Here's what the dubious leaves looked like:
|Closeup of the worst affected leaf; this was "weeping" fluid just after I cut it.|
|Underneath the leaves.|
|Upper side of the leaves. Note the top one has a long "blister" spreading out from the dark spot.|
I really really hope this is a fungus, not a bacterium, as those seem much harder to treat (and I don't think I have, or can easily get, anything that does).