Broad Mites on High Tunnel Tomatoes

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

It is unusual that I see or hear about broad mites (Polyphagotarsonemus latus) being a problem in our tomato high tunnels. A grower was having symptoms of twisted growth and browning/bronzing of their tomato leaves this spring and guessed they might have broad mites. They did, with some plants severely damaged while others were fine. The grower had a late fall crop of cherry tomatoes that they kept into December but did not clean up until 2 weeks before they planted their spring crop of tomatoes. Unfortunately, the grower had a small infestation of broad mites in the fall crop of tomatoes that was able to overwinter. I wrote an article earlier this season about the necessity of cleaning a high tunnel or greenhouse well in advance of another crop in case there was a small infestation that had started in the last crop or on weeds left in the high tunnel. Sanitation is key to keeping pest problems out of a high tunnel or greenhouse.

Broad Mite Description and Biology
Female mites are oval, 0.2 mm long and are yellow or green with a light, median stripe that forks near the back end of the body. Males are similar in color but lack the stripe. The translucent, colorless oval eggs are firmly attached to the surface of a leaf. The eggs are very distinctive and are usually used to identify whether or not broad mites are present. (Often times adults or immatures cannot be found on a sample, but the eggs will be.) The eggs are covered with scattered white tufts on their outer surface that look like round dots (Fig. 1). Immature broad mites are white and slow moving. After just one day, the larva becomes a quiescent nymph which is clear and pointed at both ends. When females emerge from this quiescent stage, males immediately mate with them. Adult females lay a total of 30 to 76 eggs on the undersides of leaves and in the depressions of small fruit over a 9-14-day period and then die. Adult males may live 5-10 days. While unmated females lay eggs that become males, mated females usually lay four female eggs for every male egg. Males and females are very active, but the males account for much of the dispersal of a broad mite population when they carry the quiescent female to new leaves.

Figure 1. Broad mite egg greatly magnified

Hosts
The broad mite has a wide range of host plants: apple, avocado, cantaloupe, castor, chili, citrus, coffee, cotton, eggplant, grapes, guava, jute, mango, papaya, passion fruit, pear, potato, sesame, string or pole beans, tea, tomato and watermelon. Broad mites also infest many ornamentals, including African violet, ageratum, azalea, begonia, chrysanthemum, cyclamen, dahlia, gerbera, gloxinia, ivy, jasmine, impatiens, lantana, marigold, peperomia, pittosporum, snapdragon, verbena, and zinnia. Their ability to attack both vegetables and ornamental plants make them especially troublesome in greenhouses that grow both.

Damage
The damage caused by broad mites can look similar to the damage caused by viruses, herbicides or nutrient deficiencies. They feed on plant cells within the leaf epidermis using their piercing-sucking mouthparts. Early feeding is mainly concentrated near the growing point on the underside of a leaf near the stalk, which tends to cause the leaf to curl and become twisted and distorted (Fig. 2). More serious infestations cause leaf bronzing leaving the main veins green against the brown leaf tissue that eventually turns black, shrivels and dies (Fig. 3). Corky patches frequently appear on fruits that often crack at the site of deformation (Fig. 4). Extensive damage can be caused by relatively low populations. Commonly, the lower leaves of a plant can remain unaffected while the younger leaves are badly damaged. Symptoms of feeding damage can remain visible several weeks after the mites have been removed. Therefore, after treatments the plants need to be checked again for the presence of the mite, even though damage may still be apparent.

Figure 2. Leaves of tomato twisted and deformed by broad mite feeding

Figure 3. Broad mite feeding causing bronzing of leaves–leaving green veins

Figure 4. Damaged and aborted cherry tomato fruit due to broad mite feeding

Management
Once the mites have been positively identified as the cause of the tomato deformities horticultural oils or sulfur can be used that produce results similar to synthetic chemical applications. The most important aspect of the application is thorough coverage. The material needs to get down into tightly wrapped growing points and to the underside of leaves. Be careful when applying the oils or sulfur as they can cause phytotoxic problems under hot humid conditions. Portal XLO has been found to control broad mites in tomato and is classified as a mitochondrial electron transport inhibitor (METI) (IRAC subgroup 21A) and should be rotated with other miticides (hort oils, sulfur, Oberon, Agri-Mek) that have a different mode of action (i.e., a different IRAC No.). As in the case of oils and sulfur, Portal is a contact miticide and for best performance uniform and thorough spray coverage is needed. The addition of a nonionic wetting or penetrating adjuvant to the synthetic chemicals is recommended to improve their performance.

