If you take a walk up Mount Royal, you may notice that about a third of all the yellow leaves have peculiar black spots. These spots are caused by Rhytisma, a black tar fungus which lives parasitically in the leaves of deciduous forest species, and is killing Montreal’s maple trees. Surprisingly, the Mont Royal maples were not always at risk of Rhytisma. In fact, this situation arose by a misguided attempt to ‘clean up’ the city of Montreal.
During the 1950s, there was a general consensus in Montreal that the city needed to be turned around. During prohibition, Montreal had become a place where Americans travelled for drinking, gambling, and prostitution—earning it the nickname ‘sin city.’
Jean Drapeau, Montreal’s mayor at the time, was particularly concerned with ‘immoral behavior,’ including displays of homosexuality and intoxication, which were occurring in the underbrush of Mont Royal. In response, he ordered that the bottom two-thirds of the maple trees on the mountain were clear-cut in an effort to expose the area. Unfortunately, Drapeau’s plan had a major unforseen consequence. Clearcutting resulted in mass erosion and landslides, as there were no longer any trees to hold the dirt in place. This forced the mayor to plant more trees in the area he had just cleared. Keeping it simple, he selected one species, the Norway maple, which is what gives Mount Royal its yellow colouring in the fall.
However, the Norway maple was a foreign, imported species that grew much faster than the red maples already populating the mountain. Since the Norway maple reproduced more efficiently than their relative—they yielded more seedpods and grew much faster—consequently displacing the native population of trees. Mount Royal’s diversity had been replaced by a single species.
Nature was not meant to support monocultures. According to Donald Sheppard, associate professor of molecular mycology at McGill University, without diversity or variation within an ecosystem, species have no chance of evolving strategies or acquiring traits to avoid pathogens. If one tree is susceptible to the pathogen, all trees are at risk since they are genetically identical.
This problem has reproduced itself throughout history. The Irish potato famine is a classic example. During the 1800s the Irish solved their problem of feeding a growing population by planting the Lumper potato variety. By only planting one species, the potato crop lacked genetic diversity. When the environment changed and an airborne potato fungus swept through the country in the 1840s, the entire crop of potatoes was devastated.
Fortunately, the Norway maples seem to be able to tolerate the Rhytisma infection. Though the fungus has spread dramatically, its prevalence depends on the weather conditions of the spring and fall. According to Sheppard, the fungus grows on the leaves, which then die and fall to the ground. Therefore, in order for the fungus to infect neighbouring trees, the spores must be carried up and off the leaves and back up into the trees. This action increases with a long and rainy fall season coupled with strong winds.
The spring also has an impact on the infection. Jennifer Llewellyn, a horticulturalist with the Ontario Ministry of Agriculture, told the Montreal Gazette, explains that the fungus will survive the winter on the leaves infected by spores. “When you have a cold spring, leaves take longer to emerge and develop, so there is a longer period of exposure to the fungi spores.”
How can you get rid of it? Since spores can survive the winter, raking and destroying the leaves is the easiest way that we, as Montrealers, can reduce the chances of the disease reoccurring next spring.
