I have plenty of grapes on the vines, but for the last three years the grapes have all been small and fairly flavorless. I do get a lot of vine growth during the summer. I want to use my grapes for juicing.
Over-cropping is the most common cause for home gardeners. An average grapevine may have 200 to 300 buds which are capable of producing fruit. If grapevines are not pruned properly in late winter, the number of fruit clusters may be excessive. The vine is unable to ripen the large crop properly, resulting in uneven ripening of the berries within the clusters. In Iowa, 60 is the maximum number of buds that should remain on a grapevine after pruning.
Heavy rains as the grapes are maturing can cause rot and grapes won't ripen when temperatures drop below 50 degrees Fahrenheit
Where have you planted your grapes? They need full sunlight to grow and be protected from the wind. Too much shade is not good for them and will produce smaller grapes with less flavor. You may need to look at where you are growing these and they may need to be moved if you want a great harvest. You should find an area that get a lot of sunlight during the day and not a lot of shade. This should take care of the issues you are having.
You should only water the plants once a week and make sure you water them with at least an 8 inch deep trench filled with water. This is best for the plants and over watering is very bad. I remember my grandmother had grape vines going out of her house and they were filled with tons of grapes. She hardly ever watered the plants and the plants stayed in full sunlight and it was very warm where she lived.
Are you sure you are pruning correctly as this is most important or you will be cutting off your new grapes before they start. Ask neighbors for advice and read online instructions.
"Only fertilize your grape vines if the soil is very poor. Too much fertilization results in green top growth and little fruit."
If you do not wish to do this then maybe you could contact your county extension service and ask them for advice. They probably will have some suggestions but it would be best to have answers ready about fertilizer - type/how often/how much as well as watering system.
pickyourown.org/countyextensionagentoffices.htm
extension.umn.edu/.../growing-grapes-home-garden
homeguides.sfgate.com/grow-blue-concord-grapes-43292.html
A common problem that plagues concord grapes is uneven ripening. This is when some (or most) of the berries within a grape cluster ripen unevenly. Some of the berries remain sour, hard and green, while others continue to develop normally. The grapes that fail to ripen may grow to a full size, but they don't soften and they never see the increase in sugars and decrease in acid that occurs during the normal ripening process.
Uneven ripening can affect a small or large portion of the total grape crop and seems to happen to grape crops every few years. This problem is only associated with concord grapes and although the reasons are not completely understood, it's thought to be due to hot temperatures, overcropping (allowing the vine to produce more fruit than it is capable of bringing to maturity) or canopy shade. Concord grapes do not tolerate high temperatures, which is why commercial productions of this variety of grape is not common in your area.
Still, many people have success growing them in the backyard garden. Have you had some unusually warm weather this summer? For more information on diagnosing grape diseases, check out this bulletin from North Carolina State University: http://www.ces.ncsu.edu/depts/pp/notes/Fruit/fdin012/fdin012.htm
Doug the Cook