A  It sounds like the infection is bacteial. When this occurred on my equitant oncidiums, it would start on wounds left by insects or other sources. Unless the infected area was quickly cut off, the rot would spread through the stolon to the next fan, and soon the entire plant was dead.

 If the fan has more than one leaf affected, I usually cut the entire fan off, sacrificing one fan to save the rest of the plant. If it gets into the stolon, you have to remove one extra fan to prevent the spread. If it’s still on one of the outer leaves, you can cut the leaf at the base and often stop it. When making a cut, position the blade of the sterilized cutting tool at least 1/2 inch beyond the noticeable rot on the leaves.

I grow many equitants here in Miami, and have been through a similar infection. I used a fungicide called Dithane M-45, and mixed it with RD2O for the bactericide. They are available from OFE in Miami, a good supply house that will ship. There is also a product called Banrot or Stop Rot that can be applied to control this type of bacterial infection. Before using any product, however, make sure it is recommended for use on orchids. Prompt action is of the essence, since this is usually a fast-moving rot. Until you can get one of the above products, apply ground cinnamon powder to the cut area. A plant label can be used to scoop some from the bottle. Dip the label into the container to get a small mound of cinnamon and then rub it into the cut so it gets good adherence. Do it over piece of paper, so that any cinnamon that spills will be saved. I use a new razor blade and a new label for each cut, so that the infection does not spread. I use TSP (tri-sodium-phosphate), a saturated solution, from a paint store (economical) to sterilize the razor blades. You don’t get the same amount of rust on the blades that you get with chlorine bleach. The main disadvantage to the cinnamon is that it is only a spot treatment, like a Band-Aid. It does not pre vent the spread of the rot as the others will.   — Jim Watts, 7713 North Kendall Drive, Miami, Florida 33156.