The application of magnesium (Mg) fertilizer to the foliage of ‛Pinot Noir’ was studied over three years in a vineyard that historically developed moderate to severe Mg deficiency symptoms on leaves in late summer in western Oregon. Three foliar sprays of MgSO4 between fruit set and veraison at either 11.2 or 22.4 kg.ha-1 for each spray (providing a total of 3.36 or 6.73 kg.ha-1 of actual Mg per year) improved Mg concentrations in leaf blades at veraison, but not in petioles, and decreased Mg deficiency symptoms. The high Mg rate had higher leaf blade Mg concentrations than the low Mg rate in year 3, and a lower prevalence of Mg deficient leaves in years 2 and 3 than the low rate. Improving vine Mg status with foliar Mg had no influence on vine growth, other physiological parameters, or fruit yield. Increasing Mg status also did not increase sugar levels in must, nor increase Mg or S concentrations in must. Leaf blades collected at veraison or thereafter were effective in diagnosing low vine Mg status, but petioles were ineffective in diagnosing Mg status. Critical values for leaf blade Mg concentrations for ‛Pinot Noir’ are discussed based on the concentration of Mg in leaf blades of symptomatic leaves and also in routine veraison samples where Mg deficiency symptoms subsequently occurred in experimental plots. Since there was no negative impact of low Mg on vine productivity or must chemistry, critical values for Mg that actually impact economic production or fruit quality for ‛Pinot Noir’ grown in the region remain unclear.
Schreiner et al. (Sun,) studied this question.