Model Based Humidity Control of Botrytis in Greenhouse Cultivation

Botrytis cinerea is one of the most common plant pathogenic fungi that affects a wide range of greenhouse horticultural products and decreases their market value. B. cinerea can strongly decrease yield and post-harvest quality through latent infections. It is a highly water dependent fungus and a certain leaf wetness duration seems necessary for spore germination. However, during germination before building of the inocolum durations of dryness (desiccation) can kill the spores. To avoid grey mould disease incidence, the relative humidity set point is generally kept low. This costs high amounts of energy through dehumidification by heating and ventilating the greenhouses. Therefore, to control grey mould with climate control in a more specific way to save energy, a mathematical model simulating the microclimate in the greenhouse crop was used. The model calculates durations of leaf wetness and leaf dryness. The prediction of leaf wetness incidence is based on modelled leaf temperature at different locations in the crop and the dew point temperature. Duration of leaf wetness (i.e. time to dry out) is computed from the energy balance of latent heat on the leaf surface. Climate control is then optimised for energy saving and grey mould prevention. In the present paper, a simulation study was performed to test the performance of the model on greenhouse microclimate and energy saving. Simulations with a set of yearly reference climate data of Denmark showed high amounts of energy saving. INTRODUCTION In modern greenhouse climate control systems, options for energy saving with dynamic climate control are usually implemented. In those regimes (e.g. temperature integration), greenhouse ventilation is reduced to the minimum. However, humidity control is a limiting factor in those energy saving regimes (Körner and Challa, 2003b) and growers often adopt a strongly risk-averse management strategy, with frequent calendarbased fungicide applications (De Kraker et al., 2000). In the present situation, humidity is controlled to a relative safe level by ventilation and heating. This means that controlling humidity can counteract energy saving within dynamic temperature regimes (Körner and Challa, 2003b). One of the major reasons for humidity control is the avoidance of Botrytis cinerea incidence. In practice, B. cinerea in ornamentals is next to climate control actions usually also controlled with fungicides. However, their use is often restricted through development of fungicide resistance by the pathogen (Köhl et al., 2000), negative side effects of fungicides on plant growth, or visible residuals on plant surfaces (Henseler, 1981; Köhl et al., 2000) or environmental issues. Research resulted in model based decision support systems (DSS) (De Kraker et al., 2000; De Kraker et al., 1999; Tantau and Lange, 2003). Those approaches used models of Botrytis epidemiology, from which decisions on various control measures were taken. However, the focus of these DSSs was on a more directed application of fungicides rather than an improvement of the climate control strategies. A dynamic microclimate model based regime that calculates set points for the greenhouse climate computer based on fungus biology can improve that. One of Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005 142 the major characteristics of B. cinerea is the dependance of germination and infection from leaf wetness duration (LWD). Therefore, to prevent B. cinerea incidence with climate control in order to reduce the impact of biocides in greenhouses and to reduce energy consumption, rather than using a risk averse low set point for relative humidity (RH) and biocide applications according to a calender scheme or a DSS, LWD must be controlled first. In the present paper, we therefore present a climate control regime based on LWD rules. Periods of condensation and drying of leaves are calculated with a greenhouse microclimate model, and according to that the greenhouse climate is controlled to avoid B. cinerea incidence. The system (microclimate model and climate regime) was implemented in a greenhouse climate and control model, and simulations with average Danish yearly climate data were performed. It was shown that high amounts of energy could be saved, especially in combination with a dynamic temperature regime.