Cercospora Leaf Spot. You get brownish and-or purplish spots. Some lesions decay to the point where the leaf ends with a hole. Usually caused by the presence of the fungi and overhead watering; common in the middle of or in late Summer. There are some fungicides that you can use but they are expensive so in my two infected shrubs, I use these practices: water the soil early in the mornings and not the leaves; replace the mulch if the infestation is large; dispose of infected leaves whose affected surface area is large (say 1/3 of the leaf? 1/4?); throw away infected leaves in the trash and not in the compost pile; dispose of the blooms and the dried out leaves in the Fall also in the trash; dispose of plant debris in the trash; do not plant other shrubs nearby to improve air circulation; if using fungicides, start at the first sign of trouble (or as soon as the rainy season starts) as the fungicides only control the issue and do not prevent it. When using fungicides, it is best two use two fungicides with different active ingredients: apply the first one; if the label says to reapply after 7 days, apply the second one instead; if the label of the 2nd one says to reapply after 10 days then after 10 days, apply the first one... and so forth.

Here is more info:

http://www.aces.edu/pubs/docs/A/ANR-1212/ANR-1212.pdf