Jeff,

One of the top Masdevallia growers in my OS feels Masdies are prone to various leaf-spot fungi. He, accordingly, treats his plants once a month with a systemic fungicide. (He uses Funginex, but says any systemic will do.) His technique is to drench the potting media with it--he does *NOT* apply it to the leaves themselves. My opinion, never humble but always honest, is that once per month is overkill (once every 3 months would probably be plenty), but I cannot argue with his success--his Masdies are always just bursting with vitality!

The only other thing I can suggest is to raise the humidity around your plants. Masd. nidifica is supposed to be one of the Masdies more tolerant of warmer conditions so I don't think it's a case of your grow area being too warm.