by Sharon Oosthoek
Researchers at the University of Guelph are discovering that playing matchmaker to lonely elm trees – as with lonely humans – requires no small degree of perseverance. Nevertheless, they are determined to track down and eventually mate mature white elms, or American elms as they are also known, with a circumference greater than 18 centimetres. Such trees withstood the ravages of Dutch elm disease when it swept through the province in the middle of the last century. Native and non-native beetles spread the disease after feeding on wood infected with the lethal fungus imported from eastern Europe.
The immune systems of the scattered survivors were strong enough to seal off branches attacked by the fungus and stop it from spreading. But because resistant trees are isolated from one another, there is little opportunity for them to produce the next generation of resistant elms.
“It’s an astoundingly beautiful tree with a classic umbrella shape on the crown,” says Alan Watson, director of Guelph’s arboretum, where cuttings taken from surviving trees are propagated. “They died so quickly, causing such a rapid change in the landscape that people were shocked. Within a decade in certain places, the elms were gone.”
So far, Watson and his team have tracked down more than 1,800 survivors across the province and visited about a third to assess them as suitable trees for cuttings. Of those 600 trees, researchers have grafted cuttings from nearly half onto rootstock to create clones, and have already begun testing some of those clones for resistance to the fungus. Trees with the strongest immune systems will be planted in a “seed orchard.” When the trees reach maturity, they will be crossbred to create families of resistant elms. To make doubly sure of the trees’ ability to resist Dutch elm disease, researchers will expose them to the fungus once again.
There may come a time, believes Watson, when no longer lonely elm trees will again flourish throughout our fields and along our roadsides.