The earliest grapevine varieties are starting to bloom at the Kearney Agricultural Research and Extension Center in Parlier, near Fresno, California. Some of the grapevines here, as in other parts of the state, are susceptible to mild zinc deficiencies, the only obvious symptom of which may be reduced fruit set. Varieties having 'Muscat of Alexandria' parentage are particularly prone to zinc deficiency, as are vines on sandy soils, or those grafted to rootstocks having Vitis champinii parentage, including 'Freedom' and 'Harmony'. All vines at the station are subject to an annual zinc foliar spray during a two-week pre-bloom window that closes when 60 to 70% of flowers have opened. This year's treatment is scheduled for Monday, May 9.
Neutral zinc is the most effective product. Two to three pounds of neutral zinc per acre are suggested to correct moderate deficiencies, though lower rates may be acceptable for minor deficiencies. Dilute sprays are most effective, using 100 to 150 gallons of water per acre. For more information on the diagnosis and correction of zinc or other micro or macro nutrient deficiencies, review L. Peter Christensen's excellent book chapter Mineral Nutrition and Fertilization.