White, yellow or brown blotches on fruit raise alarm bells, but they may be sunburn rather than a disease or chemical injury. This article lay out clues to identifying fruit damage as sunburn or another issues.
Hot weather, strong sunlight, and low humidity are a recipe for sunburn on fruit crops. But another key ingredient is at play here too, which is lack of shade on the fruit itself. Sunburn can happen during any stage of fruit development if the conditions are right. While sun is needed to ripen the fruit, too much direct sunlight beaming down on the fruit during low humidity can cause the fruit to burn. The science behind sunburn is described here.
Sunburn on fruit crops shows up as large white or brown blotches on the side of the fruit that is directly exposed to the sun. The specific symptoms depend on the type of fruit.
On grape clusters, sunburn looks like a white, brown, or yellow blotch on the side of the cluster that is directly exposed to sunlight. This happens more often on the south- or west-facing sides of a grapevine. The blotch may be soft at first, but after a period of time, the affected area dries up and creates a berry that is hard on one side and partially mummified. Sunburned grapes do not ripen as well as normal grapes. Small amounts may not impact wine quality, but severely sunburned clusters should be dropped.
Because grape clusters ripen better with some direct sunlight, we often recommend that grape growers do leaf pulling between bloom and veraison. However, leaf removal should only involve removal of 2-4 leaves on each shoot. Furthermore, skip leaf removal if the vines lack strong vigor. Most importantly, the leaves should be removed earlier (before mid-July) so that the shaded clusters are not suddenly exposed to strong sunlight late in the season. Removing leaves around the clusters now, in late July, can cause sunburn to clusters that developed in the shade and are not accustomed to direct sun. If leaves are removed now in order to help the grapes ripen, be conservative and ensure that the clusters are still somewhat shaded to reduce the risk of sunburn.
If brown, soft spots are observed on many berries on a vine, and are on all sides of the cluster rather than just the side exposed to strong sunlight, it is more likely a disease such as black rot or anthracnose. Refer to the Growing Grapes in Minnesota guide for more information on recognizing grape diseases.
The most immediate solution for preventing sunburn on fruit is to drape a shade cloth over the plants to decrease the amount of direct sunlight hitting the fruit. This solution is straightforward and can be done in the middle of the season.
A longer term strategy is to provide adequate nutrients (especially nitrogen, phosphorus, and potassium) to the plants, to support strong canopy growth. Fruit on unhealthy trees that lack sufficient vigor are more susceptible to excess sun exposure. Fertilizer should only be used at certain times in the season, and the timing depends on the crop.
Third, when planting a new orchard or vineyard, orienting the rows north-south rather than east-west will help ensure that there are not fruit on the south side getting direct sunlight all day. However, multiple factors go into decisions about row orientation, and sunburn is just one of them to consider.
Finally, vines can be watered using overhead irrigation like a sprinkler or hose during periods of hot temperatures. This helps because it creates “evaporative cooling” – in other words, as the water evaporates, it decreases the temperature around the fruit. This method is not for everyone, because it requires small, frequent sprinkler application throughout the period of hot temperatures. Still, some growers may consider this during very hot, sunny weather depending on the scope of the issue.
Using Shade for Fruit and Vegetable Production – Utah State University
Author: Annie Klodd, University of Minnesota Extension Educator – Fruit & Vegetable Production