My choice of tomatoes all grown in a unheated greenhouse are, Alicante, Moneymaker, Ailsa Craig, Gdnrs Delight, and Bejbino (DT Brown). Bejbino is similar to Gdnrs Delight. My tip to prevent mildew is to ventilate well even leaving the greenhouse doors open 24 hrs a day when conditions allow,my other tip is to keep seedlings on the dry side