Looks like scab or a fungal infection as a result of another issue that is making the leaves turn yellow and maybe even dry out.

Check out the soil acidity and amend the soil if it needs to be made more acidic. Maintain the soil as evenly moist as you can (no periods of dry-moist-dry-etc) and apply 3-4" of mulch. The soil looks a bit dry but I cannot tell if it is that has been like that for long periods of time. I would also trim some of the plants growing around the camellia since the camellia roots tend to be on the top 4" and these other plants' roots will disturb the camellia and "steal" nutrients and water. Since the camellia roots are near the surface, the roots may not get water if the surface soil on the top 4" dries out. Finally, make sure the shrub gets afternoon shade. Japonicas prefer morning sun only or dappled sun.

http://www.afn.org/~camellia/leaves.html