Modelling the Climate in Unheated Tomato Greenhouses and predicting Botrytis cinerea infection

Abstract

Abstract

Botrytis cinerea Pers.: Fr. is the causal agent of grey mould disease and is one of the most important diseases affecting tomato crops in unheated greenhouses. Ventilation is the technique used for environmental control in Mediterranean unheated greenhouses. Many growers tend to restrict nocturnal ventilation in order to increase air temperature, forgetting that humidity is a very important factor affecting plant development and most of all that high humidity is favourable to fungal disease development. Growers usually apply large quantities of chemical fungicides with disadvantages such as commercialization problems due to chemical residues on tomato fruits, high production costs, risk of fungicide resistance and negative environmental impacts. Nocturnal (or permanent) ventilation is an effective way to reduce high relative humidity inside greenhouses and could be a useful tool to minimise chemical use in unheated greenhouses. The main purpose of this research was to study the effect of nocturnal ventilation on B. cinerea occurrence in unheated tomato greenhouses and to develop a disease predictive model. Experiments were carried out at the Instituto Superior de Agronomia in Lisbon in two identical adjacent double-span greenhouses. The structural material was galvanized steel and the covering material was a three layer co-extruded film. Each greenhouse had a floor area of 182 m2, eaves height of 2.8 m and ridge height of 4.1 m; the orientation was north-south. The climate was controlled by natural ventilation, using continuous apertures located on the roof and side walls over the entire length of the greenhouses. Two different natural ventilation treatments were randomly assigned to the greenhouses. One treatment was permanent ventilation (PV), with the vents open during the day and night, while the other was classical ventilation (CV), in which the vents were open during the day and closed during the night. A spring tomato crop (Lycopersicon esculentum Miller), cultivar Zapata was grown directly in the soil between the end of February and the end of July in both 1998 and 2000. The growing technique was the usual for greenhouse tomatoes in Portugal. Trickle ferti-irrigation tubes were located between each two rows of plants. Climatic data were measured with three meteorological stations, one located in the centre of each greenhouse and one outside. Air dry and wet bulb temperatures were measured using a ventilated psychrometer. Soil temperatures were recorded using thermistors, the leaf temperature was measured using infrared temperature thermometers and the cover temperature was measured using a thermocouple attached directly to the inner film surface. Global and photosynthetically active (PAR) radiations, wind speed, soil moisture content and water draining from the lysimeter were also recorded. All data were averaged and recorded on an hourly basis using two data logger systems from Delta - T Devices. Data on the evolution of the crop, such as plant growth, leaf area, flower production, fruit production, fruit weight and yield were also recorded. The number of leaflets with lesions caused by B. cinerea were counted and removed from the greenhouse from the randomly selected groups of plants, five times in 1998 and 10 times in 2000. Experimental microclimate parameters recorded over the two years in the two greenhouses with different ventilation management are presented and analysed. It was shown that greenhouse air temperature was not significantly influenced by the night ventilation management. On the contrary, a significant reduction of air humidity occurred in the nocturnally ventilated greenhouse, even with unfavourable outside conditions that occurred during the spring of 2000. A dynamic climate model was tested, modified step by step, parameterised and validated for the conditions which occurred during this research. The modifications were mainly related with the crop and the soil characteristics, the heat transfer coefficients and the ventilation sub-models. The good agreement between the predicted and measured data showed that the revised model can be used to estimate the greenhouse climate conditions, based on the weather conditions and on the greenhouse-crop system characteristics. Also, it was shown that the modifications to the original model improved its performance. Nocturnal or permanent ventilation was shown to have a great contribution to reducing disease severity on tomato leaves caused by B. cinerea, in both years of the experiments. It was shown that nocturnal ventilation management is an environmental control technique which can be used as a prophylactic control measure, since it reduces the severity of B. cinerea on tomato crops grown in unheated greenhouses. This is a very important result since it permits a reduction in chemical use lowering both production costs and environmental impacts. A model that predicts grey mould severity caused by B. cinerea on tomatoes grown in unheated greenhouses was developed as a function of the time duration with air temperature and relative humidity within certain ranges. This model was validated, and comparison between predicted and observed disease data showed good agreement. Integration of the climate and the Botrytis models was tested and reasonable results were obtained, showing that integration of both models is possible. This combination permits the prediction of when the climate conditions would be favourable for disease development and what would be the expected grey mould severity. A warning system, defining disease risk levels based on disease severity was developed and could be a useful tool for technicians, advisors and growers, helping them to decide what are the adequate actions and the correct timing to avoid favourable conditions for disease development. A more practical and immediately implementable application was presented, defining disease risk levels based on the number of hours per day with relative humidity higher than 90%, which is a useful tool for growers, helping them to identify the risk of disease occurrence and making it possible to act in order to reverse or to avoid disease favourable conditions.