Condensation forming on tunnel plastic causes 2 problems:
When condensation forms on plastic, it creates droplets that always fall at the same place. The leaves on which it drips never fully dry, which creates an ideal habitat for Gray Mold.
To prevent condensation, use anti-condensation-treated plastic in your tunnel. It is commonly available from tunnel manufacturers.
If you do not need to change your plastic soon, you can buy a bottle of anti-condensation liquid. You can apply it even during the season. No pretreatment required!