Flea Beetle Feeding and Tomato Early Blight

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

I visited a few tomato fields this week and found 2 to 4-week-old tomato plants with some early blight (Alternaria solani) and in some cases bad early blight lesions. This is very early in the season to be seeing this level of early blight. Many of the plants had a few flea beetle adults on the plant (Fig. 1) and in the areas where the early blight was found also had moderate to high flea beetle feeding (Fig. 2). In some cases I could not find any flea beetles after the rains we have had and in other cases I could find a few of them. Normally the amount of flea beetle feeding I saw would not have been of much concern, but flea beetles can cause increased infections of Alternaria leaf blight in tomatoes and potatoes and possibly other early blight susceptible crops. I found that there was a strong relationship between the amount of flea beetle feeding and the amount of early blight on tomato plants in different fields of a few farms. If you have moderate flea beetle feeding damage to your Solanaceae plants and you see any early blight starting you’ll need to control both the beetle and the disease. Pyrethroids should work well in controlling flea beetles. There is not much organically that will control flea beetles once they are causing economic damage (there are some things that can be done though, to reduce flea beetle problems before flea beetles cause damage, more at: https://attra.ncat.org/attra-pub/download.php?id=135). Using kaolin clay (Surround) before beetles begin to feed on plants is one organic possibility as is using spinosad on beetles after they start to feed.

Flea beetle adults are generally small and range in size from 0.05 to 0.15 inch. They overwinter as adults on weed hosts surrounding the field, on residues of a previous tomato crop, or in the soil if the previous crop was a flea beetle host. Some flea beetles (Systena blanda – the pale striped flea beetle being one) can feed on amaranths or pigweeds (Fig. 3) and will readily move from them over to your crops. Other flea beetles are more host specific (the eggplant, potato and tobacco flea beetles feed on Solanaceous plants while others prefer broccoli, cabbage and other cole crops). However all adult flea beetles have similar damage patterns, they chew small round holes in leaves, which make them look as if they have been damaged by fine buckshot, called “shot-holing”. The white larvae feed on underground parts of the plant, but this damage is usually not economically significant. There is normally a second generation during the summer and at times even a third depending on species. Normally foliar damage to larger plants is not considered to be economically important but feeding damage to small plants or seedlings can reduce stand or vigor of the plant. The other exception about flea beetles not being economic pests is when Alternaria is associated with their feeding on smaller tomato plants.

Figure 1. Underside of tomato leaflet with two flea beetles (Epitrix sp)

Figure 2. Tomato leaf with old flea beetle feeding and early blight

Figure 3. Pale striped flea beetle feeding on amaranthus weed

Recent Vegetable Trends

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

Vegetable growers that direct market or that target marketing programs to entice buyers should be on the leading edge of food trends.

Food trends are driven by many factors such as health benefits, dietary shifts, public values, celebrity recognition, and customer diversity.

The great thing about food trends is that you, as a grower and marketer, can help to start trends, invent new ways to market your produce, develop tastes in your customer base, and help define new eating habits.

One of my goals as a vegetable specialist located in Delaware is to reinvent one of our most important regional crops, the lima bean, by promoting different specialty types. We have been testing a range of potential specialty lima beans from our breeding program and other diverse sources that have different sizes, shapes and colors for cooking, eating, and taste attributes.

In recent years other vegetable trends have waxed and waned. The word on the street is that kale’s best days are now behind it. However, Brussels sprouts are still going strong, cauliflower is being put into everything, arugula is still hanging in there, and beets are on the upswing (2018 was the year of the beet).

Beets are an interesting study in trendiness. Five years ago, you would see small sections of beets in the fresh, canned, pickled, and frozen sections of the supermarket, maybe 10 selections at most. Now there are beet products in the juice, snack, and health product sections. Why? Because beets are being promoted by the “health” industry as a superfood.

A current question that is being asked by trend analysts is what will replace kale in the “greens” arena. One group that is gaining traction is chard and beet greens. Chard is now being sought by chefs as the new greens item to add menu selections. Other trend followers suggest that “wild” tasting plants will be part of the new trend driven by chefs. This includes sorrel, dandelion, an amaranth.

There are dozens of types of dandelions from the common weed to cultivated types. All parts of the weedy dandelion can be used as food and as a medicinal. Dandelion greens are very nutritious and can be added to salad and soups or cooked as a greens side dish. I expect to see some growers start to provide this as an actual crop. It is also perennial.

Edible amaranth is close relative to pigweed and makes a rich flavored cooked green. It is very easily grown and loves the summer heat. In addition to this new trend, it is a favorite of many immigrants from the Caribbean and Africa.

