instant black spot on Cattleya!
 

I can't tell the difference between sunburn and rot. I moved this cattleya to the south side of the house on Sunday after removing some rot on another leaf. It is sunny here in tampa but it is October and I wanted it to get some more light. The temp dropped to 45 last night from 80 last week and is a big surprise. Now on Wed night I see this on one of the big leaves on one side that is facing the sun.

What do you think and what should I do now?

It looks like a sunburn. It shouldn't spread any, you can just leave it as is. The rest of the leaf that is still green is still functioning so I would leave it on. Some would take it off due to it not looking so pretty like that, but I would advise against it. I have seen a sunburn on a plant develop into a bacterial infection so I would just keep a close eye on it. If it starts to spread I would then remove the leaf.

:(( That certainly looks like sunburn to me! Too bad it takes such a long time for a leaf to fall off.

:)

Hello, that looks like burn to me, and unfortunately that is dead tissue so with time will rot or lead to decay, I would personally work my way with a sterile razor blade around the black gunk and be more cautious with the lighting- I think it is that you moved it immediately to south window, gradual movement to areas of higher light is the best way for them to adjust :)

That is a good idea, trim the brown/black area out as mentioned above. Make sure to dust the cut area's with cinammon afterwards, this will help fight off an infection.

You should not move a catt from a dark spot to a lighter area suddenly as you have discovered the consequences will be, burnt foliage. If you have to move the plant to a position were it gets more light give it some initial protection gradually acclimatizing the plant.
Please do not cut those leaves the plant needs all the leaf area it can get and very often the underside of a sunburnt leaf is still functional . It will also be a stark reminder of not to do that again !!!
I have personally never known these sunburnt areas to become infected with fungus , but if you want to make sure, rather spray the plant with a systemic fungicide

thank you everyone
 

I think that I will go with it being sunburned. I will keep a watch on it and see that they get less immediate sun in future.

I have some fungicide and maybe I will use that.