Bacchus (Vitis vinifera L.) vines trained to a vertical canopy were subjected in the 1991 to 1993 growing seasons to one of three spray materials for the control of powdery mildew: distilled water (control); Kumulus S (micronized sulfur; commercial control); and potassium silicate (K2SiO3). In 1991 infection on berries and shoots of K2SiO3-treated vines was equal to that on Kumulus S vines at veraison and mid-Stage III, but exceeded Kumulus S by harvest in 1991. In 1992 and 1993, both spray materials exceeded the control treatment in terms of mildew control, based upon harvest infection ratings; K2SiO3 equalled Kumulus S in 1992 but failed to provide the same level of control under heavy disease pressure in 1993. Triangle tests on 1991 and 1992 wines showed no aroma or flavor differences between Kumulus S and K2SiO3 treatments, but tasters could distinguish between control wines and the other two treatments in the 1991 wines (tank samples and aged wines), and between 1992 control and Kumulus S wines. Yields were lowest in control plots in all three seasons, and control vines also produced lowest cluster weights and berry weights in 1993. No noteworthy differences were observed between treatments in terms of composition of berries, juices, or wines. Scanning electron microscopy and energy-dispersive X-ray analysis of the control and Kumulus S-treated berries showed large quantities of Si deposited near infection sites and hyphae, suggesting that grape berries may utilize endogenous Si to help fight disease. This suggests that exogenously applied silicates may act to augment the activity of their endogenous counterparts. At appropriate application intervals and concentration, K2SiO3 has potential as an alternative spray material to sulfur for powdery mildew control, because: (1) material cost is lower; (2) risk of H2S in wines is reduced; and (3) it would potentially fall within guidelines for "organic" winegrowers as a natural substance.
- Copyright 1996 by the American Society for Enology and Viticulture