Yellowing leaves on Oncidium

[Shamu]

Hello all! I have a 6 year old Oncidium Twinkle 'Fantasy' in my care. About a year and a half ago, the perfect storm of low light, too much water, and aging media left the plant vulnerable to rot. The older psuedobulbs yellowed and became soft and mushy, dropping leaves. In attempt to save the plant, I removed all the yellowing and mushy psuedobulbs, leaves, and roots; repotting in a fresh bark mix.

Fast forward, the plant is still recovering and has been slow to bounce back. It began sending out new growth this past January, and things were looking up. However, in the past few days, the plant has started to yellow again. The tissue is still firm. Nothing has changed in the care. It is currently on a S facing windowsill with some extending LED light for ~6 hours a day. After a brief scare of underwatering (in my attempt to prevent further rot, I held off watering for too long, leading to a brief week of being underwatered), I've been closely monitoring its moisture levels with the bamboo skewer technique, using distilled water (the pictures show wet media because I watered it right before the pictures). I do not currently fertilize.

Does anyone have some helpful advice or thoughts as to what may be causing this downturn in health? Could it be temperature related by being on the windowsill? Could the brief underwatering period have killed off the root system and the consequences of which being this yellowing? Is it under-fertilizing? Might it be some sneaky pest munching on my beloved plant? How do I test out these different hypotheses without stressing the plant further?

Thanks for the help!

[Salixx]

It looks under watered and no fertilizer will slow its growth and could also lead to nutrient deficiencies. I don't know if that's what is going on, but I would have been fertilizing as soon as it had some healthy roots. Maybe someone else will know?

If the roots are in doubt, I think you should look at them, gently.

I don't think there are any pests involved.

I grow my Twinkle using Semi-hydro and it loves it. I think it's a great method for Oncidium types. You don't have to worry about over watering and you don't have to worry about media break down. I would definitely check it out.

[estación seca]

The pot is enormously too big. A plantlet this small would be better handled in a tiny pot, just big enough to hold it plus a few growths, in a terrarium to keep the humidity up. Don't let it get dry, but don't keep it soggy wet, either.

I grow Oncidiums in semi-hydroponics, but I wouldn't move this plant to S/H unless it is actively making a lot of new roots.

[Dollythehun]

I agree, you are grossly over potted. A smaller pot and finer bark are in order. These really don't like to get dry either.

[Shamu]

I gently took a look at the roots. The ones that I could touch without jostling the plant too much were firm. There appears to be a few green roots. I'm out of fertilizer, so once I get some, I'll start a weak dose to help boost growth.

The mix its currently in is the finest mix that I have (other than a spagnum moss based mix that I use with great effect for my phalaenopsis). Do you have any suggestions of where to find finer mix? As a student, I'm often moving, so it's impractical to start a semi-hydroponics set-up currently, though I will look into it once I stay in place for more than 9 months at a time!

[estación seca]

It will be OK if you are careful with watering - not dry, not wet too long.