Do not know the answer, looks like sunburn, but you are in zone 8 and I assume you are going into winter.

Do you know if it is a Miltonia or a Miltoniopsis? Although related they need different growing conditions.

Plant probably doesn't have a name tag, but if you do a bit of googling you will see the difference.

Thanks for your reply , I will do some research to find out .

My feeling is that the plant is in complete shock. Miltonias are very sensitive as to their water supply and sphagnum moss has a very specific structure that allows for an abundance of moisture plus little air pockets all through it. Medium bark has a completely different structure. For a plant that needs such a continuous supply of moisture, having its roots adjust to a completely different medium (and, sorry, an inappropriate one at that) is probably enough to send it over the edge. You can see, not sunburn, which is localized, but general water deprivation from the curling under of the leaves. , much more open and airy and inappropriate for fine miltonia roots

I don't know why my computer keeps re-arranging the wording of my texts, but I hope you get the "gist." The last part "much more open and airy and ainnappropriate for fine miltonia roots," was meant to be a description of medium orchid bark, compared to sphagnum.

Almost looks like it got frozen or very chilled.

Odd, never ran into this before. My guess, because it happened suddenly, something either over chilled the plant or cooked it.

Jane

John B, If it were a root problem, it wouldn't show up in a week. Looks like the whole plant was damaged by environmental conditions. My guess is heat or cold.

Jane

Maybe. More details needed.

Jane , you are correct , It was out side for 1 night , maybe it cold .

Yes, I concede. Sorry I got your diagnosis wrong. I recognize the color as temperature stress, but when you said you repotted a miltonia from sphagnum moss into medium fir bark, that's still a big red flag for me, unless it's a moisture-retentive mix. What's the saying? "That's my story and I'm stickin' to it." Probably because I've always had trouble with miltoniopsis and their fussiness with constant water supply. Yours is a warm-growing miltonia?

Here in the "sub tropics" the warm growing Miltonia are easy to grow and sometimes you see a Miltoniopsis at an orchid society meeting but you hardly ever see a repeat blooming the following year.

Most orchids grown here are outside all year in the yard or in shade houses.

Humidity is all over the place depending on wind direction. I think Miltoniopsis are mist forest type orchids.

You don't mention how cold it was that night that the orchid was outside. 50-55 would be OK, but if you had temperatures in the 30's, or frost, that could be a problem. It does appear to be damage from cold. But it may recover. I brought back to life a Strelitzia (bird of paradise) plant that was a Christmas gift. Problem was that the grower shipped it in December, and I am in zone 6. All the leaves were frozen, but near the base it was solid. When I tracked the package, it had been in transit for FIVE days, and all through northern states!. But right now it has two flower spikes forming. So don't give up.

Overwatered?

Thanks 1beautylover , I saw someone on you tupe even grow them in cup of water ..https://www.bing.com/images/search?view=detailV2&ccid=3AOpxGHt&id=8DE8C12701C5BE6BAA1E02C2FED06C98DE34F1B0&thid=OIP.3AOpxGHt-k_o9uXyR02JeQHaJ4&mediaurl=http%3A%2F%2Fwww.rv-orchidworks.com%2Forchidtalk%2Fattachments%2Fgeneral-orchid-culture%2F5115d1157723296-orchid-water-culture-dsc00602.jpg&exph=700&expw=525&q=Orchid+Grow+in+Water+Culture&simid=608001783806755530&selectedindex=19&ajaxhist=0&vt=0