Urgent Orchid Advice - Withering/Yellow Lower Leaves

[phalorchidto]

Hi OrchidBoard!

I've grown orchid flowers for awhile. My phalaenopsis orchid plants have gone from green leaves to yellow leaves within a few weeks and many leaves have dropped off of all the plants. The leaves turn yellow/light green and drop. The plants receive bright, indirect light and the upper leaves don't show light burn.

I water once every one/two weeks during the winter and every 9 days depending on the moisture in the pot. So, I removed the sphagnum moss from one pot and the bark media from the other pots which have many perforations for circulation.

Positives: There were no rotting or mushy roots to cut. All the roots are green and firm. No aphids/mealybugs/pests were found. I water the plants with distilled or rain water at 0 ppm.

Cons: The leaves continue to turn yellow and dozens have fallen off of the 6 orchids.

I appreciate any advice.