I don't know if you guys have County Ag Extension offices or something similar in Canada but if you do you can take medicine jar sized soil samples from the concerned areas around the yard and they can test the soil and tell you what you need to treat the affected area with ... which, if it were my guess I probably say a manganese or other similar type mineral.
Way down here in south Florida another concern with yellowing like that would be an insect or worm issue, which is often mistaken and mistreated as a fungus.
