Is the phrase disagree to grammatically correct?

No, "disagree to" is not grammatically correct. The correct phrase is disagree with.

Here's why:

* Disagree is a verb that means to have a different opinion or belief.

* With is a preposition that indicates the object or person you are disagreeing with.

Example:

* "I disagree with your assessment of the situation."

* "She disagreed with the proposed plan."