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A Cumulative Degree-Day-Based Model to Calculate the Duration of the Incubation Period of Guignardia bidwellii

July 2012 , Volume 96 , Number  7
Pages  1,054 - 1,059

Daniel Molitor, Section of Phytomedicine, Geisenheim Research Center, 65366 Geisenheim, Germany, and Department Environment and Agro-Biotechnologies, Centre de Recherche Public – Gabriel Lippmann, 4422 Belvaux, Luxembourg; Cathleen Fruehauf, Centre of Agrometeorological Research, Deutscher Wetterdienst (German Meteorological Service), 38116 Braunschweig, Germany; and Ottmar Baus and Beate Berkelmann-Loehnertz, Section of Phytomedicine, Geisenheim Research Center, Germany

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Accepted for publication 17 February 2012.

The duration of the incubation period of Guignardia bidwellii on leaves and clusters of Vitis vinifera strongly correlates to temperature. To describe this relationship mathematically, a new, cumulative degree-day-based model was developed. According to this model, first symptoms on leaves appear after reaching a threshold of 175 cumulative degree-days (calculated as the sum of average daily temperatures between 6 and 24°C starting on the day after the infection). On clusters, the duration of the incubation period is additionally affected by their respective developmental stages. For ‘Riesling’, the duration of the incubation period on clusters corresponds to the duration on leaves until reaching the phenological stage “berries beginning to touch” but extends continuously with ongoing phenological development. Therefore, a correction factor recognizing cluster phenology was derived to calculate the cumulative degree-day thresholds for the occurrence of first symptoms on clusters after reaching “majority of berries touching”. Hence, this present model allows the estimation of fungal development and forecasts the appearance of new symptoms on leaves as well as on clusters, enabling growers to more precisely schedule curative as well as protective fungicide applications against grape black rot.

© 2012 The American Phytopathological Society