Reduce greenhouse humidity to avoid disease problems

The highest relative humidity in a greenhouse is generally found inside plant canopies, where moisture is generated from transpiration and trapped due to insufficient air movement. Univ. of Mass. extension floriculture specialist Tina Smith and Univ. of Conn. extension horticulture and greenhouse IPM specialist Leanne Pundt said it’s important to properly water plants and reduce humidity in greenhouses to prevent and manage Botrytis (gray mold).
Plants should be watered early enough in the day to allow plant surfaces to dry out before evening. Only enough water should be applied to avoid excess water on the floor. Plants should be spaced adequately to improve air circulation around the plants. Mesh benches can also help to increase air circulation at plant level.
Heating and venting can be effective technique to reduce the greenhouse humidity even if it is cool and raining outside. The method and time it takes for heating and venting varies according to the heating and ventilation system in the greenhouse. To vent the humid air in greenhouses with vents, the heat should be turned on and the vents opened an inch or so. The venting and heating cycle should be done two or three times per hour during the evening after the sun goes down and early in the morning at sunrise.
The time it takes to exchange one volume of air in the greenhouse depends on whether or not fans are used and the size of the fans and vents. For some greenhouses it may take as little as 2-3 minutes for an air exchange. For naturally ventilated greenhouses, it may take 30 minutes or longer.

Pictured: By controlling the amount and time of watering, providing adequate plant spacing and using heating and venting to reduce greenhouse humidity, problems with Botrytis can be minimized.
Photo by Leanne Pundt, Univ. of Conn.