Zygopetalum
 

I just noticed this on my oldest bulb on my zygopetalum. Is this the old bulb just dying or is it some sort of disease? http://uploads.tapatalk-cdn.com/2016...2ad51c8229.jpg

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From the photo it seems your zigo is dehydrated, all the mature pbulbs are shriveled and the medium seems dry too. Zigopetalums should never dry out.

And the older pbulbs also die.

Yeah, I was out of town for a few days. I just watered it yesterday. But I was talking about the black part of the bulb.

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It seems you have more than one new growth. I think it's normal for the oldest bulbs to die as their reserves are being consumed by the new growths. After all that's where the plant stores energy.
But let it dry out can make it happen faster, I think.
I'm no expet on Zigos so let's see what others have to say.

It looks like normal die-back to me, possibly accelerated by the dehydration. If it is soft remove the whole pseudobulb to keep any rot from spreading, but if it is firm/dry it can stay until it dies back completely. You've got healthy new growth and roots coming so the plant should recovery quickly from any lingering effects of dehydration, but the old pseudobulbs may never plump up again.