Hello Guccisimo85,
The fact that you say it is dry and brittle makes me think it is not black rot. To be honest, I don’t think I’ve ever seen black rot on anything other than a cattleya and it usually starts around the pseudobulb in my experience. Dry spots thankfully are usually not a terrible problem like black rot or erwinia.
What you have looks like either a fungal leaf spot or a bacterial spot. To my eye, it looks like a bacterial spot.
It is very common for Zygopetalums to get this kind of spotting but I also have something similar looking spots on one or two of my Dendrobiums.
In my experience, it comes from water getting left on the leaves. I really have not had too many issues with these spots harming the plants.
That said, I would watch it to make sure it doesn’t expand. Some people might also suggest spraying the plant with Physan which couldn’t do any harm. If it’s a fungal spot I think cleary’s is the best thing.
I don’t remember either of these things harming flowers but I’m not positive. I wouldn’t recommend using copper though as I’ve heard bad things about copper and dendrobiums.
I’ve also heard of some people mixing peroxide and cinnamon and putting that on the spots but I’ve also heard some negative thing about peroxide on this board.
Alternatively, it could also be a water quality issue/fertilizer burn based on where it is on the leaf.
Based on the fact that it is only the bottom leaf tips, I think this is also a very likely diagnosis. In that case, maybe it just needs better water or less fertilizer.
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