Would be.. , Tranzschelia pruni-spinosae , causes the leaves to fall prematurely.

Look in Google Images.

The usual treament is Sulfur.

I googled it and it sure looks like that. It is a young tree and doesn't have any fruit on it this year. Would it do any good to spray now or just wait til fall? We had apple scab on our apple trees last year and of course, peach leaf curl on the peaches. We sprayed for those diseases, but the plum rust caught us by surprise. Thanks for your help.

If you are in organic growing, apply sulpur (late evening) now. And the next year, one and two months after petal fall sulphur as well.
If the problem is severe, apply (now) azoxystrobin or pyraclostrobin but an limited number of times (not more than 4).
The usual control method would be preventive, with sulphur.

Sorry for my English

Thanks, santi2. I am a commercial cut flower grower by profession, but haven't had any experience with fruit trees. All help is appreciated.