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How should horizontal airflow fans in high tunnels be set up to help prevent foliage diseases on tomato, pepper, and eggplant crops?

Answer: First, the purpose of using high-airflow fans in your tunnels is to keep the air moving mostly at night to even out the temperatures throughout the tunnel. This decreases the chance that the tomato foliage temperature will drop below the temperature of the air around it, and hit dew point temperatures, which causes condensation to form on the foliage. Moisture on the foliage is a vector for fungal and bacterial diseases. To work well, the fans need to move the air along at between 50 and 100 feet per minute.For example, in a 30×96-foot high tunnel, four 20-inch fans rated at 1600 cfm would be more than adequate. The fans cost about $100 to $150 each. Place two fans on one side of the tunnel and two on the other side about a quarter of the distance from the sides of the tunnel. You are trying to get the air to move down one side of the tunnel and back up the other side so the fans on the opposite sides of the tunnel should point in the opposite direction of one another. Place the first fan about 10 to 15 feet from the end wall and the next one about 50 feet from the first fan. Do the same with the other two fans on the opposite side. These fans should be run when the whole-tunnel ventilation fans are not running. This will usually mean from the evening, when the sun starts to go down, until the morning. For more information, check out the ATTRA publication Organic Greenhouse Tomato Production. This publication covers some management practices to reduce foliar diseases in a tomato crop.