1) It's really hard and we just don't know how yet
2) Creating an engineered biological weapon and releasing it into the environment just might have some unintended consequences when it starts interacting with all the things out there which could cause it to mutate
I doubt our understanding of biology is strong enough to be sure that we could do something like that without causing major problems. It would, possibly, be more viable to construct an altered version of the fungus itself which could outcompete the one that kills the bananas, which is the kind of approach that has been investigated for controlling other pests such as mosquitos (although in that case they engineer mosquitos which produce sterile female offspring but viable males, so the males keep breeding and producing more sterile females... eventually you run out of fertile females and tada! No more mosquitos).
1) It's really hard and we just don't know how yet
2) Creating an engineered biological weapon and releasing it into the environment just might have some unintended consequences when it starts interacting with all the things out there which could cause it to mutate
I doubt our understanding of biology is strong enough to be sure that we could do something like that without causing major problems. It would, possibly, be more viable to construct an altered version of the fungus itself which could outcompete the one that kills the bananas, which is the kind of approach that has been investigated for controlling other pests such as mosquitos (although in that case they engineer mosquitos which produce sterile female offspring but viable males, so the males keep breeding and producing more sterile females... eventually you run out of fertile females and tada! No more mosquitos).