I found this on the Mo. Bot. site . the problem may be the first ailment listed .
Several fungal leaf spot diseases attack orchids. They usually begin as yellow areas on the leaf undersides, but as they darken the spots are visible on both sides.
This starts as a sunken, brown, sharply-defined discoloration. Fruiting bodies develop in large numbers on the dead tissue. The causative fungus is Colletotrichum.
This leaf spot disease is caused by numerous Cercospora spp. and starts as yellow spots on the undersides of leaves that enlarge and eventually engulf the whole leaf. Old spots are purplish-black. Leaves fall from the plant prematurely. Once a leaf is infected the condition cannot be reversed.
The spots start as small, sunken, yellow lesions on either leaf surface that enlarge into circular or irregular patches. Leaves fall off prematurely.
Rust is caused by several fungi genera. Orange, yellow or rust brown spore-bearing pustules develop on the leaves. Rust fungi have high reproductive rates, so plants should be isolated.
Other leaf spot fungi include Diplodia, Guignardia, Phyllosticta, Sooty Blotch (Gloeodes), Fusarium, Flyspeck (Schizothyrium)
Here is a link to the whole page .. Kemper Center Factsheet
A picture .. Plant Disease Diagnostics, University of MN
If it progresses and cannot be stopped removing the leaf is an option . First I would try a fungicide , Captan , or even powdered sulfur might work . Maybe someone else can come up with a better one . ... Gin