Separate (the diseased Catt) from ALL of your other orchids, like right now! Seriously, b/c if it is black rot, (or something similar), it can spread, really really easily... Thru water droplets splashing from one plant to another... This link should help you, (it's long, but seriously great info, I'd suggest for you to bookmark it, for future reference... I did awhile ago, when I started out, & I'm glad I did!)
Good Luck! And keep us posted... it will help other newbies to see what you are dealing with, and the steps u take to fix it!
http://www.houstonorchidsociety.org/...ySueBottom.pdf
Also, she has a chart for common fungicides/bactericides on there (pg 36), but you can't really read it, so here it is in its own link:
http://www.staugorchidsociety.org/PD...idDiseases.pdf
Here's the one for pesticides too, while I'm at it, Lol. (Its the chart on pg 12) Don't think this is your problem, tho...
http://www.staugorchidsociety.org/PD...rchidPests.pdf