QUALITATIVE APPROACH FOR MODELING OUTBREAKS OF GRAY MOLD EPIDEMICS INNONHEATED CUCUMBER GREENHOUSES

Disease records and microclimatic parameters were analysed in order to develop a model for predicting outbreaks of grey mould (Botrytis cine rea) epidemics in non-heated cucumber greenhouses. A quantitative stat istical approach failed to produce a reliable model. However, when emp loying a qualitative approach, an accurate, biologically sound predict ion model was developed. Outbreaks of grey mould occurred when, on a w eekly average, the wetting period of the foliage exceeded 7 h day-1 an d the duration of temperature in the range of 9-21-degrees-C during th e night (18:00-08:00) exceeded 9.5 h day-1. Data recorded in two growi ng seasons were used to construct the model and its predictions were e valuated with an independent data set. Controlled-environment experime nts provided biological interpretations for these greenhouse-originate d thresholds: when free moisture was present, germination of B. cinere a spores occurred within 7 h; the period during which the pathogen is exposed to environmental microclimatic influences (germination and pen etration) is 9-10 h. The potential for outbreaks of grey mould epidemi cs in commercial greenhouses could be reduced by restricting the wetti ng period to <7 h day-1 by measures such as ventilation and heating.