Maybe edema?
From my favorite pest/disease reference, a pdf by Sue Bottom:
"•Excess water is absorbed by the roots quicker than it is lost by the leaves, causing swelling of plant cells and producing a blister-like lesion.
•Occurs when plants watered during warm days and the nights turn cool or during periods of cool weather when water quantity and/or frequency is not reduced.
•The blister-like symptoms can appear on upper or lower leaf surfaces, stems, petals or sepals."
Here's a link to the rest of the pdf-
http://www.staugorchidsociety.org/PD...ySueBottom.pdf
The pictures kind of look similar to yours, perhaps at later stages.