polytunnel

Blight is caused by wam and moist condtions (it was very prevalant last year on outdoor tomatoes). If you don't want to spray I'd advise some way of getting air movement in the polytunnel to try and prevent it

As Andrewr suggested, you need to get air circulation. Had this problem myself, but Im not sure if it was just the incresed airflow that helped - I had a hedgehog family move into the garden!

Agree with Andrew. Your PT is the problem. Seriously consider adding vented sides.

thanks very much for your suggestions.can i buy vents for my pt sidas