I live in North York, and come to the Botanical Garden regularly. Have two, one in front of the house and one behind. The soil is regular, we don’t feed them anything, water the plants around them. Every year they start red with abundant leaves (though this year somewhat less), then many of the leaves, esp. under the canopy, shrivel. I checked always for bugs but did not find any until last week when found a grey 1 cm long thin worm, could not zoom in close enough to take a good picture on the leaf. Could not find more of them. By the way, the leaves turn green in summer, not a problem though we would like them to stay red, but I guess this is what we got years ago. Your response is much appreciated.
Thanks for contacting the Toronto Master Gardeners regarding your concerns about your Japanese maples. What you describe is most likely verticillium wilt, a common fungal disease of Japanese maples. It is caused by a fungus in the soil that occasionally will infect and clog the xylem of the tree, not allowing water to reach the leaves–that causes them to wilt. If the branches die, they should be pruned from the tree. For further information, please see both of the attachments.
Master Gardener Guide–Japanese Maples