Breeding new hope: American Chestnuts in the Ontario Landscape

A trail leading into the Ash Woods at FWG. photo: C Hanrahan

Foresters, landscape architects and conservationists are familiar with the seemingly never-ending parade of native trees on the road to being endangered. In the 70s, Dutch Elm disease changed the Ontario landscape forever as native elms (Ulmus americana, in particular) died, leaving gaping holes in the urban forest and newly shade-less streets. Today, the same bleak outlook applies to ash trees, another long-time landscaping stalwart. The Emerald Ash Borer is the culprit here, whose larvae feed on the inner wood of ash trees creating extensive galleries, slowly killing the tree through an effective girdle as water and nutrients can no longer reach the leaves. Expensive inoculation programmes are the current solution. For example, selected trees on the Experimental Farm are being treated but not all – and protecting street trees around the city would be prohibitively expensive for the city alone to cover (though they are engaged in a programme of selective inoculation with TreeAzin, and encourage ash tree owners to do the same). At Fletcher, we can only wonder for how much longer our Ash Woods habitat area will retain that name.

While less common in the Ottawa Valley, the American Chestnut (Castanea dentata) is a native tree useful in the landscape for its appearance, for its wood and for its edible nuts. More frequently found in Southern Ontario as part of the Carolinian Forest Ecozone, this once plentiful tree was listed as endangered by 1950, some 50 years after the killing culprit of chestnut blight was first recorded in New York. Blight is caused by an Asian bark fungus, Cryphonectria parasitica, imported with Asiatic chestnut trees at the turn of the century. Highly infectious, this air-borne fungal disease can spread rapidly where populations of chestnuts grow closely, such as nut orchards. Isolated stands of trees do remain though under constant threat.

Chestnut blight. Photo from Tree Canada. Visit their website for more information.

Infected trees develop fungal girdles, as the hyphae spread and eventually kill the cambium (the live area of a tree’s tissue just under the bark) all around a twig, branch or stump. A canker forms in the damaged areas, and the fungus secretes various toxic exudates which lower the pH to a level that further kills the tree’s cells.

Trees never reach maturity and appear sickly before dying, and re-sprouting stumps never produce a healthy tree. Like the Butternut blight resistance breeding programme, efforts are underway to develop trees capable of withstanding infection. A virus was developed to kill the fungus, but its success in inoculated trees was such that it killed the fungus and died before it could move on to other trees.

In Ontario, the Canadian Chestnut Council is leading a recovery project similar to one in the United States; the goal is to develop blight-resistant chestnuts. Research with the University of Guelph’s Simcoe Research Station by Dr Adam Dale’s team is looking to breed a resistant strain through traditional breeding techniques. Nuts are collected from trees in the landscape that show signs of resistance. These are sown and saplings purposefully infected with the fungus. The hardiest survivors are identified, and their flowers pollinated from resistant trees before being protected by bags to prevent pollination with undesirable trees. Those nuts are then collected and sown, with the process repeating itself. The research tree stands are limited to a few locations, including the Tim Horton Onondaga Farms in St George, Ontario.

Eventually, the hope is to plant the resistant trees in the landscape to breed with surviving chestnuts. Ideally, the genes for resisting the blight will spread and inter-breeding will prevent genetic bottle-necks. While results are promising, researchers warn that the re-population of trees will be slow and likely a forest full of mature chestnut trees isn’t something any of us will see in our lifetimes. Landscape-level natural research science projects operate at time-scales outside of human convention, so with a lot of patience, more research and work, we will hopefully one day again enjoy all the benefits of a large population of our native chestnuts.

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