My old "Bible" recommends dusting with sulphur for downy mildew on vines.
I would think if you could leave both ends of the poly tunnel open all the
time and if you could put a fan about 1/3rd of the way down the house and
fit a timer to bring it on for 15 mins every hour to prevent a build up of
stagnant air then you could well avoid the problem.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk