Loss of foliage due to poor growing conditions or disease can cause fruit to be exposed to the sun. Hot temperatures and direct sunlight can lead to areas of the fruit that appear bleached or sunburned. Sunburned fruit may not be marketable.
To reduce the probability sunburned fruit, every effort should be made to maintain foliage throughout the season. Early wet weather encouraged foliar disease and recent hot, dry weather may have restricted foliar development. Orienting vegetable plantings to minimize damage from the prevailing winds and providing windbreaks such as strips of rye or wheat may help to reduce sunburn.
Several products are available that are labeled for use as a preventive for sunburn. These products may be broken into two groups: kaolin (clay) based products and calcium carbonated based products.
Kaolin based products include Surround®. Some Surround® products are labeled for use as sunburn protection, while others are not. For example, the label for Surround WP® includes language about reducing sunburn damage, whereas Surround CF® lacks such language. These products are designed to place a layer of the clay product on the surface of the fruit. The clay will reflect the sunlight, thus reducing the sunlight that reaches the fruit. Kaolin based products should be applied in sufficient spray volume to obtain ‘near-drip coverage’. Growers should be prepared to wash off the kaolin product if necessary prior to sale.
Products with the active ingredient calcium carbonate represent the other major category of sunburn protectant. Products include Purshade® and Sombrero®. These products are also designed to reflect sunlight away from the surface of the fruit. Read the label to make sure it is labeled for sunburn protection. The label for Purshade® specifies NOT to apply to runoff. As with kaolin products, the grower should be prepared to wash the product off the fruit surface.
Since both the kaolin and calcium carbonate based products work by reflecting sunlight away from the fruit surface, there is some concern that these products may reduce sunlight that reaches the leaves and therefore the photosynthesis that drives plant growth. However, a study of the use of kaolin in apples found that the reduction of sunlight to leaves may be compensated for by the reflection of sunlight into the interior of the canopy. The benefit of these products for managing sunburn may out-weigh any reduced photosynthesis. However, growers must balance the possible benefits and risks of using any of these products.
A study in Michigan looked at the use of kaolin to reduce shoulder check in fresh-market tomatoes, a disorder described as a surface roughness that appears on the shoulder area of the fruit. The use of a kaolin product actually increased the amount of shoulder check found in tomatoes.
Vegetable growers should avoid using products to manage sunburn unless the label specifically states such a use on the label. For example, anti-transpirant products (e.g., Vapor Gard®) do not list on the label anything about reducing sunburn on vegetable crops.
Some pesticides may aggravate sunburn problems. For example, products with the active ingredient chlorothalonil (e.g., Bravo®, Echo® Equus®) have a warning that applying the product to mature watermelon fruit may result in sunburn to the upper surface. In general, it is best not to apply any pesticides during the heat of the day.
Sunburn or sunscald damag of vegetables can be a problem, especially in years with as much sun and heat as we have witnessed this season. Avoding sunburn on vegetables involves maintaining good foliage cover and the judicious us of the right product if necessary.
Vegetables such as this watermelon may become sunburned.