There are various fungal organisms that can produce leaf spotting as shown....septoria, phyllosticta, cercospora. They are often associated with sunscald or winter damage and are typically encouraged by wet weather. I'd guess this to be septoria, as yellowing and foliage drop, especially of the lower, older foliage, is common with this.

Take samples into your local extension office for recommended treatment options for your area.

Thank you, gardengal for being so generous with your knowledge. I'll get in touch with my local extension office on Monday.