Thanks for the tip. I hadn't heard of that before.
You too, @em3

But it is probably a result of leaf spot disease that has survived winter. That would explain why everything was infected last year... Unfortunately, there's not a whole lot you can do about it if you want to stay away from chemicals.
For now, I've cover all leafs with a thin layer of lava rock meal, which keeps the leaves dry and prevents fungus from spreading. Fingers crossed!