Masdevallia mystery leaf loss, multiple plants, help!

[Paphy]

I was also thinking that your slow release fertilizer could be contributing to the leaf demise. Although maybe fine outdoors now with the new indoor condition it may be triggering a different uptake scenario.

Usually, with Masde's it can be best to simply regular water (no fert) normally (which also gives a flushing of salts) with intermittent very weak, 1/4 strength, fertilizing, about once a week or 1/2 tsp per gal every two weeks. With time release you never really give the plant a break. It's constantly getting fertilized. Even if it is portioned out minutely I don't think Masde's particularly like that. In native habitat they tend to get the flushing routine naturally, especially higher elevation plants growing between rocks, etc.

To that end, what ratio fertilizer are you using, standard equal parts NPK? or?? And have you changed water type as well since the indoor transition? What water are you using? In my area, tap water is ridiculously high phosphorus so adjusting fertilizer balance and/or having RO/rainwater is preferable for my 'picky' plants.

[braxtonds]

I have spoke in depth with members and the president of my orchid society as well as some local vendors and the general consensus was that it is was a fungal problem. I have never grown in the basement so this problem is new to me. I did a systemic treatment a few weeks ago and I haven't lost a plant since. I have also increased the air flow and decreased the watering slightly which I think helped alot. Thanks for all of the suggestions and I will keep you posted on the progress.