predict the risk of a mildew infection throughout the season. Controlling mildew will also assist you to control Botrytis because mildew creates lesions that assist a Botrytis infection to initiate. Pathologies are seldom from a single cause.
Secondary infections from other fungi can cause sour rot, complicated by bacterial infections that contribute volatile acidity and attract fruit flies. If it’s bad enough your vineyard can decline into a horrid and evil mess. Grapes can be dehydrated from sunburn, so be careful where and when you remove leaves to expose fruit. The leaves close by each fruit cluster provide the greatest contribution of photosynthate to the berries, so be conservative with their removal. If you are leaf-stripping to open up your canopy to light and air movement, take leaves off as soon after fruit set as possible in order to stimulate the fruit to develop thicker skin, enhanced tannins and increased flavonoids to resist UV radiation.
The later you strip, the greater your chance of damaging your fruit. Don’t strip leaves from the south or west side of your vines!
Some grapes, especially reds, are subject to late season dehydration shrivel. In this case, the berries begin to dehydrate, but still have green pedicels and are not infected. Berries affected by this disorder can have very high sugar levels, thus it may also be referred to as sugar accumulation disorder (SAD). If the pedicels are brown, then water has been cut off and the berries are dehydrating. This is likely bunch stem necrosis. This could affect either the whole cluster or parts of the cluster. It can occur either early in the season or after veraison.
This is generally accepted to be a nutritional problem rather than an
infection. If there is cold weather during fruit maturation and there is a nitrogen/magnesium imbalance, then late-season bunch stem necrosis can occur.
In vineyards with a history of this disorder, a pre-veraison foliar magnesium spray might minimize the problem The disorder is more likely to occur in coarse soils than fine soils. When you walk your vineyard, don’t ever ignore wrinkled clusters. Check them out and try to figure why they are wrinkled. It might not be a serious problem this year, but you stand a good chance of it becoming a bigger problem next year if you do nothing about it.
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30 British Columbia FRUIT GROWER • Spring 2015
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