USE OF POTASSIUM SILICATE FOR THE CONTROL OF POWDERY MILDEW [UNCINULA-NECATOR (SCHWEIN) BURRILL] VITIS-VINIFERA L CULTIVAR BACCHUS

Bacchus (Vitis vinifera L.) vines trained to a vertical canopy were su bjected in the 1991 to 1993 growing seasons to one of three spray mate rials for the control of powdery mildew: distilled water (control); Ku mulus S (micronized sulfur; commercial control); and potassium silicat e (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 I II, but exceeded Kumulus S by harvest in 1991, In 1992 and 1993, both spray materials exceeded the control treatment in terms of mildew cont rol, based upon harvest infection ratings; K2SiO3 equalled Kumulus S i n 1992 but failed to provide the same level of control under heavy dis ease 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 betwee n 1992 control and Kumulus S wines. Yields were lowest in control plot s in all three seasons, and control vines also produced lowest cluster weights and berry weights in 1993. No noteworthy differences were obs erved between treatments in terms of composition of berries, juices, o r wines. Scanning electron microscopy and energy-dispersive X-ray anal ysis of the control and Kumulus S-treated berries showed large quantit ies of Si deposited near infection sites and hyphae, suggesting that g rape berries may utilize endogenous Si to help fight disease, This sug gests that exogenously applied silicates may act to augment the activi ty of their endogenous counterparts, At appropriate application interv als and concentration, K2SiO3 has potential as an alternative spray ma terial to sulfur for powdery mildew control, because: (1) material cos t is lower; (2) risk of H2S in wines is reduced; and (3) it would pote ntially fall within guidelines for ''organic'' winegrowers as a natura l substance.