Tens of thousands of trees were destroyed in the Fraser Valley after a disease long known in eastern Canada but not in British Columbia erupted in a poplar plantation.
Now, a researcher with the University of British Columbia and Natural Resources Canada has helped discover how the deadly tree fungus that causes Septoria leaf spot works and how it can be identified through DNA tests.
Richard Hamelin said the findings could head off a catastrophic outbreak of "tree killer" diseases that can be devastating when they are new to an area and encounter forests that have no natural resistance.
The Septoria leaf spot infestation was detected in British Columbia in 2005, when a forest company called Dr. Hamelin to say poplar trees on a densely packed plantation (trees grown for logging) near Harrison Mills in the Fraser Valley were showing signs of malformation.
"The fungus grows around the stem and then kills the living part of the stem and … the whole living part of the tree dies," he said.
"I like to think of it as strangling the trees."
The fungus also produces small purple, grey, brown or white lesions, or spots, on leaves, which gives the disease its common name.
"By the time we saw it, [the outbreak] was extensive and the company had to cut entire plantations," Dr. Hamelin said. "It was a pretty severe outbreak."
He said tens of thousands of trees were cut down in an attempt to eradicate the fungus and keep it from spreading to surrounding forests.
"For now, we think we've contained it. This coming summer, we'll do a mapping survey … and see if it has extended or if we've contained it to a few sites in the Fraser Valley," Dr. Hamelin said. "That would be our hope, because that would mean it's not spreading to the native poplar. But if it did, it would ring an alarm bell. If it has expanded, it has the means to spill out into the native poplar and that would be a big concern."
In his research, Dr. Hamelin mapped the DNA of the deadly fungus and compared it to another closely related fungus that does not harm trees.
"What we found out was a little bit surprising. We found out that the tree killer had a bunch of extra genes, genes that are absent in the non-killing version," he said.
The toxins produced by the genes kill tree cells and degrade wood.
"These two adaptations are what make this pathogen a tree killer," Dr. Hamelin said.
He said foresters can now use DNA tests to determine if Septoria leaf spot is present in organic material and to identify other fungi that have the toxic genes.
"What is the next big disease that might come our way? Having those DNA tests will help us to predict that and assess the risk," Dr. Hamelin said.
"The other way we use [this information] is to ensure this particular pathogen doesn't spread," he said. "We know a lot of pathogens are propagated by us humans … when we plant trees we take cuttings to use as planting material … so having these DNA tests will actually allow us to test the material before it's propagated to ensure it's not carrying the fungus."
Dr. Hamelin said it is not known how Mycosphaerella populorum, the fungus that causes Septoria leaf spot, came to be in British Columbia.
But he said its presence is worrying.
"That fungus is not native, it's been introduced here … and there's a danger it might spread to native poplar … and then start killing black cottonwood, which are an important species ecologically," Dr. Hamelin said.
The study, published this week in Proceedings of the National Academy of Sciences, was done by a team of researchers from several universities and research institutes.
Mark Hume