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ISHS Acta Horticulturae 555: II International Symposium on Edible Alliaceae

EVALUATION AND USE OF A SIMULATION MODEL FOR LEAF WETNESS TO FORECAST DOWNY MILDEW (Peronospora destructor) IN ONIONS

Author:   C. de Visser
Keywords:   Peronospora destructor, downy mildew, leaf wetness model, advisory system, prediction
DOI:   10.17660/ActaHortic.2001.555.17
Abstract:
An evaluation of an existing and validated dew model is carried out using observed weather data and leaf wetness in six onion crops grown in four different years. Leaf wetness was measured with a resistance leaf wetness sensor. Data on temperature and relative humidity were measured in the crop, while rainfall, wind speed and cloud cover were obtained from a nearby automatic weather station. Hourly calculated leaf wetness values in mm were converted to mV-values using an exponential model. Calculated leaf wetness duration was compared with observed leaf wetness duration. On average the calculated leaf wetness duration overestimated the observed leaf wetness duration by 0.30 hours with a standard deviation of 3.89 hours. The effect of the dew model on the weather based advisory system DOWNCAST was evaluated. The sporulation-infection periods calculated daily by DOWNCAST on the basis of calculated leaf wetness contradicted the sporulation-infection periods calculated on the basis of measured leaf wetness on 6% of the days. The combination where a sporulation-infection period was not estimated using calculated leaf wetness while a sporulation-infection period was estimated using measured leaf wetness, occurred on 1.5% of the days. This means that the use of calculated leaf wetness in forecasting downy mildew could potentially lead to some unnecessary spray advices but less probably to unpredicted outbreaks of downy mildew.

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