Posted: 26/07/2012 at 09:05
Coralie, "blight" has become a generic term for any fungal or bacterial problem. There are actually only two Blights - Early and Late - but there are many more fungal or bacterial diseases, some of which resemble Early Blight in particular.
Can you be more specific about the symptoms - or even post a photo - so we can try to work out what the problem is? And have the plants been indoors or outdoors?
But, in general, unless you spray preventively, all you can do is try to minimise the chance of infection via housekeeping practices. I say minimise because, without preventive spraying, you can't actually stop infection. Fungal spores are invisible to the naked eye, they travel in the air, and they're everywhere. You can't avoid them.
Housekeeping practices include: keeping plants well apart to aid air circulation; judiciously removing foliage to avoid great clumps of leaves to aid air circulation; keeping the foliage as dry as possible because damp leaves are the perfect incubator for the spores. It's also a good idea to remove the lowest branches in order to maintain a gap of at least a foot between the soil and the lowest foliage. Spores can and will fall from the leaves to the soil and can be splashed back up again onto the leaves when watering. The gap helps against this.
When you say well-watered, how often were you watering? Over-watered plants - and over-fertilised plants, for that matter - can be more vulnerable to disease.