Comfrey does not suffer from many diseases or pests. In fact, if you listen to some people it does not have any. Possibly, the primary disease it faces is Rust. Symphtyum Officinale suffers greatly from rust, to a point where it is probably not commercially viable to grow it except perhaps in a small garden setting. Bocking 14 however, is very resistant to Rust, thankfully. In terms of pests though, the ones I have come across in my region are only occasionally briefly mentioned in literature. I have only come across a couple which I find extremely annoying because of the damage they cause. I must admit I do enjoy the sight of rapidly growing comfrey, but these pests activities can turn a kind hearted fellow into a cold blooded killer !