Sunburn in Fruiting Vegetables and Fruit Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Recent weather has produced conditions where there is high potential for sunburn in fruits and fruiting vegetables. Growers may need to consider ways to protect against sunburn. Sunburn is most prevalent on days with high temperatures, clear skies and high light radiation. We commonly see sunburn in watermelons, tomatoes, peppers, eggplants, cucumbers, apples, strawberries, and brambles (raspberries and blackberries).

There are three types of sunburn which may have effects on the fruits. The first, sunburn necrosis, is where skin, peel, or fruit tissue dies on the sun exposed side of the fruit. Cell membrane integrity is lost in this type of sunburn and cells start leaking their contents. The critical fruit tissue temperature for sunburn necrosis varies with type of fruit. Research has shown that the fruit skin temperature threshold for sunburn necrosis is 100 to 104°F for cucumbers; 105 to 108°F for peppers, and 125 to 127°F for apples. Fruits with sunburn necrosis are not marketable. Injury may be white to brown in color.

The second type of sunburn injury is sunburn browning. This sunburn does not cause tissue death but does cause loss of pigmentation resulting in a yellow, bronze, or brown spot on the sun exposed side of the fruit. Cells remain alive, cell membranes retain their integrity, cells do not leak, but pigments such as chlorophyll, carotenes, and xanthophylls are denatured or destroyed. This type of sunburn browning occurs at a temperature about 5°F lower than sunburn necrosis. Light is required for sunburn browning. Fruits may be marketable but will be a lower grade.

The third type of sunburn is photooxidative sunburn. This is where shaded fruit are suddenly exposed to sunlight as might occur with late pruning, after storms where leaf cover is suddenly lost, or when vines are turned in drive rows. In this type of sunburn, the fruits will become photobleached by the excess light because the fruit is not acclimatized to high light levels, and fruit tissue will die. This bleaching will occur at much lower fruit temperatures than the other types of sunburn. Damaged tissue is often white in color.

Storms that cause canopies in vine crops to be more open will expose fruits to a high risk of both sunburn necrosis and photooxidative sunburn.

Genetics also play a role in sunburn and some varieties are more susceptible to sunburn. Varieties with darker colored fruit, those with more open canopies, and those with more open fruit clusters have higher risk of sunburn.

Control of sunburn in fruits starts with developing good leaf cover in the canopy to shade the fruit. Fruits most susceptible to sunburn will be those that are most exposed, especially those that are not shaded in the afternoon. Anything that reduces canopy cover will increase sunburn, such as foliar diseases, wilting due to inadequate irrigation, and excessive or late pruning. Physiological leaf roll, common in some crops such as tomato, can also increase sunburn.

In crops with large percentages of exposed fruits at risk of sunburn, fruits can be protected by artificial shading using shade cloth (10-30% shade). However, this is not practical for large acreages.

For sunburn protection at a field scale, use of film spray-on materials can reduce or eliminate sunburn. These materials are kaolin clay based, calcium carbonate (lime) based, or talc based and leave a white particle film on the fruit (such as Surround, Screen Duo, Purshade and many others). There are also film products that protect fruits from sunburn but do not leave a white residue, such as Raynox. Apply these materials at the manufacturer’s rates for sunburn protection. They may have to be reapplied after heavy rains or multiple overhead irrigation events.

While particle films have gained use in tree fruits, their usefulness in vegetables is still unclear. Research in a number of states has shown reduced fruit disorders such as sunburn in peppers and white tissue in tomatoes when applied over those crops. Watermelon growers have used clay and lime based products for many years to reduce sunburn in that crop in southern states.

There are some drawbacks to the use of particle films. If used for sunburn protection on fruits, there is added cost to wash or brush the material off at harvest. Where overhead irrigation is used, or during rainy weather, the material can be partially washed off of plants, reducing effectiveness and requiring additional applications. Produce buyers can also have standards relating to the use or particle films and may not accept products with visible residues.

