I've no idea what stem blight is, never heard of it, but it occurs to me that, as this plant likes damp soil, it may be simply suffering from drought - is that possible? Have you had a drier year than usual?

That's what I thought, Bamboo, especially if the problem started during the hot, dry summer. I'd get in there with the hose and give it a real drowning as often as you can, Ann, and cut out all the dead stuff. It's a hardy plant, so it'll soon start to shoot again. Annie (Cumbria)