Overwintering form of Erysiphe necator, the causal agent of grapevine powdery mildew in southern Syria

Th e main aims of this study were to determine the overwintering form and the primary inoculum of Erysiphe necator, the causal agent of grapevine powdery mildew in southern Syria. Eleven vineyards located at fi ve diff erent geographical sites were visited every week from March to November in 2014 and 2015. Th e results of fi eld and histopathological studies showed that E. necator survived as mycelium in dormant grapevine buds during the winter season. Th e fi rst fl ag shoots were observed shortly aft er bud break in spring. Th e number of fl ag shoots varied greatly according to vineyards, cultivars and years, being present on 27.4 to 61.9% of the grapevines in 2014, and on 5.2 to 40% of the grapevines in 2015. Th e percentage of fl ag shoots on the same grapevine also varied according to the year, cultivar and location. It was between 4.3 to 9.4% in 2014, and 2.1 to 3.6% in 2015. Th e disease was observed only on Balady and Black cultivars. Conidia were released from the second week of May to early September. Th e fi rst conidia were trapped around mid-May, and the fi rst secondary symptoms were observed on leaves from mid-May to early June according to the site. Chasmothecia were observed on leaves in 45.5% of the studied vineyards. Th e fi rst observation of chasmothecia on leaves was in July, and their numbers varied greatly between vineyards and years. Chasmothecia were not detected on bark, nor were ascospores trapped at the beginning of the season. Th ese results confi rmed that the ascospores did not have any role in the initiation of spring infection. To the best of our knowledge, this is the fi rst report of the overwintering form of E. necator in Syria.