Black tar spot of maples (Rhytisma acerinum) is one of the few excuses I can accept for sending your leaves off in the yard waste truck. Toronto gardeners have likely noticed this unsightly fungal disease over the past few years. It manifests as dime- or quarter-sized black spots on leaves, hitting many kinds of maple (Acer), including Norways (the sample above), red, silver and sugar. Tar spot isn’t a threat to the life of the tree, but it is not pretty to look at, especially now when the leaf colours change.
In theory, the only sensible treatment is to avoid letting infected fallen leaves remain in the garden, so that the spores can’t reproduce the following spring. That requires hot composting – which, for most of us, means bagging the leaves and putting them out for the compost pick-up. Municipal compost piles get hot enough to kill most of the fungal spores.
However, in areas where maples are heavily planted, such as the east end of the city where entire streets are shaded by Norway maples, bagging is probably a futile endeavor. There are just too many leaves, and they and their spores can hide everywhere. I still leave mine to provide humus to my sandy soil. Tar spot is thought to be a cyclical problem that will eventually run down.
A healthy tree is the best defense against diseases that might otherwise weaken it through repeated early defoliation. Make sure your trees are watered during drought, and try not to damage the root system through reckless digging or damage the trunk by raising the soil level like this.
For more information, here are fact sheets from University of Guelph, the Ontario Ministry of Agriculture, Food and RuralAffairs (which is what OMAFRA stands for) and from Cornell University.