Looks like sunburn to me. Move it to a place where it gets shade during the most intense period of sunlight during the day. Here, that's between 11 am and 1 pm.
Dendrobium is a very large genus. It is best to identify the kind of Dendrobium you have to the best of your ability in order for others to figure out how to give you the best advice for the type of orchid you have in question.