There are quite a number of different fungi that set up on our Trees and shrubs that can look alarming but are quite harmless. Everybody's got to eat. Nectria or coral spot, apple scab, fungal leaf spots and powdery mildew are the ones we are going to discuss.
Perhaps it is the bright orange like color that catches people's eye. Most gardeners will at some time notice nectria somewhere in their garden. Not a primary or pioneer pathogen, but a secondary one, nectria has never killed the branch you see it on. It may set up as the branch is dying. Like many fungi, nectria is living off the stored energy that is in wood, cellulose. Common inside a cotoneaster hedge, nectria will help you spot the deadwood.
The other three are all fungi that take part of the year to set up, and flourish when they find conditions they like. They can be unsightly but never do much damage to the host. Once noticed, it is already too late to be effectively dealt with.
Apple scab is uncommon in Calgary. The scab needs wet and humid conditions early in the year to set up. Once in the leaves the fungus moves from there.
Fungal leaf spots can affect many species of Trees and shrubs. If the season goes long into the fall, the tired leaves of every Tree will be affected with something. And that's OK, and completely natural. Frogeye leaf spot is one we see on crab apples.
Powdery mildew can be present on a number of Calgary's Trees and shrubs. Caragana is perhaps most strongly affected. Some crab apples are also affected; Strathmore, not a particularity strong Tree, is quite susceptible.
If you have persistent annual problems you may want to fight back a bit. Fungicides have to be applied early, usually at bud break, before the fungus gets into the new leaves. After that, grin and bear it. Fall and Spring clean ups can help a lot by significantly reducing spore populations on old dead leaves and fruit. In general, if these plants are kept healthy these problems will not be significant. Remember, leaves are only here for a six-month visit, do their work and are gone; they are not a permanent part of your Tree.