You often hear winemakers lamenting the weather during flowering and complaining about a poor fruit set. What does that mean?
The tiny flowers of grape vines must be pollinated to create grapes, just like any other fruit. In the case of grapes, bees certainly assist, but if all goes well, they can pollinate with the wind as long as the flowers stay dry so the pollen can blow freely from one flower to the next. When the weather gets rainy and the flowers are wet, the pollen won't blow around because it's all soggy. The bees are not out there working the flowers much in the rain either. So basically, nothing gets pollinated.
The picture below shows some clusters of Marquette all photographed on the same day. (Click the pic for a larger view). On the left, a nice healthy cluster of small grapes that had sunshine during flowering. In the middle a cluster created by a vine that flowered a couple days later, when it was rainy. You can see how very few flowers in the cluster got pollinated so the cluster has only a small number of grapes, plus a few tiny grapes that probably will not mature. On the far right, an extreme example of a cluster that really didn't pollinate well at all. Fortunately, I only have a few vines that flowered late and ended up with clusters like this. Who knows why they flowered late, but I'm grateful for the ones that flowered when the sun was out. This was a rough spring for grapes...very rainy in spite of early warm weather and a mild winter.
That, ladies and gentlemen, is why we pray for sun during flowering!