Wayne Jones wrote:
Hi
I have a Canadice seedless grape vine and a Stueben vine (like concord)
planted side by side.

The first year I got grapes from the Canadice they were seedless as they
should be.
Then, we had 2 bad winters where both vines died right back to the ground.
This year, I got about 6 clusters off the Canadice and some grapes had
seeds. Some seeds were very small but some were normal size.

The local nursery says that it shouldn't happen but I think some cross
pollination is going on or the Canadice is a plant grafted onto a base plant
which does have seeds and after the die back I'm getting branches and grapes
from the base plant.

Some kinds of grapes are grafted but the kinds you have almost never
are. Canadice almost could be grafted because that variety is very prone
to a canker disease low on the trunk. Possibly grafting could help
prevent that.
I would think it would be very obvious if you were now getting grapes
from a root stock. Canadice grapes have a unique taste (which I don't
happen to like... I replaced mine with something better). If you tasted
the Canadice grapes before the die down, do this year's grapes taste
about the same or not? Should be a simple question to answer. If they
taste like Canadice, I think you can expect the grapes to be seedless
again in future seasons. Cross pollination could be the problem, though
I don't believe I have ever heard of that causing trouble.
I've been eating Venus grapes this week. They are considered to be
seedless grapes that sometimes produce a few soft seeds. This year they
all have seeds and some of the seeds are not soft either. We had a
warmer than normal summer, by our northern standards, but I don't know
why that would matter. Most people growing Venus grapes would live where
it gets hotter than this every year. By the way, there absolutely,
positively are no seeded grapes growing anywhere near my yard.
If your vines sometimes die back to the ground, you should be doing what
I am forced to do every year: Prune the vines in the fall, after the
leaves fall off, and then lay them down flat on the ground. Use things
like bricks and boards to keep them flat. You have to plan ahead for
this by developing a trunk that is long enough to be flexible and
emerges from the ground at an angle that allows it to lay down. A thick
trunk growing straight up isn't going to bend down.

Steve, in the Adirondacks.
(growing 4 or 5 kinds of seedless grapes where the winters get at least
30 below zero every year)