In one case I have seen dragon fruits suffered from N deficiency symptoms. This occurred in Kelantan, in the NW state of Malaysia - the soil is of bris type i.e. sand which is lack of organic matter. The symptom is shown in the yellowing of the whole mature branch and no disease detected.
If you encounter that problem, one should give a lot of organic manure or foliar spray .
The pic above how the symptoms look like. It is not actually disease but it is lack of N. If this matter continues the branch might dies and attack by pathogen which finally caused severe disease infection. Therefore, it should be prunned and cut off.
Should you have the same problem then you can email or contact me.