Investigations into the effect of pre-bloom defoliation on quality and yield of organically grown 'Riesling' grapes as well as 'Zweigelt' grapes grown according to integrated plant protection guidelines.

With the varieties `Riesling` and `Zweigelt` in a two year trial the effects of defoliation at different stages of development on the occurrence of sunburn and botrytis and on cluster compactness, yield, cluster weight, berry weight, must weight, acidity, pH-value and the concentrations of phenols and nitrogen in the must were determined. In the year 2012 with the variety `Riesling` damages caused by sunburn increased because of defoliation and were significantly highest in the treatment `early removal of 6 leaves and of 2° shoots at pre bloom at the development stage BBCH 57`. In the year 2013, because of the climatic conditions, damages caused by sunburn were significantly lower and occurred only on individual berries, just as with the variety `Zweigelt`. With the grape variety `Riesling` occurrence of Botrytis was significantly highest in the control treatment, were no leaves and shoots had been removed. The removal of six leaves and the 2° shoots at pre bloom (BBCH 57) had a significantly reducing effect on the incidence and severity of Botrytis. With the variety `Zweigelt` Botrytis infestation was very low and occurred only on individual berries. A reduction of cluster compactness caused by the defoliation before flowering could not be determined with the variety `Riesling`. Cluster compactness was classified `medium` to `dense`. With the variety `Zweigelt` cluster compactness was significantly reduced from `dense` to `medium` by the treatment `removal of six leaves and 2° shoots at pre bloom`. With both varieties yield and cluster weight were significantly lowest in the treatment `removal of six leaves and 2° shoots at pre bloom`. Year-dependent reduction of yield was 16 % respectively 21 % with the variety `Riesling` and 28 % respectively 35 % with the variety `Zweigelt`. Cluster weight decreased year-dependent by 20 % respectively 15 % with `Riesling` and by 24 % respectively 29 % with `Zweigelt`. In both years but only with the grape variety `Riesling` must weight significantly decreased in the treatment `removal of six leaves and 2° shoots at veraison` namely from 0.8 to 1°KMW respectively from 0.4 to 0.7°KMW. However the treatment `removal of six leaves and 2° shoots at pre bloom` did not influence must weight with both varieties. Only in 2012 and only with `Riesling` the content of phenols in the must of the treatment `removal of six leaves and 2° shoots at pre bloom` was significantly increased and had a value of 0.039 g/l. With both varieties the nitrogen levels in the must dispersed strong within the different treatments but without significant differences between the treatments. In the wine tasting no significant differences could be determined between the vines of the four treatments.