Whether it is worth treating and rescuing back to health is something only you can decide, but if it does have leaf Septoria, (or possibly powdery mildew) moving it far, far away from your other plants would be my advice to you.
If it is growing in the ground directly and can not be moved, and if she does not interest you that much, then yes, cull it and throw it in the garbage and make sure it is as far away from your other plants as possible.
Cheers, Organoman.