Citrus acid or alkaline soul
Yellow leaves black spots -- soil problem?
Certified GKH Gardening Expert
It is likely a fungal problem.
They can tolerate just about any range of pH, but prefer to be around 6.5 to 7. Dolomitic lime will help keep the pH balanced, but you will need iron sulfate to bring that down just a bit, along with killing the fungus responsible for the issue.