I pruned my vine in early spring but after the fruit set I pruned it again so the grapes could get more light. I pruned mainly the top of the vine as those were the branches hiding the fruit. Now I have another set of fruit growing on the new growth. Never had that happen.
Susan

That's really cool, Susan. You must have pruned it at just the right moment. I hope it made lots more leaves, too though.

Btw, the fruit itself doesn't need to be in the light. It will ripen and develop sugar even in the full shade of the leaves. It's the leaves that collect the sunlight, make the sugar for the fruit, and supply all the energy for the plant to do its thing. So the more big, green, healthy leaves the plant has, the more sugar the fruit will have.

Photosynthesis: 6 CO2 + 6 H2O + Photons -> C6H12O6 + 6O2 (carbon dioxide + water + sunlight gives sugar and oxygen) That's what plants do. Leaves collect the CO2 and sunlight, roots and leaves collect water, we (and the plant) eat the sugar and breathe the oxygen, thank goodness. The fruit is the plant's way of reproducing itself but it doesn't do any of the work.

It's also a fallacy that store-bought fruit will get 'sweeter' if you ripen it in the sun on a windowsill. The amount of sugar in a fruit when it is picked is all you're going to get. The color will change, and the flavor may change too as the fruit ripens, but no more sugar will develop. It takes leaves to do that. Leaving it in the sun may dry out the fruit somewhat, which will intensify the flavor a bit. But fruit will ripen as well in the shade, (but much slower in the fridge). Some fruit will ripen faster if you put it in a paper bag with a piece of ripe fruit like an apple or banana that gives off ethylene.

Although they are a different type of grape, I grow several dozen merlot and cab vines and this is a frequent occurrence for us. We always remove these extra clusters for several reasons. If we let them hang, it would stress the vines as they now would be setting too much fruit. Also, the new clusters would be several weeks behind the primary flush- making the wine process impossible to time. Last, the cooler weather would most likely arrive before the brix/TA reached proper level. Like you, we thin the leaves every year so that the bunches hang in light but we need to shade a little more than other parts of Cali as our sun is more relentless. We do this so that all of the fruit will ripen close to the same time and to lessen the incidence of powdery mildew and bunch rot. Also, it is worth noting that many vintners advocate sun exposure in order to develop the more complex flavors associated with a particular grape or style of wine. Canopy management is such a science within itself.

Wow! I received two great answers on grapes. I have three different grapes growing and last year I had a big harvest but the Scuppernongs were hard to find because of all the leaves but the fruit was so sweet and delicious. I made juice and froze it in coke bottles, it too was delicious. So I guess I will cut the new crop of scuppernongs out, I can see where it would make the bigger grapes taste better and they would be large. I need to start writing my questions down as I get them because when I get busy working the land I forget.
Susan

Frogbit, the juice sounds delicious! to ATP!