Yellowing leaves on Onc.

[Reg22]

Hello,
I bought this Onc. Taka in December in bloom. It was indoors for a month and I just put it outside on an east-facing balcony two weeks ago. Now, two of the largest leaves are turning yellow, but the new growth is still growing.

Should I be concerned? It's in a medium I'm not familiar with...looks like very fine tree fern fiber. I haven't wanted to take it out of that because it was doing just fine for the grower (a very reputable one, RF Orchids).
Is this normal or sunburn or something else?
Thanks!

[FLBob]

The yellowing of the leaves on the back bulbs should raise some mild concern. My guess is that as a result of moving the plant outside you have introduced it to dryer conditions. During the recent days the relative humility has probably dropped to sub 50%. Tree fern does not hold much moisture. You might think about adding a top dressing of sphag to help keep moisture in the root zone.

[Reg22]

Thanks FLBob.
Maybe the medium is not tree fern fiber after all because it seems to hold moisture for many days in a row.
Does anyone else have any ideas? I hate to disturb the plant when there is new growth coming in, but should I check the roots? What are Oncidium roots supposed to look and feel like anyway? I don't have a lot of experience with this genus.
Please help!

[BGGraham1]

Hey Reg22!

OK, I had a similar problem with a couple of my Onc. alliance plants. It was a twofold issue: 1) I had root rot. I overwatered, and nearly killed them; 2) when I tried to revive them, I almost killed them completely by putting them inside the house. I had read that when you are trying to get new roots to grow, you need to put them in a shady area. It was shady, but the circulation was poor. So I took them back out to the sunroom where I always have a fan going and put them under the table. Still shady, but much better circulation. Got new growth coming in with a nice little batch of new roots.

I notice that the pbulbs are a little shriveled. So, if it was my plant, I would take it out of the pot and check the roots. They kind of look like angel hair pasta to me, and should feel pretty firm (al dente, if you will ). I always give them a gentle tug to see if the root comes apart. They should be white, not brown or black (although sometimes the medium may tint it some.)

Cut off any of the mushy roots, and see what you have left, then repot.

The ones I have in recovery are in my "ICU mix", which is mostly sphag, with a little CHC and a few pieces of LECA. I lightly water them in with a mix of 1 capful of liquid seaweed extract and a 1/2 tsp of K-L-N rooting hormone to a gallon of water. I like to bottom water with the sphag - it kind of freaks me out a little. I wait for the medium to mostly dry out before I water again, and I don't use fertilizer until there are some roots coming in.

You have the new growth, so there may be some good roots coming in on that one.

Good luck, and let us know how it goes. Oh, and when you pull it out of the pot, take a picture of the roots after you've rinsed them off so we can see what you have going on there.

[orchids4me]

Looking at the pic I see the new growth is attaced to that back bulb with the yellow leaves. Its possible its using its energy to support the new growth by sheding the leaves to give it all the energy. I could be wrong but it looks that way to me.
Hopefully more ideas will come along along with the 2 above me I see their point also but I really think the new growth is taking its stores of energy by the back bulb supporting it to grow.

[Reg22]

Thank you very much, Brigitte. I will follow your instructions and let you know how it goes.

[jsehorn]

I agree with those who suggest checking out the root system. I have noticed on my Oncidium alliance plants, yellowing leaves can be one of the first signs of root loss. Pop it out of the pot to check the condition of the root system and the medium. If the medium is mushy, repot in fresh medium, and if the root are in bad shape, keep the plant humid and warm with less light and wait until new growths and roots start growing, and which point you can resume normal watering and culture.