If symptoms are localized to mostly mature leaf sets (those on stems already carrying blooms), see which one of the following deficiencies your plant may have.
Symptoms over Whole Bush
Nitrogen Deficiency: Leaves lighter green to yellow, with random leaf spots. If soil is too acidic (pH 5.8 or less), apply lime (1/4 to 1/2 cup per bush). If pH is OK, fertilize with high nitrogen fertilizer (1 to 2 tablespoons per bush).
Phosphorus Deficiency: Leaves dark green developing dark red and purple colors, mainly within leaf (colors can also spread to outer edges). If soil is too acidic, apply lime. If pH is OK, fertilize with high-phosphorus fertilizer (20 percent), 1 to 2 tablespoons per bush.
Localized Symptoms in Smaller Areas
Potassium Deficiency: Dead tissue, mainly at edges of leaves. If soil is too acidic, apply lime. If soil pH is OK, feed with 2 tablespoons per gallon of potassium nitrate.
Zinc Deficiency: Large areas of dead tissue at tips and between veins. Apply lime to correct pH. If pH is OK, then apply zinc chelate (1 teaspoon per bush).
Magnesium Deficiency: Yellowing starting from center of leaf, with signs of dying tissue overlaying the affected parts. Apply Epsom salts, 1/2 cup sprinkled around the base of each bush.