The cell protects its DNA from foreign attack with multiple defensive barriers. Breaching those barriers successfully is called "cancer", that is, cancer is what happens when the DNA's normal function is compromised, either by a virus, or some environmental factor, like random mutation, radiation or toxins.

I can't think of any method today that would target a specific DNA without being hazardous to other organisms as well, for example, if we sprayed Melaleuca stands from the air with a toxin targeted to its DNA, it might be disastrous if some other organism's nucleic acid could be affected by the toxin.

It could probably be done in the lab, or one tree at a time, but that wouldn't be very effective as a means of biological control. Broad-spectrum genetic poisons sound like a potential nightmare to me. Too many things could go wrong.