Venom Approved for BMSB in Some Fruit

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

A Section 18 label for Venom (dinotefuran) has been approved for use on Delaware stone and pome fruits for brown marmorated stink bug management. This chemical has lower risk to beneficial insects. Maximum size of fruit orchard treated with Venom is 415 acres. There is a 12 hour REI, 3 day PHI.

Guess the Pest! Week #9 Answer: Plum Curculio

Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Congratulations to Sylvie Childress for correctly identifying the ovipositioning scar and larva in the photos below as plum curculio and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!

Guess the Pest Week #9 Answer: Plum Curculio

The plum curculio is a pest of apples, peaches, plum, and other stone fruit. The adult beetles are about ¼” in length, dark brown with patches of white, and have a protruding snout. They belong to the weevil family, commonly referred to as the snout beetles.

Adult Plum Curculio

The primary damage to fruit is caused by the ovipositioning or egg laying behavior of the female plum curculio. The females lay eggs in the developing fruit and cut a crescent shaped slit beneath each egg to prevent the rapidly growing fruit from crushing the egg. As the fruit continues to grow, the slit the female beetle cut below the egg develops into the classic, crescent shaped scar that you see in the photo above. These scars are usually only cosmetic. However, if the egg hatches, the larva will bore into the fruit, which will usually cause the fruit to drop from the tree. After a couple weeks of feeding on the fruit, the larva will exit the fruit to pupate in the soil.

In addition to the crescent shaped ovipositioning scars and fruit drop, the adult beetles will also feed on fruit, creating numerous round puncture holes in the fruit skin. It has been estimated that a single beetle will average over 100 feeding and/or puncture wounds during its lifespan.

Fun Entomology Fact: The family Curculionidae (true weevils), are the largest family of insects with the most species described worldwide.

Sunburn in Fruiting Vegetables and Fruit Crops and Sunburn Protection

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

With the expected high temperatures this weekend and next week, there is high potential for sunburn in fruits and fruiting vegetables. Growers may need to consider ways to protect against sunburn. Sunburn is most prevalent on days with high temperatures, clear skies and high light radiation. We commonly see sunburn in watermelons, tomatoes, peppers, eggplants, cucumbers, apples, strawberries, and brambles (raspberries and blackberries).

There are three types of sunburn which may have effects on the fruits. The first, sunburn necrosis, is where skin, peel, or fruit tissue dies on the sun exposed side of the fruit (Figure 1). Cell membrane integrity is lost in this type of sunburn and cells start leaking their contents. The critical fruit tissue temperature for sunburn necrosis varies with type of fruit. Research has shown that the fruit skin temperature threshold for sunburn necrosis is 100 to 104°F for cucumbers; 105 to 108°F for peppers, and 125 to 127°F for apples. Fruits with sunburn necrosis are not marketable. Injury may be white to brown in color.

SunburnNecrosis

Figure 1. Sunburn necrosis on pepper fruit.

The second type of sunburn injury is sunburn browning. This sunburn does not cause tissue death but does cause loss of pigmentation resulting in a yellow, bronze, or brown spot on the sun exposed side of the fruit. Cells remain alive, cell membranes retain their integrity, cells do not leak, but pigments such as chlorophyll, carotenes, and xanthophylls are denatured or destroyed. This type of sunburn browning occurs at a temperature about 5°F lower than sunburn necrosis (i.e. 115 to 120°F in apples). Light is required for sunburn browning. Fruits may be marketable but will be a lower grade.

The third type of sunburn is photooxidative sunburn (Figure 2). This is where shaded fruit are suddenly exposed to sunlight as might occur with late pruning, after storms where leaf cover is suddenly lost, or when vines are turned in drive rows. In this type of sunburn, the fruits will become photobleached by the excess light because the fruit is not acclimatized to high light levels, and fruit tissue will die. This bleaching will occur at much lower fruit temperatures than the other types of sunburn. Damaged tissue is often white in color.

Photoxidative

Figure 2. Photooxidative sunburn on pepper fruit.

Recent storms have caused canopies in vine crops to be more open, exposing fruits to a high risk of both sunburn necrosis and photooxidative sunburn.

