That's a good question.

Green and yellow mottling could be just old interior leaves dying of old age and neglect by the plant. I see this (apparently) on The Fairy.

However, on a couple of plants, Lavender Lassie and Rosarium Uetersen, I've seen the mottling in areas where other leaves are blackspotted and there's no good reason for the mottled leaves to be discarded. So possibly some black spot infections never get around to forming the sporulating body. But if there is no blackish spot, there's no way the leaf could infect other leaves, so no need to pick off.

If the plant does have black spot you will always see a small black spot surrounded by yellowing. If there is not black spot in the yeloow area its not black spot.

This can also be cause by a mineral defficiency in the soil. Have a soil analysis performed.

jill