Answer: Some yellowing of older leaves is usually a sign of soil moisture extreems. Either too wet or too dry can cause such yellowing. If you had burned it with fertilizer, you would likely see burning of leaf tips and edges.
You may also be seeing the lasting affects of blackspot problems. Some leaves that did not show symptoms before the treatment but which were affected may now be succumbing to the disease. Blackspot sprays are not very curative but rather are preventative. Repeat spraying is usually needed on susceptible varieties.
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