Procrastination enhances risk of sunburn
Grapes aren’t as likely to be damaged if exposed early in the season, and some varieties are inherentlymore resistant than others.
By Gary Strachan W
ouldn’t it simplify our lives if there were a suntan lotion for grapes? Think about it. You could pull leaves and shoots around each cluster and get all of the benefits of fruit exposure, mildew control, and leafhopper suppression without a worry about getting wrinkles and dark blotches on exposed clusters.
Sunburned grapes can even deteriorate to the point of getting fungal and bacterial infections that lead to volatile acidity and crop loss —as if loss of fruity character wasn’t enough!
Sunburn, or sun scald as it is sometimes called, isn’t easy to pin down. It depends on where your vineyard is located, row orientation, grape variety, timing of fruit exposure, and I don’t think I have enough fingers to count the rest of the factors which affect sunburn. Shaded fruit doesn’t get sunburned, but opaque canopies have many other problems that I won’t cover in this column. Exposed fruit can develop resistance to sunburn, sort of like we do with our own exposed skin. Berries aren’t as likely to be damaged if exposed early in the season, and some varieties are inherently more resistant than others: Red varieties are more resistant than white, and thick-skinned varieties are more resistant than thin-skinned.
I suspect there is a relationship between mildew resistance (which often occurs with thick-skinned varieties) and sunburn
resistance but I don’t recall
having seen such a study.
It may not be immediately obvious, but pre- planting vineyard design affects sunburn
susceptibility. Slope, altitude, and orientation determine the heat and direct sunlight exposure that will fall on the operating vineyard.
tal predisposition toward sunburn is then modified by trellis design, variety selection, and vineyard management. For example, compare the differences between a vertical
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shoot positioned (VSP) canopy and a hanging cane such as California Sprawl (CS).
With typical VSP vines the fruiting zone is shaded. Leaves should be
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British Columbia FRUIT GROWER • Summer 2010 GARY STRACHAN
Dense, vigorous canopy, above, has very little light penetration. In addition to blocked air movement, this dense canopy is at risk for sunburn unless leaves are pulled at fruit set. Open canopy, below, has filtered shade. Light can penetrate and grapes are less likely to become sunburned, especially if leaves are pulled early in the season.
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