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White Pine Decline
Several articles ago, I wrote about white pine trees and how I was getting so many calls about their decline in the landscape across southern Ontario. These symptomatic white pines have a few things in common. They were all transplanted into the landscape as fair-sized trees (200 cm and greater), which puts them at a greater risk of transplant stress than younger trees. Secondly, they all seem to be exhibiting some level of sap weeping on the main trunk and also on larger branches. If you cut into these weeping cankers, you will see that the cambium has been killed and has turned a bright brown colour (smaller inset photo). Tree death is gradual; it often takes 3-5 years. As decline progresses, the tree shows increasingly obvious signs of chlorosis, stunted growth, needle drop and dieback.
I have been consulting with Dr. Tom Hsiang, University of Guelph, and Dr. John McLaughlin from the Ontario Ministry of Natural Resources since they are both forest tree pathologists. We went out to investigate a site near Guelph to more closely examine symptomatic white pine trees to see if we could unravel any patterns to decline development and cause. We saw trees in varying stages of the disorder and tried to correlate them with the symptoms and level of vigour. One thing that was very evident was how good white pines are at hiding their pain. Just like an old dog, they will hang on to the bitter end, showing only general chlorosis and stunting with a bit of needle drop when actually only 25% (or less) of their cambium is still functioning. They can really fool you, which can sometimes make them a hazard tree in residential areas. By the time you see general chlorosis, needle drop and stunting on white pine, it’s often way too late to do anything to save the tree.
When we sliced into the weeping cankers, we found bright brown cambium that was obviously dead. I had thought that the decline in the white pine was caused by some organism (e.g. fungus) that was entering the bark and killing cambial tissue underneath. I have been sending samples of cambial tissue (partially alive) to the diagnostic clinic in Guelph and was very puzzled when several samples came back as “no pathogens detected.” During our site visit, Dr. Hsiang told me that the cankers themselves may not be the cause of the decline or disease, but they may actually just be an expression of stress for the tree. To which I said “Oh,” and thought to myself how lucky I was to know him. We took samples of cambial tissue at the leading edge of the killed cambium, just to double check. We also removed large portions of bark that had sap weeping from them and the extensive amount of cambial death was incredible. The brown cambium ran in long streaks from the roots, extending two metres or more up the trunk (see top right photo).
Staff arborists felled the tree and cut several “cookies” for us so we could examine the growth rings, signs of injury and look for fungal staining. We did in fact see a distinct blue stain in the cookies from high up in the main trunk. The samples were catalogued and packaged for submission to the lab for pathogen analysis. This is about the time that Dr. McLaughlin, who was using a Pulaski (a specialized axe and digging tool in one head) to expertly uncover the cambial tissue, started using it to chop away at the roots so we could examine them. My comment “Oh my, are you ever strong,” has come in handy several times in my life and somehow, I never get tired of saying it. Much to everyone’s surprise (and delight – these are plant pathologists after all), the blue stain did extend well into the root flare. In addition, some insect bore holes were also observed as well as a live beetle on the roots, which got the OMNR Health Technical Specialist, Eric Cleland, so excited that he did a little jig. No wonder these trees are dying. The fact that we’ve had more hot, dry growing seasons than those with average rainfall and good growing conditions for landscape trees is likely another big part of the explanation for large-scale decline.
So the next steps are to analyze the samples and try to isolate and identify any potential plant pathogen or pest complexes that might be the cause of the problem. Just because a pathogen is found, it might not be responsible for the original cause of decline. Sometimes fungi are there because they found a crack in the bark and the tree was too weak to fight off their infection because of other problems it was dealing with. And THIS is why I like consulting with plant pathologists when I’m uncertain about diagnosing tree problems. White pine decline is one of those mysteries that we are still trying to solve. I thought it might interest you to know that there are still plant pathologists in Canada and although they are few and far between, they are deeply concerned about tree health in Ontario. I’ll keep you posted.