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

During a 5-year period (1993-1997), the release of Uncinula necator (Schwei niz) Burrill ascospores was monitored under natural conditions in the Borde aux region. Ascospore release always began after bud burst and generally en ded before blossoming. The release periods of ascospores were always associ ated 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 di d not appear to be important for the increase of the powdery mildew populat ion; on the other hand, the weather conditions of April (rainfall and tempe rature) seemed to strongly influence disease severity on berries by enablin g good growth of the pathogen on leaves. These findings could be used to de termine the optimal dates of the first fungicide treatments for powdery mil dew according to the weather conditions.