Help! Tan Splotches on Healthy Cattleya

[realoldbeachbum]

My Cattleya mossiae (‘Blanca’ x Aurora’) suddenly developed large tan splotches on top surface of two most mature leaves. There are no yellow halos around the splotches. They are recessed. Bottom of leaves are fine.

I bought the plant two months ago (mail order from Hausermann’s) and repotted it immediately. It has thrived, putting on an excellent new growth.

LIGHTING: Tip of top leaf is 6” from T8 full spectrum LED light strip (along with 20 other Cattleyas that are NOT splotched), 6 hours per day, plus little light from East window.

FERT/WATER: Very weak 'Better-Gro Orchid Plus' (1/4 teaspoon per gallon), mixed with few drops of CalMag and a few drops of seaweed extract – about once a week.

MEDIUM: Cattleya orchid mix from Repotme. Root tips are beginning to appear around the see-through plastic pot.

TEMP: In grow room with exterior door and windows open to Alabama heat and humidity.

The splotches occurred quickly (within two days) and do not appear on any nearby Cattleyas.

Please help me figure out what this is and what I can do. I searched the web for pictures, but found none that looked exactly like this. Thanks so much!!!