Posted: 10/05/2012 at 20:56
It sounds like they may have got too wet, possibly in the recent heavy rains. Does your container have good drainage?
The bad news is that it can spread to other plants, so if it is what you think I would be inclined to get rid of the lilies and the compost. Next year, try and keep the planter dryer, if necessary covering it during periods of heavy rain.