Seyval Blanc is a white wine grape that has been extensively grown in eastern Canada and the North-Eastern U.S for several decades. It is sometimes known simply as "seyval" and was bread in the early 1900's by Seyve-Villard of France.
This vine produces large clusters of grapes and almost always requires crop thinning to reduce crop load. It can produce massive crops of grapes but in short season areas and cold climates needs to be thinned to ensure the remaining grapes ripen and the vine has sufficient time and energy to harden off for winter.
The vine grows fairly upright and vinifera like in a trellis however, not so much that it can not be adapted to various style suitable to vinifera or hybrid varieties. It has moderate resistance to the mildews and has little disease pressure.
Seyval Blanc is definitely a mid season grape. It can be picked in some locations in approximately 100 days from bloom to harvest but it is more likely a 110 day period. Often ripening to 22-24 brix, it can be picked earlier at 18-19 brix and still makes nice wine. Of particular cultural value, especially in areas prone to late spring frosts is it's tendency to bud out later than most other varieties.
The wine from Seyval Blanc can be pale yellow (in under - ripe fruit) to golden colour and highly acidic, green apple like (in under-ripe fruit) to rich full tropical fruit flavours in fully ripened seyval grapes. However, even the Seyval's that I have tried that were from cooler years or from vintages that did not fully ripen the fruit, the wine was still quite nice.
One of the finest wines made from Seyval comes from Jost vineyard in Nova Scotia. Year over year, Jost produces a consistent high quality wine from this grape. They leave a little residual sugar for an exceptional wine suitable for the deck or dinner.
More on seyval http://viticulture.hort.iastate.edu/cultivars/Seyval%20blanc.pdf