It's been incredibly dry here in southern ontario. Yesterday I found my amur maple had a number of yellow leaves on it, with brown patches. It was also drooping. I thought I'd been watering it enough, however I watered it and then it rained here 20mm. The tree is standing right up straight now. I hope that it holds the rest of of it's leaves, but maybe, if you're weather has been like ours, water might be the answer.