We have had a very cold spring and I have noticed black/brown spots on my two autumn blaze maple trees leaves. Some of the leaves have dried and fallen to the ground. I thought this variety was suppose to be quite hardy and not susceptable to most diseases. I have watered and fertilized the trees and do not believe the cause would be lack of care on my part. Any ideas what the problem would be? I purchased them from a very reliable nursery and they were about 8 feet tall when planted two years ago.