Thanks for your question about your oregon grape (Mahonia aquafolium). Our diagnostic guide in Oregon indicates that red/white/brown spots may be caused by several fungi carried in the air which, when deposited on wet leaves, begin go grow. Eventually, they will develop spores, rather like seeds of plants. Here's a link to a brief article about it, as well as suggestions to eliminate/control it. Please note that only the Bronide is available to the home gardener.
Hope this is helpful. Good luck!