Answer: What you describe is called "shot grape" in the industry. To minimize this in commercial orchards where immature grapes in a cluster is not acceptable, the growers thin the developing grape clusters or painstakingly pinch off the undeveloping grapes so the rest of the grapes in the cluster fill in the empty spaces as they are maturing. I'd vote for thinning the grape clusters as they are beginning to form. This will result in fewer clusters, but larger and sweeter grapes. Give this method a try this year and you'll see a big difference in your grape harvest.
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