Practice sanitation in the greenhouse, especially when it is vacant. Kill all weedy vegetation growing in the greenhouse. Remove and dispose of dead plant debris. Use fresh, heat-pasteurized soil mixes or new soilless potting media. Avoid importing live bedding plants or other plant materials into a greenhouse where vegetable transplants are being grown.
Bedding plants are notorious sources of tomato spotted wilt virus.
Assume that crucifer, pepper, tomato, and watermelon seed are contaminated and apply a bleach treatment. Place seed in a loose-fitting cheesecloth pouch. Wash seed in running water for 5 minutes, then dry seed thoroughly. Use treated seed as soon as possible. Long storage times will reduce seed germination.