We need more information. Pseudobulbs turning black is not usually caused by infection, so chemicals likely won't help.
What is the plant?
How long have you had it?
Where are you growing it? Inside? Outside?
How much light is it getting? Sun or electric?
In what is it potted?
What are the temperatures day/night?
How and how often are you watering it?
What is the humidity?
__________________ May the bridges I've burned light my way.