Effect of weather factors on the release of ascospores of Uncinula necator, the cause of grape powdery mildew, in the Bordeaux region

Abstract

During a 5-year period (1993-1997), the release of Uncinula necator (Schweiniz) Burrill ascospores was monitored under natural conditions in the Bordeaux region. Ascospore release always began after bud burst and generally ended before blossoming. The release periods of ascospores were always associated with a rainfall higher than 2 mm, a wetting duration greater than 2.5 h, an average temperature generally above 11°C and a daily mean temperature sum from November 1 to the first ascospore release above 1100°C. There was no relation between earliness, number of ascospores released, and disease severity on grapes (Vitis vinifera L. cv. Merlot). The primary infection did not appear to be important for the increase of the powdery mildew population; on the other hand, the weather conditions of April (rainfall and temperature) seemed to strongly influence disease severity on berries by enabling good growth of the pathogen on leaves. These findings could be used to determine the optimal dates of the first fungicide treatments for powdery mildew according to the weather conditions.Key words: cleistothecia, ascospores, release, weather conditions, powdery mildew, Uncinula necator, grapevine.