Genetics also play a role in sunburn and some varieties are more susceptible to sunburn. Varieties with darker colored fruit, those with more open canopies, and those with more open fruit clusters have higher risk of sunburn. Some varieties have other genetic properties that predispose them to sunburn, for example, some blackberries are more susceptible to fruit damage from UV light.

Control of sunburn in fruits starts with developing good leaf cover in the canopy to shade the fruit. Fruits most susceptible to sunburn will be those that are most exposed, especially those that are not shaded in the afternoon. Anything that reduces canopy cover will increase sunburn, such as foliar diseases, wilting due to inadequate irrigation, and excessive or late pruning. Physiological leaf roll, common in some solanaceous crops such as tomato, can also increase sunburn.

In crops with large percentages of exposed fruits at risk of sunburn, fruits can be protected by artificial shading using shade cloth (10-30% shade). However, this is not practical for large acreages.

For sunburn protection at a field scale, use of film spray-on materials can reduce or eliminate sunburn. These materials are kaolin clay based, calcium carbonate (lime) based, or talc based and leave a white particle film on the fruit (such as Surround, Screen Duo, Purshade and many others). There are also film products that protect fruits from sunburn but do not leave a white residue, such as Raynox. Apply these materials at the manufacturer’s rates for sunburn protection. They may have to be reapplied after heavy rains or multiple overhead irrigation events.

While particle films have gained use in tree fruits, their usefulness in vegetables is still unclear. Research in a number of states has shown reduced fruit disorders such as sunburn in peppers and white tissue in tomatoes when applied over those crops. Watermelon growers have used clay and lime based products for many years to reduce sunburn in that crop in southern states.

There are some drawbacks to the use of particle films. If used for sunburn protection on fruits, there is added cost to wash or brush the material off at harvest. Where overhead irrigation is used, or during rainy weather, the material can be partially washed off of plants, reducing effectiveness and requiring additional applications. Produce buyers can also have standards relating to the use or particle films and may not accept products with visible residues. For example, some watermelon brokers will accept watermelons where calcium carbonate protectants have been used but will not accept watermelons sprayed with clay based products.

Section 18 for Brown Marmorated Stink Bug (BMSB) Management on Apples, Peaches and Nectarines Approved

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Our Section 18 request for the use of three bifenthrin products (Brigade WSB – FMC Corporation; Bifenture EC and Bifenture 10DF – both from United Phosphorus) to control BMSB on apples, peaches and nectarines has been approved by EPA. This use expires on Oct 15, 2016. You must have a copy of the label in your possession before making an application. Please contact either Chris Wade at the Delaware Department of Agriculture (Christopher.Wade@state.de.us) or Joanne Whalen (jwhalen@udel.edu) for more information.

Revisiting Fruit Losses Due to the Late Freeze and Fruit Drop in Tree Fruits

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu and Emmalea Ernest, Associate Scientist – Vegetable Crops; emmalea@udel.edu

Losses in tree fruits due to the late freeze are now more obvious. As one grower put it “a light crop becomes even lighter”, that is, the damage is not fully known for several weeks. Fruits that seemed to be set often do not develop because the embryos have been damaged.

Plum, pluot, plumcot, and apricots have 80-100% losses. Peaches and nectarines are more variable depending on location and variety. In discussions with growers, some orchards are carrying only a 20% (or less) crop, others are in the 40-60% crop range. However, certain varieties, such as Redhaven and White Lady, and later flowering varieties are carrying a heavier (near full) crop. What remains to be seen is if there is hidden freeze damage to existing fruit leading to continued drop.

Fruit drop is a result of unfertilized or poorly fertilized seeds, freeze damage to buds and flowers (as in this year 2016), competition between fruits, or shading. In other years, fruit drop may be due to poor pollination as a result of cold, rainy weather during bloom in self-fertile fruits such as peaches, or poor insect pollinator activity during flowering in insect pollinated fruits such as apples. In stone fruit, some fruit that is not fertilized will remain on the plant for 25-50 days after bloom and then will drop before pit hardening starts. This is what we are seeing now in cold damaged peaches and nectarines.

