There are a couple of possiblities.* Bacterial leaf spot is one.
From this site:
"BACTERIAL LEAFSPOT, Pseudomonas syringae
Symptoms. Dark gray to black leaf spots up to Â¼ or Â½ inch in diameter. Leaves tear or become tattered.Another possibility is a fungal disease - Cercospora leaf spot.
Survival and Spread. Favored by rain and storms, spread by splashing water. Infects through wounds.
Control. None developed. Usually none needed.
Remarks. Usually appears in late June or early July, slightly before Cercospora leafspot. Severe bacterial leafspot may confound early season diagnosis of Cercospora. Cercospora leaf spots have tiny black dots in them; bacterial leaf spots do not."
You can use vinegar as a fungicide.
These pics might be helpful too.