Proper climate control can reduce the risk of botrytis. Air humidity and temperature play an important role in this. The physical variable ‘enthalpy’ is a measure that takes these two factors into account. During a meeting with rose growers it was explained how this concept could be used in the fight against botrytis.
A complex of climate factors is determinative of botrytis development. Spore germination in particular depends on free water on leaves or flowers. This free water can result from condensation. During a meeting of rose growers at the end of January researcher Peter van Weel introduced the concept ‘enthalpy’ as predictor of the risk of botrytis infestation. Botrytis is directly associated with condensation on the leaf and condensation is caused by differences in enthalpy.
Enthalpy originates from physics and stands for the heat content of air at a certain combination of temperature and moisture. Enthalpy is expressed in kJ/kg.
Avoid condensation
“There is a high chance of condensation at a small difference in enthalpy between inside and outside air. It is very difficult to avoid this by means of ventilation”, explains researcher Peter van Weel. “In such a situation opening the screen to remove moisture is hardly effective. This may even increase the risk of botrytis; research has shown that especially in autumn radiation to the cold sky may cause the flower to cool so much that it may under high RH reach temperatures below dew point for longer periods of time.”
Van Weel therefore advises to prevent condensation by increasing the enthalpy of the greenhouse air while at the same time avoiding radiation. Van Weel: “This can be done by keeping the screen closed while maintaining a higher greenhouse temperature. A better alternative for a small vent is to keep the screen fully closed and to vent with outside air warmed at the outer greenhouse wall to the temporarily higher greenhouse air temperature. This air is then blown underneath the crop via air tubes. The moist greenhouse air is removed via openings in the wall.
According to Van Weel enthalpy control as well as a radiation meter installed between greenhouse cover and screen are valuable additions in the fight against botrytis.