Another cause of fruit drop is cloudy weather during the period 5 to 7 weeks after bloom. A continuous 4 day period of cloudy days during this period will cause fruit to drop. In addition, defoliation due to disease such as peach leaf curl, chemical injury such as copper fungicide damage, or severe storms can cause fruit drop during this critical period.

Another wave of natural fruit drop occurs in late May or early June. This fruit drop is due to competition between fruit for sugars stored and produced by the tree. A tree can only carry a certain load of fruit and will naturally drop smaller and weaker fruit during this period. With the light crops in 2016, competitive drop should be minimal. However, when there are large differences in timing of fruit set (late set fruit on the same twig near to earlier set fruit), the larger fruit will become a sink for resources and the smaller fruit will abort leading to further crop reduction.

In 2016, Asian pears and European pears had some flower losses due to the freeze but still have good crops. Most apples were minimally affected by the freeze.

Sweet cherries in the open have high losses; however, those in high tunnels have full crops if they were covered early. Sour cherries have a much reduced crop. One of the issues with sweet cherries is that cross pollination is needed for many varieties. If a pollinator variety suffered damage, the varieties dependent on the pollinator will have reduced crops, compounding the freeze damage.

Marginally hardy fruits such as figs were killed to the base and persimmons were also damaged.

Strawberries also had some fruit losses in both plasticulture and matted row systems and have a higher than normal amount of deformed berries in early pickings. Brambles (raspberries and blackberries) have more tip dieback this year than usual. Early flowering blueberry varieties (mostly Southern highbush) have some freeze loss in our trials at Georgetown, but it will be difficult to estimate yield loss until closer to harvest.

Assessing Freeze Damage in Fruit

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

On Wednesday, April 6, I evaluated a mixed orchard for damage from several freeze events below 25 over the last 3 weeks. Apricots and Pluots were in the post bloom stage and had greater than 90% loss. Plums were also in the post bloom stage and while significant freeze losses of 50% or more are evident, final fruit load is still uncertain. Peaches and nectarines have severe losses but because they flower at very high numbers, final fruit load is still uncertain; however, the early flowering peaches and nectarines that were inspected showed the most damage and are expected to have very light fruit loads or no fruit. All early flowers on cherries were killed. Asian pears in bloom had over 70% losses (see photo below). Apples are pre- bloom and damage is expected to be low.

FreezeDamageAsianPear

Freeze damaged Asian pear flowers. Note the black centers of flowers that have been killed by freezing.

Flowers and young fruit that are dead will be discolored (yellow, brown, black) or shriveled and often will pull off easily from the tree. Viable flowers and fruit will have green ovaries inside and out. You may have to cut open the ovaries with a razor blade to see if they have been damaged.

Freezes, Frost and Frost/Freeze Protection

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Fruits and fruit flowers are damaged by temperatures below 28°F and by frost. Temperatures in Delaware reached 21°F or lower on April 6. Another freeze is predicted for Saturday, April 9. The term freeze means that temperatures dropped below 32°F. Frost is the formation of ice crystals on crops and occurs when the dew point is near or below freezing. You can have a freeze without frost and a frost without a freeze. Both are damaging to plant tissue.

Frost and freeze protection methods vary with fruits and the type of freeze expected. Advective freezes occur with freezing temperatures and high winds. This is the most difficult to protect against. For strawberries, two layers of floating row covers may be the most effective strategy for advective freezes. Double covers have been shown to be more effective than single heavy covers in this case. Irrigation along with double covers can provide even more protection if done properly.

Radiational freezes occur on cold, still nights. In this case cold air is near the ground and warmer air is above. Wind machines and helicopters have been successfully used to stir the air and raise the temperatures in orchards in this case. Row covers in strawberries will protect against radiational freezes too.

Irrigation has also been successfully used for frost protection but it has to be done properly. How irrigation works is that as ice forms on plants heat is released. The key is to keep ice formation occurring through the night and continue through melt in the morning. Remember that initially, until ice starts forming, there will actually be evaporative cooling of the plant. The latent heat of fusion (water freezing) will release heat (approximately 144 BTUs/lb of water), whereas evaporative cooling will absorb heat from the plant (absorbing approximately 1,044 BTUs/lb of water) and lower plant temperatures. Therefore, irrigation must start well above critical temperatures. Also, the volume of water needed needs to be matched with the expected temperature drop and wind speed. In addition, uniformity of water application is critical. This is difficult to do in high wind situations.

