The image you sent shows a white powder on the leaves and this is a sign of powdery mildew a fungus disease encouraged by dry soil, poor drainage or plants grown where there is poor air circulation. Add a fan to the greenhouse and remove the most infected leaves. Give the plants room so they are not crowded together. If this does not work try a fungicide such as Funginex following label directions. When you water keep the foliage dry. I think your plants will survive and still give you tomatoes. Good Luck, Marianne Binetti