A dull, dark green color will appear on the leaves.  In mid-to late summer, leaves may have a bronze color, especially on the west-facing side of the trellis.  Some leaves may have dark spots or blotches.  This symptom often has been characterized as black leaf of grapes Marginal chlorosis, browning, and drying may occur as the deficiency becomes more severe.  Other possible symptoms include brown dead spots or areas throughout the leaf.  In sever cases, more than half of the leaves on a vine may show these symptoms.  Severe potassium deficiency greatly reduces vine vigor, berry size, and crop yield.  Symptoms of potassium deficiency generally develop in mid-shoot leaves followed by older basal leaves.

Foliar sprays of K2SO4or KNO3 can be effective to temporarily reduce a severe K deficiency.  Soil application in general 40-160 kg per acre have been adequate.  Foliar spray KNO3 1% is recommended.