Sorrel is lemony flavored and there are selections that have been made for specific leaf attributes. It is this flavor that has brought it back to the table. Expect to see it more on plates in the future.

Other interesting trends include:

Vegetable “steaks” – these are vegetables that can be sliced and grilled like steaks (eggplant, squashes, tomatoes). This is a new way to market “old” crops.

Small sizes – small versions of popular vegetables. Snack peppers, snack cucumbers, mini eggplants, mini squashes and much more are becoming more and more popular. This follows the past baby vegetable trend but with new crops.

Color and color blends – Colorful vegetables are very trendy, especially in blends or mixtures. Everything from chard to cauliflower, carrots to peppers.

Fermentable foods – Grow foods for your customers to ferment. Cabbage, Napa, Pak choy, daikon, cucumbers, peppers, and many more.

Ugly produce – Off shapes and types are now sought. An example would be the ugly tomato that has been marketed as such in grocery stores.

Hot Water Treatment of Seeds

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

We have had an increase in bacterial diseases of vegetables that are seed transmitted such as black rot of cole crops.

Hot water treatment of seeds is a method to eliminate certain seed borne diseases of vegetable crops. This treatment has the benefit of killing pathogens that may be found on and within the seed coat.

From the Mid Atlantic Commercial Vegetable Production Recommendations:
“Seed heat-treatment follows a strict time and temperature protocol, and is best done with thermostatically controlled water baths. Two baths are required: one for pre-heating and a second for the effective pathogen killing temperature. The initial pre-heat treatment is 10 minutes at 100ºF (38ºC). The effective temperature treatment and time in the second bath differ between crops; protocols for several important crops are listed in Table E-10. Immediately after removal from the second bath, seeds should be rinsed with cool water to stop the heating process and dried on screen or paper. Seeds may be re-dusted with fungicide if desired. Pelleted seed is not recommended for heat treatment. Heat treat only seed that will be used during the current season. See crop sections for specific seed treatment recommendations.”

List of seeds that can be treated, treatment times and temperatures, and diseases controlled can be found at https://ag.umass.edu/vegetable/news/hot-water-treatment-of-seeds

The University of Delaware Extension Vegetable program has the equipment to hot water treat seeds. Please contact Gordon Johnson gcjohn@udel.edu or Emmalea Ernest emmalea@udel.edu to arrange to hot water treat seeds.

Guess the Pest! Week #22 Answer: Helicoverpa zea, Corn Earworm

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

Congratulations to Amanda Heilman for correctly identifying the insect as an adult corn earworm 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 #22 Answer: Helicoverpa zea, commonly known as corn earworm

The moth in the photograph is an adult Helicoverpa zea, commonly referred to as a corn earworm. The adult moth is a nectar feeder and not considered a pest. However, corn earworm larvae are considered by some to be the most economically important crop pest in North America. They are highly polyphagous meaning they feed on many different species of plants. Corn, especially sweet corn, is a preferred host plant. However, they also attack soybean, sorghum, snap bean, tomato, and cotton to name a few. Larvae prefer to feed on reproductive plant structures including blossoms, buds, and fruits. It is because of this large host range, and the fact that Helicoverpa zea larvae are so destructive that they are known by several other common names including tomato fruitworm, cotton bollworm, and podworm.

Tomato Problems

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

To no one’s surprise this week has seen a great number of problems pop-up in tomato fields throughout our area. The first, of course, has been disease with bacterial and fungal diseases spreading. The fungal pathogens usually can be contained with timely (as difficult as that is) fungicide applications. Bacterial spot or speck on the other hand can be much more difficult to control once it gets started and the weather remains wet and warm. In Figure 1 on the right hand side is a row of tomatoes that had been sprayed with fungicides and copper for the last several weeks. Bacterial spot has ravaged this row. The row to the left has been sprayed with the same material plus Actigard. Actigard is a plant activator that stimulates systemic acquired resistance, reducing the symptoms of bacterial and even fungal diseases. If you have problems with bacteria spot or speck Actigard can help reduce symptoms (as too can hot water treating your seed before you plant). You must use Actigard before the problem arises in the field, it is not a rescue material. This is the year in which the use of Actigard and heat-treating seeds would have greatly helped reduce losses to bacterial and fungal diseases.

Several fields had similar symptoms as seen in Figure 2, where the bottom foliage had been decimated but the top of the plant’s foliage looked good. If fungicide sprays are applied as needed the top foliage should be OK. The problem arises with the green fruit being exposed by the missing foliage. These fruits are in danger of sunburn, sunscald (Fig. 2) and rain check (the many, tiny concentric cracks that form on the shoulder of fruit and can expand over time (Fig. 3). This is where using shade cloth (20-30% shade-inducing) would greatly reduce the chances of sunscald and rain check. Gordon Johnson also had tips on preventing sunburn in vegetables in an earlier WCU article.

