Posted: 21/09/2012 at 15:30
If it's a blight - let's call it fungal - issue, it has no devastating impact on the soil. If Late Blight, the spores don't live on in soil. If Early Blight, much more common, spores can have fallen to the soil. Turn the soil over and bury them. Buried, they're harmless. Ditto the other fungal diseases. A scrub of the greenhouse with a 1:10 bleach solution will deal with any lingering nasties.
Given reasonable precautions, reinfection from spores from the previous season is pretty rare. Most infections are new ones. Fresh spores arrive every season. They travel on the breeze. There's really no avoiding them.