Too much water, not enough water, and fungus can cause them to fall off.
Some grapes will do well in acid soil, others wont. I am not sure about
Concord, but Mars is a take off of Concord and they do well. I know
Thompson didnt make it in Arkansas, and we had acidic soil there.

Dwayne

"Quin Filips" wrote in message
...
I planted 3 grape vines 3 years ago and this is the first year I am
getting grapes. I'm so excited! Now the grapes are about half the size
of a marble and half of them have fallen off the bunches already. Is
this a sign of my vines lacking something or getting too much of
something?

The Concord vines are covered with bird netting just to keep those
suckers in check and the pH of the soil is around 6.0 - just checked
today.

Any advice would be great.
Thanks,
Quin