect fruit in the orchards are released under the drier conditions of the Okanagan Valley," he concludes.
CONTACT: Dr. Jim Rahe, 291-4801/4105