Brooks Prunes or Plums are well suited to this area. Autumn rain brings many plant diseases to light. Your tree has a fungal disease such as leaf spot or rust. I suggest that you apply a fall fungicide spray and then a dormant oil spray early next year. Here is the OSU Extension publication Managing Diseases and Insects in Home Orchards. It will give you the precise spray information that you need to care for your tree. Enjoy the fruit from your tree.

Thanks Anna. 

 In referring to the publication and specifically to your suggestion that we use a fall fungicidal spray:   On the "Prunes and Plums" page,  the only reference I see for leaf spot is during petal-fall stage or shucksplit.  Can we still go ahead and spray the fall fungicide?  If so, could you please direct me to fall fungicide products, as I don't see leaf spot or rust listed under "some uses".  Thanks!

 First order of business is to plan to prune your tree for good air circulation once the tree is fully dormant, Training and Pruning Your Home Orchard). Do not leave the leaves as mulch. Rake them and destroy them. Once the leaves fall, you have a few spray options. I’m including an additional publication from OSU Extension, Prune and Plum Leaf Spot which has a list of sprays available to a homeowner. They are marked with an “H” for homeowner. The “O” stands for organic growers.   As always, follow the label for specific directions, but I think you’re committed to that already.