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  • Writer's pictureThe Low Down

Paint our way out of a corner to save oaks

It’s normal to tune out the Chicken Littles who run around squawking about the latest forest pest or disease. I mean, how many times can the sky fall, anyway? But the real danger is feeling helpless. There’s a pithy fable about a kid who rescues starfish from the beach after storms, and some busybody informs them they can’t save all starfish.


The child hurls another into the ocean, saying “I saved that one.” Right now, we have a chance to save oaks from a devastating disease, not by tossing them in the sea, fortunately, but by adopting a simple practice when pruning or removing oaks.

Oak wilt, a virulent fungal pathogen first identified in Wisconsin, can turn healthy oaks to crispy critters in two to six weeks. It spreads through root grafts, by insects transferring spores, and by moving firewood long distances. Underground tree-to-tree spread near outbreaks is less important overall than airborne transmission and movement of firewood.


Healthy red-type oaks succumb in a matter of weeks, while white-type oaks take one to three years to die. After an oak is killed, the pathogen forms mycelial spore pads on the trunks. Spore-laden ooze is secreted, which attracts native sap beetles that normally feed on sugars from sapwood of newly cut or wounded hardwoods. The beetles, which have a flight range of about 1.5 kilometres, get covered in disease spores at these pads, and can infect healthy oaks if a fresh wound is present.


From April 1 - July 1, the risk of spreading oak wilt is extreme, and from July 1 - Sept. 30 it’s moderate. Exposed fresh wounds on oaks puts trees at risk. Never cut oaks, or allow them to be accidentally wounded, from March through September. Okay, now stop laughing. It’s impractical to stop pruning and harvesting oaks all season. The work-around is to paint wounds and stumps immediately with interior latex paint (wood only needs coverage a few days, and it’s best that wounds don’t remain sealed) that will flake off in several weeks. Treat wounds with interior latex paint right away, as beetles can locate fresh sap in just minutes. On pruning wounds, paint the whole thing.


With stumps, only sapwood needs coverage. Stumps need painting because they may have root grafts connected to nearby healthy oaks. Spores deposited on fresh-cut stumps could infect many trees, given that roots extend 3x the branch length.


Oak wilt is not far away. In southern Ontario, two new outbreaks in the past month were due to folks who brought firewood home from trips last summer. It could show up anytime, anywhere. Painting oak stumps and wounds between March 1 and Sept. 30 must become standard practice. Also, don’t move wood long distances!


Here’s our chance to hold a critical line. Implementing these strategies will vastly reduce the risk of seeing oaks go the way of the American chestnut. Let’s do our part to prove Chicken Little wrong.


Paul Hetzler is an ISA-Certified Arborist.


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