Thank you for your question and for reaching out to Ask Extension.

There could be one of several things going on with your vines, namely nutrient deficiency or a virus disease. It will be important to first rule out major nutrient deficiencies, which may be causing the yellowing and marginal reddening of leaves. I would suggest submitting a leaf sample as the grapes start ripening (i.e., turning red or softening). You can read more about how to sample here: https://grapes.extension.org/monitoring-grapevine-nutrition/.

There are many laboratories serving the west coast that provide leaf nutrient testing. One such lab in California is Dellavalle Laboratory: https://dellavallelab.com/agricultural-services/.

Once you get results back, the grapevine nutrition guide above can help you interpret the results. Additionally, I would be happy to help interpret the results or you can reach out to UC Cooperative Extension.

If there are no major nutrient deficiencies to correct, you may consider submitting a sample this fall for virus diseases, namely red blotch and leafroll. I would not have the vines tested for disease until you can rule out nutrient deficiencies.

Young vines require adequate water and fertilizer. Be sure that they are watered weekly during the summer heat and fertilized regularly, especially during the spring and early summer months. Please reach out if you have additional questions about watering or fertilizing young vines.

Cody Copp, Extension Horticulturist, Oregon State University