Hello, rjinga. Can you post a picture(s) of the affected leaves? A close-up of a leaf (front and back) plus a wider shot showing a few affected leaves?

You could be having mite damage, scale damage or even lichens. Of the three, mite damage is probably what you have due to the time of the year in which the damage showed up. But, pictures would really help!

Mites are controlled with a miticide but it requires two applications: one followed by a 2nd one 10-14 days later.

Scale can be controlled using early morning oil emulsion sprays when temperatures are warm but under 85 degrees F.

Lichen are not organisms that damage camellia; instead they show up when the camellia (or some other plant/tree) is stressed for another reason. It is, thus, an indicator of a problem rather than a cause. Determine if the camellia soil is kept wet too long such that the roots are developing root rot.

Luis

You wrote: "like they have a film of dirt over them"

If that film rubs off, the plant has pests. Perhaps camellia scale.

It was said: Lichen ..."is, thus, an indicator of a problem rather than a cause."

Lichen grows on slow-growing plants but isn't the cause of a problem. Nor does slow growth always indicate a problem. It's a case by case investigation to determine why growth is slow.

A 12 year old camellia produced masses of blooms until last year. The normally deep green leaves have turned yellowy and not a flower in sight last year or this. We have tried to raise the pH by spreading pine needles around the base but to no apparent effect. Any ides?