Caveat: I am no pathologist, and have killed all my tropical bamboos, so have no authority, but come to think of it I'm not really a botanist either and I've killed plenty of temperate ones too, so here are some thoughts.
These are necrotic lesions, but just an extreme form of what happens to older culms as they lose their resistance to infection with old age. This would probably be caused by a fungus, the lesions spreading from where the airborne spores land and grow. These are much more noticeable on cultivars with yellow culms, but there still looks like a major difference between these enormous spots and the mass of much smaller spots that you normally see with age or the larger more irregular blemishes around scratches and cuts
Similar large lesions are seen on other plants, eg spur blight on raspberries, caused by Didymella applanata, see http://www.pisvojvodina.com/RegionPI/Li ... gavost.JPG
So this might be an opportunistic growth rather than a real infection, encouraged by a humid climate, spreading from the primary host to bamboos
On the other hand it could well be something unique to bamboos, carried on vegetatively propagated material, but something the bamboo can live with. Are others seeing it on their culms? Are there tissue cultured yellow-culmed bamboos around to compare?
Anyway, puzzling though it is, it looks like the bamboo is not seriously affected by it? If you see it on young, otherwise healthy culms that would be the time to really start worrying