It sounds to me like you possibly have a heat and/or overexposure problem there with your vines. I’ll share a little personal anecdote with you about a similar situation that I have some experience with in hopes that it relates to your own situation and helps you out.
I have a Malbec grapevine in a pot, which my husband and I recently transplanted when we moved from our old house in downtown Napa, California to a new property off on the west side of town. At the old place, the grapevine used to be in a big 1⁄2 barrel planter that didn’t have a bottom, so roots had the opportunity to reach down into the soil underneath.
When we moved, we dug it up and put it into a large 30-gallon (114 L) pot and put it in our new driveway, which faces due west. This is a very sunny spot but my Malbec vine, which is about three years old, is facing some challenges. Like your vines, the fruit shriveled earlier than expected after ripening on the usual trajectory earlier in the season. See if you recognize any of the same things in your vineyard as happened to my vine:
• Shallow root system: Because of the limited root space in the pot, as opposed to the old bottomless wine barrel, the roots just weren’t able to get enough water or nutrients to support a healthy canopy and the few grape clusters that developed. Solution: Plant in the soil, not in a pot and make sure you don’t have a “hardpan” situation where a hard layer of clay or rock keeps roots from going deep.
• Not enough water-holding capacity in soil: I made the mistake of transferring old, tired garden soil from the old house into the pot as we were in a rush to move. This most likely caused a nutrient deficiency, was not “living soil,” and provided soil with limited water-holding capacity. If you have poor soil you may really have to work in lots of compost over time. Solution: I should’ve added more organic matter to my potting soil to help retain water and provide additional nutrients.
• Insufficient canopy to shade grape clusters and develop healthy fruit: Because of the shallow root system and poor soil, not enough good nutrients were taken up by the vine as it came out of dormancy and budded out leaves. Over time, fewer than normal leaves developed and those that did were smaller than normal. This meant that those few clusters that did develop were at increased risk of sunburn and raisining after they turned purple and the heat really turned up. Solution: Do all you can to take care of and enrich your soil but also leave enough canes and leaves on during the growing season for your winemaking goals and your growing conditions. The last thing you want to do is leaf thin before a massive heat spike.
• Insufficient irrigation: When our driveway gets to be over 100 °F (38 °C) on a regular basis during the summer, it’s just not a healthy situation for a grapevine with limited soil, shallow root system and insufficient canopy. I made it really hard on myself, and on this poor grapevine, to get it enough water every other day to keep it going. Solution: Stressed vines with all of the above issues will just need more water to survive and be healthy. By solving the first three issues I could lessen the amount of water I had to put on the vine. Wherever your vine is planted, it’s important to dribble enough water to keep leaves from wilting. If you see wilting leaves, it’s gotten too dry. You want to water deeply and thoroughly, completely filling the root zone each time you water. For this reason I like “drip irrigation” where you provide a small volume of water over a relatively long period of time. This lets the water penetrate deeply with little runoff. Drip irrigation in relatively deep soils, on mature vines, can be done about once a week but will need to be increased during periods of hot weather.
• Too much hangtime: And last but not least, is it possible that you are just leaving the grapes too long on the vines? Depending on the varietal and the type of wine desired, one can usually plan on harvesting grapes about 35-55 days after veraison (when the grapes turn from green to purple). Solution: Taste your grapes and harvest when Brixes are between 23-25 depending on the type of wine you want to make (rough guidelines). Look at the calendar and the weather and call the pick balancing timing, the forecast, and all the signs of ripeness including color, acid balance, astringency, and flavor.
Growing grapes for wine is not an easy task and I congratulate you on even trying (me, I have one lousy vine in a pot). Take good care of your soil. Make sure you are growing enough canopy to shade the grapes during the hottest part of the day and are dribbling in water regularly over the growing season and especially during those late summer-early fall heat spikes that can happen in many parts of the country.
