by David Wallace Barr
In recent months we have seen some troubling headlines south of the border. “Oak trees in the Mid-Atlantic region are dying. Here’s why.” – the Washington Post (Nov 25, 2021. “Why White Oak Trees on Route 1 Are Dying” – The Hyattsville Wire (Dec 17, 2019). “Essential tree species facing decline” – The Roanoke Times (Jan 9, 2022). What’s it all about?
According to several sources, the problem can be complex. Drought, poor management of urban oaks, leaf diseases and an international phenomenon termed ‘oak decline’ can all play a role. But scientists at the University of Maryland Extension Department single out a systemic fungal disease as the most dangerous culprit. It’s called oak wilt and it has been destroying red and black oaks from Texas to Wisconsin.
So what is ‘oak wilt?’ The danger signals from the hard hit states are loud and clear. Red and black oaks seem particularly susceptible. Symptoms of disease begin at the top of the tree in leaves of the crown. Some leaves begin to fade. Then they may turn yellow or brown. Signs of wilting appear. Browning of the affected leaf appears first at the tip and around the edges.
Eventually the first leaves to show signs of the disease wither, die and start to drop. Slowly the problem progresses down to branches lower in the crown. Eventually the entire tree becomes leafless and dies.
A large swath of counties in central Texas have been affected. Then northward and eastward infestation has been found in Oklahoma, Arkansas and Missouri. Maps of oak wilt distribution become particularly dense in more northern states like Illinois and Wisconsin. Frighteningly, the disease has also been found in states that border Ontario like Michigan, Ohio, and Pennsylvania. There are even a few recorded sites in New York State.
Could it happen here as well? That depends on the biology of the disease.
A Pathogenic Fungus
Oak wilt is caused by a pathogenic fungus that lives and reproduces beneath the bark of primarily members of the red oak group. That includes black, red, scarlet and pin oaks. There are no obvious external fruiting bodies, but dense mats of fungal mycelia destroy the tree’s circulatory system and eventually push up as dark masses that crack open the bark. Spores are produced and it is these spores that carry the disease to other trees.
Sap beetles become the agent of transmission. They are attracted by a sweet odour produced by the fungus. The bodies of visiting beetles become covered with fungal spores, which they then carry to other trees. Also implicated in the spread are bark beetles that normally bore into the bark of many tree species . But when they bore into an infected oak, they too can pick up disease spores and transmit them to additional trees.
Should we be worried? At present Ontario’s Invading Species Awareness Program reports that the disease does not occur in this province. As a safeguard, the Canadian Food Inspection Agency therefore regulates the importation of oak wood, particularly firewood and logs, to Ontario.
But what about sap beetles and bark beetles? Is anyone trying to stop them at the border?
What’s At Stake?
What’s at stake in Toronto? In Monarch Park, near where I live, a large percentage of the canopy is made up of huge old red and and white oaks. Toronto Parks, Forestry and Recreation has been planting red oaks to replace the older trees, some of which are so large they are endangered by age and wind damage. Toronto’s oldest and probably largest oak is a 250 year old red oak. One of this city’s most precious preserved ecosystems is the black oak savanna in High Park.
Oaks are noted as significant components of 15 of the 30 ravines and urban forests covered by Jason Ramsay-Brown in his book, ‘Toronto’s Ravines and Urban Forests.’ Four of Toronto’s largest and most breathtaking trees are oaks – two red, one black and one white.
Oak in fact, has been designated as Toronto’s official tree and now becomes one of the city’s official symbols.
“The oak tree is an important part of Toronto’s urban forest,” Mayor Tory said in the release.
“It is a great representation of the city of Toronto and plays a vital role in our history. Naming Toronto’s official tree recognizes the significance of all trees, strengthening our connection to nature.” Jennifer McKelvie, who represents Scarborough-Rouge Park, said the selection of the oak tree “reaffirms our commitment to growing, enhancing and protecting Toronto’s urban forest.”
We have a lot to lose if oak wilt ever gets here!
And we better start watching for the signs of the arrival of the disease now. Why? Because Toronto’s prevailing winds, as in all of Canada, are from the west. Now bark beetles and sap beetles are tiny insects. But they can fly. They are not strong fliers, probably only a few hundred meters at most under their own steam. But like many small insects they make use of a simple instinctive behaviour to ensure their dispersal. Under the right weather conditions they just fly upward until they are caught by the strong winds of an advancing front.
