Ken,

What I believe that is affecting your Thomcord grape plant is an abiotic (nutrient deficiencies, cold injury and damage to root systems, etc.) condition, primarily a nutrient deficiency. Below are nutrients that are often associated with red leaves on grapevines.

• Potassium (K) is a general nutrient for all plants, improving the overall health and strength of the plant. It improves the plant’s ability to withstand temperature extremes, and to a lesser degree, stress from drought. Potassium also helps plants resist diseases.

•  Magnesium (Mg) is necessary for formation of sugars, proteins, oils, and fats, regulates the uptake of other nutrients (especially phosphorous), is a component of chlorophyll, and is a phosphorus carrier.

•  Phosphorous (P) is used by plants largely for root growth and development. Flowers that are well fed with phosphorus will have more blooms, and fruits ripen better and faster.

Please read this fact sheet Red Leaves in the Vineyard: Biotic and Abiotic Causes provided by Rutgers University for in depth information regarding the causes and recommended solutions to grape red leaves.

Good luck and happy gardening.