Conversion to organic vineyards is a critical period. The changes operated in the protection strategy can have no incidence at all on the yield or a contrario yield losses can occur if adaptations are not adequate. We raised the question of the pests and diseases control during the conversion to organic vineyard. From 2013 to 2016, we followed a network of 46 in conversion and conventional vineyards (from 10 to 15 plots each year) situated in Southern France and planted with Grenache. Eight plots were managed following conventional practices, 12 were in the first year of conversion to organic farming, 15 were in the second year and 12 were in the third year of conversion. The plots presented an important diversity of management systems, type of wines, pedoclimatic situations. We assessed the severity of the five major pests and diseases in the region (downy and powdery mildew, grape moth, botrytis bunch rot and black rot). The severity was observed at bloom, at veraison and at harvest. Severity was assessed as the percentage of the bunch affected by diseases or by the number of glomerules. 30 bunches were followed regularly per plot. Results showed that the percentage of inflorescences affected by pests and diseases at bloom stayed low whatever the farming system (in conversion and in conventional farming). We did not observe more than 5% of the inflorescences affected by pests and diseases. At harvest on the contrary, we noted a significant increase of the percentage of bunches affected by pests and diseases during the first and second years of the conversion period. Respectively it reached 30% and 35% of the bunch. We did not observed high levels of severity for powdery mildew and botrytis bunch rot at bloom and at harvest. Disease pressure was controlled during conversion as efficiently as in conventional farming whatever the regional pressure. Regarding downy mildew, severity levels stayed low until bloom. Later in the season the severity levels were significantly higher during first and second year of conversion. For grape moths, they were hardly controlled the first year of conversion, where higher severities were observed. We observed an effect of the conversion on the control of pests and diseases on the first and second year of the conversion. This impact was limited in time in fact the third year of the conversion pests and diseases were controlled again. Thus learning and adaptations were established the third year of conversion. Next step will be to link these results to agronomic performances in quantity and quality. But taking into account the severities observed at harvest, we can hypothetize that an impact on yield components could be observed.