Effect of grapevine training systems on development of powdery mildew

The effects of two training systems and row spacing on development of powdery mildew caused by Uncinula necator on clusters of 'Chardonnay' and 'Cabernet Sauvignon' grapevines were examined. Disease development was monitored in blocks with two different row spacing (2 and 3 m) in vertical shoot positioned vines (VSP) and in free-positioned, topped vines (FC) with no foliage support wires. The FC vines were hedged to about one meter shoot length. No fungicides were applied and disease powdery mildew level was recorded four to seven days after appearance of the first disease symptoms. During five consecutive years (1994-1998), disease incidence was higher in the VSP system than the FC vines. The difference was high when disease level was low (30% of the clusters in VSP vines infected, compared to 5% in the FC vines) and decreased when disease pressure was high (79% in VSP compared to 46% in FC vines). In the 'Cabernet Sauvignon', in four of the years, disease incidence was higher in the narrow spacing of 2 m between the rows than that in the wider 3 m spacing. Microclimate (temperature, relative humidity and light intensity) was monitored in the cluster zone near the spurs of 'Chardonnay' vines during three weeks in the 1998 season. In VSP vines light intensity was lower then that in FC vines both four and one week before disease symptoms appeared (72% and 18% respectively). The differences in temperature and relative humidity were less than 1 °C and 3%, respectively, and most likely did not affect disease development. The results suggested that high light intensity is the primary factor, which limits powdery mildew growth development on field-grown grapevines in the Golan region of Israel. The use of the FC system might be useful in reducing the need of fungicides.