Formation of overwintering structures of Uncinula necator and colonization of grapevine under field conditions

Developing shoots of grapevine (Vitis vinifera cv. Kerner) were inoculated with conidia of the powdery mildew, Uncinula necator, at well-defined phenological stages of the host to provoke the development of flag shoots in the field and to investigate these shoots as early as possible in the following growing season for the presence of the pathogen. The disease progress was monitored and fungal growth and development on samples from a field trial were analysed by means of low-temperature scanning electron microscopy (LT-SEM). Mycelium was detected on the surface and in the interior parts of buds. The suitability of the field plot to analyse flag shoots was proven by the occurrence of such shoots in the following spring. Furthermore, early stages of cleistothecia development on berries were described for the first time. Establishment of U. necator in dormant buds of grapevine, giving rise to flag shoots in the following spring, is considered to play a significant role in over-wintering of the pathogen in the vineyards of southern Germany.