Remove that entire growth and then spray the leaves and stems (not the roots) of the rest of the orchid with Isopropyl alcohol to kill the infection (it might spread otherwise). Spray twice a day for a few days and check your orchid over carefully to make certain this doesn't spread. There are also more effective ways to control this. I have added the link for you of a good site that you should find helpful.
I had problems with Calcium deficiency some years ago and experienced something similar to what you show here as well as new growths turning black...and brown rot. I lost quite a few orchids. However, now that I add extra calcium, I have not had any problems since.
---------- Post added at 11:46 AM ---------- Previous post was at 11:44 AM ----------
Here is a link that helps you identify both the disease and explains ways to control/cure it:
http://www.parramattaorchidsociety.o...ids%20PDOS.pdf
---------- Post added at 11:57 AM ---------- Previous post was at 11:46 AM ----------
One more thing that I also do when potting up the Cattleyas is to set all but the roots on top of the medium.
Good luck!
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