When my onc. types get "rot", the p-bulb turns a yellow-brown and softens. Either I have been lucky or this type of plant only usually gets infected in one bulb at a time. Removing the bad bulb usually cures the plant.
A couple other thoughts: What kind of potting mix are you using? I ask as my biggest problems with these orchids is that I tended to buy them from either HD or Lowe's and it took me a couple plants to realize that they usually come packed in way too much sphagnum moss. At least for me, Florida growing conditions outside don't favor lots of sphag, causing the roots to fail from being too wet and the plant declines, looking alot like you describe. Mine like to almost dry out between waterings.
Another cause of your symptoms may be not enough light. Mine also likes very bright conditions (even taking a couple hours of direct sun early in the day).
Good luck.
|