Figure 1. Row on right had fungicides and Cu applied, row on left same fungicides and Cu plus Actigard.

Figure 2. Bottom half of tomato plants have lost their foliage and expose fruit to possible sunscald or rain check

Figure 3. Exposed tomato fruit with rain check.

Odd Symptoms in Tomato Plants Turn Out to be Viruses

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

In Figure 1 you can see tomato plants that look a bit squirrely. I thought it was possibly herbicide or virus or nutrient problems. After eliminating the first and third we had the plants tested for a battery of viruses. There were 3 viruses found in the plants. The most unusual one was the Pepino mosaic virus, which belongs to the Potexviruses. This virus is very easily transmitted mechanically and has a low seed transmission rate. Seed transmission occurs at rates of less than one in a thousand when seed is not properly cleaned. The virus is external, contaminating the seed coat and not the embryo or endosperm. Symptoms very greatly with fruit marbling being the most typical and economically devastating symptom. You can also have fruit discoloration, open fruit, leaf blistering or bubbling, leaf chlorosis and yellow angular leaf spots. The severity of the Pepino mosaic virus symptoms is dependent on environmental conditions. As the infected plants mature the foliar symptoms usually disappear, but not the fruit problems. Prevention of infection is through stringent hygiene measures as the virus is spread primarily by mechanical methods.

The other two viruses found were more common: Tobacco (TMV) and tomato mosaic viruses. Tobacco mosaic virus is one of the most highly persistent tomato diseases because it can remain viable without a host for many years and it is able to withstand high temperatures. Both viruses are spread primarily by mechanical methods. Workers and their equipment can become contaminated when they touch infected plants. Symptoms are rather general and appear as yellow-green mottling on leaves with flowers and leaflets being curled, distorted, and smaller than normal in size. Generally, the fruit from TMV infected plants do not show mosaic symptoms but may be reduced in size and number and may develop an internal browning that most often appears in fruits of the first cluster. Severe strains of TMV and tomato mosaic virus can cause the lower leaves to turn downward at the petiole and become rough and crinkled. Some tomato varieties when infected with TMV or tomato mosaic virus can develop dead areas on leaves, stems and roots. As with the Pepino mosaic virus, the best control for these two viruses is strict hygiene and not using contaminated seed.

The Pepino mosaic virus is a newer one but is appearing more often in tomato production areas. The overall odd thing was that the symptoms on the plants did not fit any one of the viruses well. I attribute this to the mixture of viruses in the plant making it very difficult to observe characteristic symptoms of any one of them. Add to this that the environment plays a part in the plants expression of the symptoms as does the properties of the viral isolate and it becomes clearer why symptoms were not typical. The infected tomato plants were not commonly grown hybrid plants. Growers need to be sure about where their tomato seeds are coming from before using them.

Figure 1. Tomato plants infected with three different mosaic viruses

Mites Everywhere

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

There are reports of spider mites in multiple crops, with injury in tomatoes and a variety of cucurbits, mostly watermelon. Spider mites love hot, dry weather and that is what we have been having the last two weeks or so. They tend to start out at field edges or by drive rows. Anywhere dust settles on the crop is a likely spot for the earliest infestations. In watermelon, infestations usually start in the crown and spread from there. The oldest leaves will take on a yellow color along the midrib with necrotic spots (Fig. 1). This damage can be misjudged as a disease. Check for the mites on the underside of leaves to verify their presence (Fig. 2).

Figure 1. Mite damage to watermelon crown leaves

Figure 2. Two spotted spider mites on underside of watermelon leaf

For control there are several good miticides out there, but you need high gallonages of water 70-100 gallons/A and thorough coverage of the top and underside of the foliage or you will not reach all the well-hidden mites. Agri-Mek has translaminar movement, so if it is sprayed on the upper leaf surface it will penetrate into the leaf and reside there. Portal works on all stages of mites while Acramite is primarily active on the motile stages of mites (not eggs) and has a long residual. Oberon can take longer to work (check back in 5-7 days) but will give excellent control. Zeal is a growth regulator and will not kill adults but will kill immature mites, it works especially well if you catch the infestation early on. As always be sure to check the label before spraying any pesticides.

