Cat leaf: half is yellow, half is green, what to do?
 

Looking for speedy advice on how to proceed with my Pot. Heathers Gold 'Motherload.' The first picture was taken 5 days ago, the others were taken this morning. This is the largest, newest leaf on the plant. It looks like there was some mechanical damage to the leaf, and now the yellow is quickly spreading... but only on half of the leaf. So far! What should I do? This was one of my first orchids and I nearly killed it from ignorance, but it looked like it was finally starting robust growth and now this happened! There is a new growth bud at the bottom of this cane. It's in S/H.

Help, please!

I would guess sunburn or thermal damage. Is it in the house, or a greenhouse? Did it get too close to a cold window, or to a heater?

Well, it was outside all summer, full sun. When I brought it inside, I placed it under LED grow lights on a heat mat. No exposure to cold, and the water reservoir never went dry. The leaf was growing so well that it DID get very (very) close to the lights, and at first I thought it looked like sunburn, so I moved it to a less intense location a few days ago. And the discoloration just continues to shoot through the leaf! Does it look infected? Should I take action or just leave it alone?

I've sunburned several orchids before, and I've not seen the wound grow like this one has. The others just turned black/crispy in the exposed spot, and the discoloration stayed put.

It seems to be environmental, not a disease.

Just a guess, but it looks like it sustained some damage that has either interrupted the vascular flow or allowed the introduction of a pathogen - that horizontal line seen in image 3...

I think it got too close to the lights. I don't think it's a disease. Most likely the leaf will turn partially black, then dry up. I wouldn't cut off anything until it's dry.

OK, I'll just keep an eye on it for now. Thanks!