Vine reddening is a sign of sap restriction. In young vines it is a sign of restricted growth. There are four main causes of vines like the one shown left.
1- Physical restriction. String strangulation, tendrils around the vines or wireweed wrapping around the trunk will all cause these symptoms.
2- Union Failure. In the case of grafted vines redness can also be caused by a failure of the graft union between vine and rootstock. Vines on own roots don't have this problem.
3- Insect damage. Cutworm or African Black Beetle damage the vine under the ground by ring-barking the vine roots.
4- Nutrition. Phosphorous deficiency will cause red leaves.
The healthy young vines below have been topped and will grow laterals to be trained along the wire. Watch out for Powdery Mildew on young vines as this will stunt growth and make control of disease more difficult in future years.