desiccation - there's actually a more correct name I can't think of right now

That is a fungal leaf spot, often brought about by a frost in the spring after the leaves have emerged, then turning rusty colored in the summer as the fungus develops. Typically I try to treat the garden with Daconil after a spring frost as it helps to prevent lots of different fungus problems (especially things like botrytis on lilium).

A fungus like this won't really hurt the plant, but I would make sure you clean up and throw away all of these leaves this fall after the plant starts to go dormant to prevent some of the spores from overwintering. Then consider a fungicide spray in the spring as it emerges to prevent it from getting bad again.

Thank you! I will do a thorough fall clean up and apply a fungicide next spring.