There is an old Muscadine vine at Mama's that grows the sweetest grapes I've ever tasted. The vine is strung east/west if that makes a difference. What I'm asking about is an oak tree at the west end of the vine. In the last 3 years when the vine buds out in the spring and fruit is forming it looks like we are going to have a bumper crop but most of the grapes will shed long before they are of any size. In your opinion does the oak need to be cut down or is there some additive that will stop the shedding? I have been told boron will take care of the problem.
Various factors can contribute to your problem. If grapes are forming but falling off, poor pollination isn't your problem. Insufficient boron can be a problem. It's best to determine whether there's a deficiency by having leaf samples tested. Sandy soils with high pH are most likely to be deficient. You can take leaf and soil samples to your nearest Cooperative Extension Service office to test for boron. This link can help you find yours. http://www.csrees.usda.gov/Extension/ Insufficient moisture can be a problem. Oaks tend to be heavy drinkers, so the oak could contribute to that. If the oak is casting a lot of shade on the grapes, that can be a problem, too. Powdery mildew can cause flowers and small grapes to drop. It is most active when the temperature is between 60 and 80 degrees F. It can occur in sun or shade, but shade encourages it. Powdery mildew looks like a gray, dusty powder on the flowers/fruit. Fungicides can control it, but if you can eliminate contributing factors, do so. Some insects (beetles and bugs) will feed on small grapes and flowers. Check in spring to see if they're present.
You mentioned the East/West orientation of your trellis. When planting in full sun, I prefer North/South orientation so that the vines can get sun on both sides of the trellis as the day progresses. Regardless of the orientation, full sun is always best.