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The color of the vine also indicates when to harvest Concord grapes. Throughout the growing season, the vine and stems are a light to medium green. When the vine and stems begin to turn to a light then medium brown, it means the grapes are ripening and ready to harvest.
If grapes are infected when they are small, about the size of a pea, the skin stops growing but the pulp continues to expand and the berry splits. If infection occurs during fruit ripening, purple or red cultivars fail to color properly and look blotchy at harvest. Downy mildew.