What is the Prefix for disastrous?

There isn't a single, universally recognized prefix specifically for "disastrous."

However, the word "disastrous" itself is formed using the prefix "dis-," which means "not," "opposite of," or "removal of."

You could argue that "cata-," as in "catastrophe," could be considered a prefix related to disastrous events. However, "catastrophe" is a whole word, not a prefix in itself.