Looks like sunburn. I'd cut the leaf to remove the dark half. Then sanitize your cutting tool in case of pathogens. Sterilizing cutting tools is common practice. Afternoon sun is the hottest. Reduce solar exposure...

Agreed! Sunburn.

I appreciate the feedback, but this isn't sunburn. The plant is in an eastern window. It's the only leaf affected ( so far) it happened over a day, maybe two, this leave in particular was tangled in another orchids leaves ( so no direct sun) also the window only get about 3 hours of direct sun. I've never seen a sunburn this severe with black in the center of a circle that almost looks little and bumpy. The plant is fairly new, I took it home from a nursery about 2 weeks ago. I have 10 other orchids in this window including two other dendrobium.

Was the leaf touching the glass?

No it wasn't

If there are multiple canes and this is the only affected cane, cut it off. If it moves into the other canes in might be some kind of a virus. if it moves to other canes, segregate it. Clean your cutting tools.

A few things to consider.

It does look like sunburn, and that area on the leaf ( where it bends) is where you see sunburn often. Like when you get sunburn on you nose but not your chin, because of the angle.

Sunburn can come from the combo of sun and heat. When growing in windows you can get a greenhouse effect and surface temps can be much higher than air temps. Also the angle of the sun in winter can focus light on different parts of the leaf than the plant is used to.

Another thought is that you could have had a small area of leaf damage from the sun , insect , or even banging into something. The weak spot on the leaf then lets in an infection, bacteria or fungal.

Copper will damage dendrobiums and having too much nitrogen can cause leaves to become soft.

Your damage look like it started the center of the black circle and spread outward from there which would have me lead toward some mechanical damage follow by infection.

This is not a disease. It is a burn. Just cut that leaf off.

I live in Florida and my orchids are full of sunburn. When I grew in NYC, this time of year typically would burn soft leaves because of the change in sunlight. Mine grew in windows.

Trust us, this leaf got burned. Either in your window or before you took it home.

That is not a disease.

Jane

that does not look like sunburn to me, sunburn would cook and dry cell material and it would not spread past the burnt area. this looks like a fungus problem, the dead give away is the round blotch and soft mushy center and its concentrical growth as the fungus spreads. EDIT- ADDITIONAL NOTES - This is also not a heat problem, as if it was in a very hot environment t would have turned completely black, have done it... in my early times of not knowing better about orchids, I went to a sale bought a few and kept them in car trunk while we went to lunch and shopped in the heat of august in south florida. Opened trunk to find all plants turned completely black and totally cooked. This infected leaf is not cooked. if this was sunburned you would have a dry center and some yellow portion of leaf feathering into the green cell tissue of rest of leaf, the whole leaf is dying solely due to fungus infection.

This type of problem is more widespread in phalenopsis plants, see a generic example from a google search.

http://www.phalaenopsis.net/phpBB/viewtopic.php?f=9&t=12165

In any case, removal is the first step to avoid problem spreading. get a new razor blade and cut leaf at least a half inch into clean tissue, If all leaf tissue is infected, I would remove leaf and spray plant with a fungicide safe to use on dens,.