Ken, I'm not Roberta but Black Rot (Pythium ultimum and Phytophthora cactorum) can invade mature plants (especially Cattleyas) at any plant part. The fungus can be introduced through unsterile pots, media or water sources. Water splash from adjacent infected plants will spread the disease too. Black lesions on roots, stems or leaves are a sign of the disease. The spots may expand and, if they reach the crown, the plant will die. Avoid the use of unsterile media, pots and tools. Do not use pond water. Do not set potted plants directly on soil, only on solid surfaces. Segregate infected plants to avoid splash contamination. Minimize leaf wetness. Do not allow a lesion to reach the crown. Use fungicides like Truban or Terrazole as a drench, and follow up with a good systemic fungicide like Subdue.
Jim