Blight can normally only overwinter in live tissue, so should not be a risk, but as gavinjconway says, viruses could be an issue, especially if there are bits of root left in there. The compost will be spent anyway (nutrients used up), so adding to compost is a good idea (time and microbial competition kill lots of pathogens even if the compost does not reach a high temperature).
Adding to the bottom of a bean trench or curcubit pit would be another option.