It really depends on where you live in Michigan. If it's the southern part,
summers can get pretty warm and dry.

However, from the pictures of leaves I'd suspect a blight of some sort --
fungal or viral, I can't say. This happens with moisture stress, as Iris
mentioned.

Your local agriculture extension agent or bureau of forestry (Michigan State
University should be able to help) could give you more information. If it's a
problem for you, it may be a problem for others.

C Manville