Blossom End Rot Revisited

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

Blossom end rot (BER) is showing up again this year in peppers and we expect it to be prevalent in tomatoes because of the recent hot weather. BER is a disorder where developing fruits do not have enough calcium for cell walls, cells do not form properly, and the fruit tissue at the blossom end collapses, turning dark in color. Calcium moves through cation exchange with water movement in the fruit, so the end of the fruit will be the last to accumulate calcium. Larger fruits and longer fruits are most susceptible. With fruits, the rapid cell division phase occurs early in the development of the fruit, the two weeks after pollination, and if calcium accumulation in the fruit is inadequate during this period, BER may occur. Over 90% of the calcium taken up by the fruit will occur by the time the fruit is the size of a nickel. While it may not be noticed until the fruit expands, the deficiency has already occurred and cells have already been negatively affected. We most commonly see signs of blossom end rot on fruits several weeks after the calcium deficiency has occurred.

Understanding blossom end rot also requires an understanding of how calcium moves from the soil into and through the plant. Calcium moves from the soil exchange sites into soil water and to plant roots by diffusion and mass flow. At plant roots, the calcium moves into the xylem (water conducting vessels), mostly from the area right behind root tips. In the xylem, calcium moves with the transpirational flow, the movement of water from roots, up the xylem, and out the leave through stomata. Calcium is taken up by the plant as a divalent cation, which means it has a charge of +2. It is attracted to negatively charged areas on the wall of the xylem, and for calcium to move, it must be exchanged off the xylem wall by other positively charged cations such as magnesium (Mg++), potassium (K+), ammonium (NH4+), or other calcium cations (Ca++). This cation exchange of calcium in the xylem requires continuous movement of water into and up through the plant. It also requires a continuous supply of calcium from the soil

In general, most soils have sufficient calcium to support proper plant growth. While proper liming will insure there is adequate calcium, it is not the lack of calcium in the soil that causes blossom end rot in most cases. It is the inadequate movement of calcium into plants that is the common culprit. Anything that impacts root activity or effectiveness will limit calcium uptake. This would include dry soils, saturated soils (low oxygen limits root function), compaction, root pathogens, or root insect damage. In hot weather on black plastic mulch, roots can also be affected by high bed temperatures. Low pH can also be a contributing factor. Calcium availability decreases as pH drops and below a pH of 5.2, free aluminum is released, directly interfering with calcium uptake. Again, proper liming will insure that this does not occur. Applying additional calcium as a soil amendment, above what is needed by normal liming, will not reduce blossom end rot.

In the plant, there is a “competition” for calcium by various plant parts that require calcium such as newly forming leaves and newly forming fruits. Those areas that transpire the most will receive more calcium. In general, fruits have much lower transpiration than leaves. In hot weather, transpiration increases through the leaves and fruits receive lower amounts of calcium. High humidity will reduce calcium movement into the fruit even more. Excess nitrogen that causes excess foliage will increase blossom end rot. Tissue tests will often show adequate levels of calcium in leaf samples; however, fruits may not be receiving adequate calcium. In addition, in hot weather, there is an increased risk of interruptions in water uptake, evidenced by plant wilting, when transpirational demand exceeds water uptake. When plants wilt, calcium uptake will be severely restricted. Therefore, excess heat and interruptions in the supply of water (inadequate irrigation and/or rainfall) will have a large impact on the potential for blossom end rot to occur. Proper irrigation is therefore critical to manage blossom end rot. This means a steady, even, uninterrupted supply of water in the soil surrounding the plant roots.

In high tunnels, lack of air movement can also be a factor, as transpiration is reduced, thus limiting calcium movement in the plant. In periods where tunnels are closed tight due to adverse weather, this may also increase the potential for blossom end rot. First fruits formed in early planted tomatoes and peppers are the most susceptible to blossom end rot, especially in high tunnels.

As a positive cation, there is “competition” for uptake of calcium with other positive cations. Therefore, if potassium, ammonium, or magnesium levels are too high in relation to calcium, they can reduce calcium uptake. To manage this, do not over-fertilize with potassium or magnesium and replace ammonium or urea sources of nitrogen with nitrate sources.

Applying additional soluble calcium through irrigation, especially drip systems, can reduce blossom end rot to some degree if applied prior to and through heat events and if irrigation is applied evenly in adequate amounts. Foliar applications are much less effective because fruits do not absorb much calcium, especially once a waxy layer has developed, and calcium will not move from leaves into the fruit (there is little or no phloem transport).

In conclusion, the keys to controlling blossom end rot are making sure roots are actively growing and root systems are not compromised, soil pH is in the proper range, and irrigation is supplied in an even manner so that calcium uptake is not interrupted. Supplemental calcium fertilization will only marginally reduce blossom end rot if water is not managed properly.

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.