Posted: 29/08/2014 at 23:23
GH toms are certainly not immune from blight but it is less likely. Good ventilation is a must though as there are other fungal diseases (such as the various leaf spots) which are rarely seen outside but love the hot humid conditions which can occur. Since fitting home-made solar-powered fans to my GH I have had no trouble with diseases and I reckon a louvre at one end, and 2 roof vents is ideal, as well as leaving the door open for the whole of the summer to ensure a good airflow through the plants.
When I first got a greenhouse I installed a misting system and closed the door and vents every night only to end up with my plants getting every disease I have ever heard of (plus a few more!) I honestly thought at the time I was providing perfect growing conditions; I was, but perfect only for diseases and pests, not tomato plants! Tomatoes do not like humid conditions, rather the opposite in fact.