Site Selection for Fruit Plantings

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

There has been an increase in interest in planting fruits in Delaware. This is a positive trend that matches the interest in buying local and can also provide local fruit to the steady influx of visitors in the region.

Success with tree fruits, blueberries, grapes, brambles, and other long-term perennial fruits begins with selecting a proper site. I have visited too many sites in the past 10 years where growers have lost expensive planting material because of poor locations and poor planning. Landowners most often are not buying properties with fruit planting in mind and many properties just are not suitable for fruit.

The most common issue with planting fruit is that of high seasonal water tables. When water rises in winter, it can saturate part of the root zone of the fruit plant and roots will then die due to lack of oxygen. Roots injured by waterlogging are also then more susceptible to root rot pathogens. Fruit plants with water damaged roots also have fewer effective roots which can make them more susceptible to other plant stresses such as drought. In the end, these fruit plants will die prematurely, have shorter life spans or will be less productive.

The best time to evaluate a site for the height of the seasonal water table is in late winter. Find the lowest elevation in the property being evaluated and dig a hole 6 feet deep using a posthole digger. If any free water is found in the hole then the site is not suitable for most deep rooted perennial fruits such as tree fruits and grapes. With brambles and blueberries water should not be found within 4-5 feet of the surface in these observation holes. Also examine the soil that comes out of the borings. If you see considerable amount of gray colored soil, this is an indication of water saturation. Do these borings throughout the property and map your site and avoid planting fruits on any areas with high water tables.

Another problem with water saturation and roots can be perched water tables. This is when an impervious soil layer does not allow water to drain and a saturated area develops above that layer. If perched water tables are found, the area is again not ideally suited for fruits. Subsoiling can fracture these layers if done properly but the layers may reform in a few years.

In high water table soils, it may be possible to grow some fruits such as brambles or blueberries by creating high mounds to grow on. In this case, the growing area is elevated 2-4 feet by moving soil to create a mounded ridge where fruit is planted. While this is possible, it is expensive and must be done in such a way that water does not collect between the mounds.

Another issue with fruit siting is air drainage. Our last 2 winters have had sub-zero conditions which can cause problems with winter kill in some grapes and brambles and bud damage in some tree fruits. Lower areas where cold air drains to also are more susceptible to late frost damage to flowers in the spring, particularly in peaches, nectarines, apricots, and plums. All sites should be evaluated for air drainage by doing elevations on the property. Fruit should be planted on the highest elevations and frost pockets should be avoided. Frost pockets are easily seen by looking where frost is found during late spring frost events. On Delmarva, an issue we have is that some areas are just completely flat, with low elevation. These areas will not allow for air to drain and can also have issues with cold air accumulating.

Soil pH is an issue with blueberry establishment. Blueberries require a soil pH of 4.5-4.8. Most of our soils have much higher pHs and the soil must be acidified before blueberries can be planted. This can take 1-2 years using sulfur as the acidifying agent.

Sites should also be evaluated for nematodes, soil pests that can be damaging to fruit roots, before planting.

Section 18 for Brown Marmorated Stink Bug (BMSB) Management on Stone and Pome Fruit

As indicated last week , our Section 18 request for the use of three bifenthrin products (Brigade WSB – FMC Corporation; Bifenture EC and Bifenture 10DF – both from United Phosphorus) to control BMSB on apples, peaches and nectarines was approved by EPA. Here is the label for Bifenture products from UPI (https://cdn.extension.udel.edu/weeklycropupdate/files/2014/07/Section-18-Bifenture-BMSB-DE-2014.pdf). We will post the Brigade WSB from FMC as soon as it is available. Please contact either David Pyne at the Delaware Department of Agriculture (David.Pyne@state.de.us) or Joanne Whalen (jwhalen@udel.edu) for more information.