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Frosts nip wine grapes in northern Illinois

Wine grape growers in northern Illinois took some damage from frosts on April 27 and are facing the question of what to do next.
Bill Shoemaker, senior research specialist at the St. Charles Research Center, said the damaged shoots will wilt, turn brown, dry up, and fall off, and most will be replaced by secondary buds. The key problem for growers, he said, is knowing how to manage from here on.
Fruit on shoots from secondary buds will not mature at the same time. It’s hard to manage two kinds of shoots in the same vineyard at the same time, so growers may want to choose one set of shoots to grow and remove fruit from the other.
If enough primary shoots survived, they’re the best choice to keep, he said, since secondary shoots yield only about 60 percent as much and mature several weeks later.
“If you lost all your primaries at the time of frost, there is no problem,” he tells growers. “All of your crop will be borne on the secondary shoots. You may want to go through and remove the few primary shoots (or better, their clusters) remaining so that your crop at harvest is uniformly mature. The primary shoots you remove will probably be replaced by secondary shoots anyway.
“If you only lost a small percentage of your primaries (less than 15 percent), you still have no problem. Just remove any fruit growing on secondary shoots. The remaining crop will mature uniformly and will compensate for the loss by creating larger clusters. You may end up with a crop yield similar to the original crop potential because of the ability of grapes to compensate.”
The real dilemma occurs when you lose about half your primaries, resulting in a crop that is half on primary shoots and half on secondary shoots, he said. “Removing all of the fruit on primaries is an option, but not a happy one when you know you're removing serious crop potential. But if you don't, you end up with two different crops; one that comes in early on the primaries and one that comes in later on the secondaries.
“This becomes a judgment of whether you want to harvest twice or lose crop potential by removing your primaries.”
Those who grow for wineries need to talk to their winemaker about what to do, Shoemaker said. There are about 90 wineries in Illinois, but half the grapes come from growers without wineries.
The growers’ dilemma came from rapid vine advancement after warm March and April weather. They broke bud early. They survived several close calls early, but temperatures fell to 28° on April 27 at St. Charles. Damage in Illinois was mostly in the northeast. Damage varied by site quality as well as location.
Both wine and juice grapes in southwest Michigan were damaged by a freeze April 9, according to Mark Longstroth, the fruit Extension educator there. Damage was mostly on low-lying sites.