Posted: 01/09/2012 at 11:49
Nytiri, toms are subject to a variety of diseases. They can broadly be classified as fungal, bacterial and viral.
The problem is that "blight" has become a generic term for fungal disease. There are only two actual Blights - Early Blight, and Late Blight. Other fungal diseases - Septoria Leaf Spot, etc - have similar but identifiably different symptoms, but the end result is largely the same. The big difference between the Blights - and SLS, etc - is that Late Blight can and will wipe out an entire plant within a week. Early Blight, SLS, etc, will eventually spread across the plant - unless you take preventive measures like removing affected foliage as soon as it appears - but take a long time to destroy the plant. In fact, the plant usually runs out of steam first or cold weather sets in.
All fungal spores - the nasties that carry the pathogen - are airborne. They're invisible, they travel on the breeze, and they're everywhere.
I don't know what your particular symptoms are but it's very unusual for some toms in a greenhouse to be affected by fungal problems and others not. Greenhouse toms are particularly prone to fungal problems because of the enclosed environment and lack of air circulation you get outdoors. No tomato variety is immune to fungal problems.
Do you have any photos of your symptoms?