Good morning Jill,

Thank you for contacting Iowa State University Extension and Outreach's AnswerLine. 

We shared your question with our Extension specialist and here is her response: 

I think Jill may be on the right track on her weeping willow. They do prefer moist to wet soils and this year has been very dry in some parts of Iowa. I would continue to water well once a month to keep it from losing more leaves. It may go dormant prematurely this fall – but that doesn’t mean it is dead. If you scratch the twigs and they are green underneath the bark, then it is still alive.

Unfortunately, willows are not long-lived trees compared to some other species like oaks. Therefore she might want to consider taking cuttings of the willow this winter to start some new trees for planting in the future (propagating willow from cuttings).

Hi Jill.  I would do what the horticulture specialist suggested and check the branch to see if there is green underneath and continue to water.  It is too bad that it is the type of tree that doesn't have as long a life as other trees.  

Here is an additional article from the University of Minnesota Extension on leaf spot.  It also says that it could be caused by stress to the tree during times of dry weather.
https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs