REDUCTION OF POWDERY MILDEW AND OTHER DISEASES BY OVER-THE-TRELLIS APPLICATIONS OF LIME SULFUR TO DORMANT GRAPEVINES

From 72 to 96% of the ascospores within cleistothecia of Uncinula neca tor were killed in the spring when exposed to lime sulfur (calcium pol ysulfide), copper sulfate, copper hydroxide, or copper oxychloride for 5 min in laboratory assays. Dinocap reduced viability in the same ass ay when the exposure time was more than 1 hr. Neither sulfur nor triad imefon significantly reduced viability. Cleistothecia collected in fal l were not affected by the above fungicidal compounds in a 5-min expos ure. One hour of exposure to lime sulfur or 8 hr of exposure to copper sulfate was required before ascospore viability was significantly red uced in the fall. Aqueous solutions of lime sulfur at a concentration of 120 ml/L, applied as over-the-trellis sprays at 2,800 L/ha (336 L/h a of lime sulfur) to dormant grapevines in spring, killed cleistotheci a of Uncinula necator on the bark of the vines and delayed the develop ment of epidemics of powdery mildew. A lower rate of application was n ot effective against powdery mildew. The delay of the epidemic ranged from a few days to several weeks, depending upon the rate of toxicant used, the resistance of the host to powdery mildew, and the favorabili ty of environmental conditions for disease development. In vineyard tr ials, the severity of fruit infection was reduced from 43.1 to 14.1% i n 1986, from 4.6 to 0.1% in 1988, and from 41.4 to 27.2% in 1989 by a single eradicant treatment of lime sulfur to the Vitis interspecific h ybrid cultivar Rosette, which is highly susceptible to powdery mildew. The same rate of lime sulfur reduced the severity of powdery mildew f rom 30.8 to 0.3% in 1988 on the more resistant cultivar Vitis labrusca na 'Concord'. The incidence of angular leaf scorch (Pseudopezicula tet raspora) and Phomopsis cane and leaf spot (Phomopsis viticola) was als o decreased by over-the-trellis applications of an aqueous solution of lime sulfur at a concentration of 40 ml/L and 120 ml/L. The incidence and severity of black rot of grapes (Guignardia bidwellii) was reduce d by 120 ml/L of lime sulfur in 1991, but not in 1992.