Posted: 11/10/2012 at 15:39
Greenhouses offer no protection against fungal diseases. In fact, even with ventilation the relatively closed environment can be an incubator. There are also fungal diseases specific to greenhouses, rarely if ever seen outdoors. There's really no avoiding fungal problems. The spores are airborne, they're everywhere in the air, and all you can do is try to minimise their impact by either preventive spraying or judicious housekeeping.
Blight-resistant varieties are a bit of a misnomer. They've become popular marketing devices. Unless you're hit with the destructive Late Blight, most of the everyday fungal problems don't destroy tomato plants. Providing you take some care - removing affected foliage as soon as infection appears, etc - a plant will usually live a normal productive life.