Restrepia leaves yellowing

[Ray]

Originally Posted by VERN-O

Full strength would be 1 teaspoon per gallon so I would use use 2/3 of a teaspoon to a gallon on water
Water is 18megaohm deionized water

I think the issue is gross underfeeding, and possibly pH!

By using ultra-pure water, the calcium and magnesium compounds typically found in water are missing.

Dyna-Gro 7-9-5 is not bad (besides being expensive per nutrient unit), and can replace those, but I would use a full teaspoon per gallon every week.

Orchids are not particularly demanding of mineral nutrients, but should get a steady supply as while some nutrients can be transferred from old tissue to newly-growing parts, others cannot. That plant has grown a great deal, so possibly has "tapped out" the reserves.

The first thing I would do, when seeing yellowing, is water with a teaspoon/gallon of Epsom Salts. The lack of green often means a lack of chlorophyll, which usually means a lack of magnesium.

Then, after a few days, I'd rethink your feeding regimen, and start applying some regularly. I'd also check the pH of Dyna-Gro when in such an unbuffered water supply; it's probably very low.