Many insects are dispersed hundreds of kilometres by storms and upper altitude winds. The jump from Michigan or Wisconsin to Ontario is no barrier to these meteorological hitch hikers. So the issue of oak wilt in Toronto is less about ‘if’ and more about ‘when.’
Preparing For The Inevitable
What is the city doing to prepare for this eventuality? In speaking with Jozef Ric and Hart Morrison of the Forest Healthcare section of Toronto’s Urban Forestry, I learned that city staff have been following the northward advance of this disease threat for many years. They too say that so far as they know, it has not been identified in Ontario yet. But they know it is as close as Belle Isle in the Detroit River, within a kilometre of Windsor, Ontario. In the meantime they have been preparing by taking preventive measures. Staff keep up with the latest information on the disease. They ensure that oaks are not subjected to unnecessary pruning during a generous window around the flight season of sap beetles (April – September). Pruning causes wounds that attract sap beetles. And if emergency pruning is necessary, they insist that the cut surfaces are protected with a suitable dressing sealer.
Toronto maintains an oak database with locations for the city’s extensive inventory of trees. Information in the inventory will become indispensable when the disease appears. It will be used to identify trees and oak groves at risk as well as the location of trees that need to be protected with fungicidal injections. Urban Forestry staff are the experts, after all, who are still successfully preserving Toronto’s remaining ash trees on the public land from the depredations of the Emerald Ash Borer. Since oak wilt can also spread through the roots from one tree to another, trenching to separate root systems is another extreme measure that could be used.
In the long run, preserving Toronto’s protective green canopy is best ensured by intelligent planting for the future. Toronto plants new trees on the dual principles of the right tree in the right place and increasing diversity. This way our trees remain as healthy as possible, and when deadly attacks occur, there will always be resistant trees to take the place of the fallen.
In the immediate future, lovers of Toronto’s oaks can certainly help. If you notice oak leaves changing colour in midsummer near the top of the crown, make a note of exactly where you have seen it. Urban Forestry staff need to know so they can check to see what’s going on. Each of us has a role to play in protecting Toronto’s red and black oaks, one of our most iconic natural resources.
Signs of oak wilt should be reported to the Canadian Food Inspection Agency, by email
to email@example.com. The CFIA is the agency responsible for the
control of pest species entering Canada. Naturalists can also report it to the City by
calling 3-1-1 or emailing firstname.lastname@example.org .
Dig Deeper Into This Topic:
10 of Toronto’s most breathtaking trees — our ravine queens and urban legends: https://www.thestar.com/news/gta/2018/10/14/10-of-torontos-most-breathtaking-trees-our-ravine-queens-and-urban-legends.html
Donate to the Historic Red Oak: https://www.toronto.ca/business-economy/partnerships-sponsorships-donations/donate/heritage-red-oak-tree/
Early warning research verifies the oak wilt threat is real: https://inspection.canada.ca/plant-health/invasive-species/plant-diseases/oak-wilt/science-bulletin/eng/1597413492554/1597413493195
High Park’s Rare Black Oak Savannah: https://highparknature.org/article/high-parks-rare-black-oak-savannah/
Ramsay-Brown, Jason. 2020. Toronto’s Ravines & Urban Forests: Their natural heritage and local history. 2nd ed. Lorimer, 200 pages.
Sailing with the wind: dispersal by small flying insects: https://www.researchgate.net/publication/313181339_Sailing_with_the_wind_dispersal_by_small_flying_insects
Toronto chooses oak as its official tree: https://www.cbc.ca/news/canada/toronto/toronto-oak-official-tree-1.6453659
Toronto’s oldest tree will no longer be cut down thanks to last-minute council decision: https://www.cp24.com/news/toronto-s-oldest-tree-will-no-longer-be-cut-down-thanks-to-last-minute-council-decision-1.5214228
Toronto Urban Forest Management: https://www.toronto.ca/services-payments/water-environment/trees/forest-management/threats-to-trees-diseases/
Why Oak Trees are Declining or Dying: https://extension.umd.edu/resource/why-oak-trees-are-declining-or-dying