I'm not sure what causes this, it can be from media or from nutrition or simply just from aging a bit.
Just sprayed this, but you can see it have dark spots. Mostly I see it on roots thats been in bark, but I do see it on air roots aswell. I think its rarely something to be worried about, it looks different when its rot or anything else wrong. Keep an eye on it, probably nothing to be worried about. I've never had problems with any of the orchids having darker spots like this. Its nature, they age differently and stain differently from eachother.
Looks healthy otherwise
Of course, if it starts to spread black all over the roots or looks unhealthy, then you need to do something.
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