I didn't think it was leaf spot because of the pattern I see here. Most of the examples I found online for leaf spot do not show any black up and down the spine of the leaf. Mine have developed a black center down the spine.

Did you spray something on them? The spine is a trough that collects spray liquids so when its off like that its a sign a spray may have been the problem. Besides that I don't have a good guess.

Scott

I agree, if you sprayed it with something like Safer's Soap, which I think is mainly potassium salts, those salts become concentrated as the product dries and can do significant damage to the leaves if not washed off a few hours after spraying.

I haven't sprayed these with anything.

Then the disease probably spread in a similar manner, a light rain moved spores elsewhere on the leaf to the spine. I would probably assume and treat like it is bacterial spot, if it is not spot it is a similar disease. Spot also takes on many guises depending on the weather etc, I have had many cases where I thought I had a new disease and it eventually showed its true colors. Treatment is dormant strength copper at leaf fall and again at bud swell in spring.

Scott

That makes sense.
Thanks Scott