Sunburn Protection for Apples

When you think of perils that might destroy a nice apple crop, what comes to mind? A disease, like apple scab? A freeze at blossom time? Hail? Insects, like codling moth or apple maggot? High winds?

Well, how about sunburn?

Research in the last several years has identified sunburn as a multi-million dollar crop destroyer...in the neighborhood of $100 million annually in Washington state alone. That’s the bad news. But the good news is that a spray application of a new patented product containing carnauba wax and certain additives can greatly reduce sunburn, and, when used in conjunction with properly-timed evaporative cooling, can virtually eliminate losses.

Dr. Larry Schrader, Horticulturist/Plant Physiologist with his team at Washington State University’s Tree Fruit Research and Extension Center in Wenatchee, Washington, has developed the two-pronged approach. The carnauba wax spray, called RAYNOX^TM and developed in Schrader’s lab, protects against harmful ultraviolet-B rays from the sun that cause sunburn and reflects solar radiation to decrease heat stress. It alone can reduce losses due to sunburn by 50%. The evaporative cooling reduces the temperature of the apples. Various degrees of injury occur when apple surface temperatures exceed 114 degrees Fahrenheit for as little as one hour, or 126 degrees for just ten minutes.

In addition to treating the crop while it’s on the trees, growers are advised to protect harvested apples in bins by shading them from the sun. Schrader has found that apples suffer sunburn at much lower temperatures when the "back" side of the apple, the side that hasn’t experienced direct sunlight, is suddenly exposed to the harmful ultraviolet rays. The injury may not show up for several days, but it may greatly affect the condition of the fruit in storage.

All About Apples discovered RAYNOX^TM in an article back in late March. We contacted Dr. Schrader about his work, and following is an interview conducted by Topper Sponsel, contributor to All About Apples.

All About Apples (aaA): Others you've cited in your research documents have done sunburn research in the past, but you have obviously moved the science quite significantly ahead. You've identified three types of sunburn, identified the fruit surface temperatures necessary to inflict damage, identified the functions of UV-B (and an effective agent to block it), wind, humidity, evaporative cooling, maximum daily temperature, and much more. How did your initial idea (to expand this line of research) come about, and how did carnauba wax enter into your thinking?

Dr. Larry Schrader (LS): Through personal observation and discussions with leading apple growers, it became apparent that sunburn damage is a significant problem under our environment in Washington State (and also several other parts of the world). Our research has shown that, on average, sunburn damages 8% to 12% of the crop in Washington state each year. With a crop valued at $1.2 billion at the farmgate in this state alone, sunburn was costing our growers over $100 million annually.

As I thought about ways to decrease sunburn damage, I needed something that could be approved for food use and that could be applied easily by growers. I considered natural plant products and decided to try carnauba wax, a natural wax produced on leaves of palm trees in the tropics. The wax is produced on palm fan trees to protect them from damaging Ultraviolet-B (UV-B) radiation. Carnauba wax is used in car waxes to prevent oxidation of paints. Therefore I reasoned that this wax might help protect apples from damaging UV-B. I first tried a wax emulsion on apples, and it looked promising. The formulation was further improved by adding some other constituents, but the wax is a major component.

aaA: Have the losses due to sunburn been at those levels historically, or have they increased in recent years due to the ongoing shift from Red Delicious to Granny Smith, Gala, Fuji, Braeburn, and other cultivars?

LS: The shift to high density plantings on size-controlling (dwarfing) rootstocks has resulted in smaller trees with less foliage (leaves) to protect the fruit from solar radiation. Consequently, the incidence of sunburn has increased as the shift to these new cultivars (such as those listed below) has occurred. The acreage devoted to old large Red Delicious continues to decline, so sunburn damage is increasing.

aaA: Granny Smith entered the scene in the mid-1970's at the same time plantings of size-controlling rootstocks increased dramatically. The grade and eating quality of Grannies are reduced when they get a blushed cheek on the sunny side. They're supposed to be grass green. So growers had to change their pruning practices to provide some shade. So that's a case where it's not even sunburn, but protection is needed. Do you think RAYNOX^TM could potentially provide protection against that undesirable blush (which is usually an asset on other varieties) so growers could revert to the old pruning regime?

LS: I do not have an answer for this question. We have not conducted a careful study of this issue yet.

aaA: How often is RAYNOX^TM applied?

LS: RAYNOX^TM is applied every 3 to 4 weeks depending on the cultivar and local environmental conditions. For an early maturing variety such as 'Gala', the interval between applications may be 3 weeks, but for most varieties an interval of 4 weeks is recommended. A total of three to four applications is recommended. In Washington State, the first application is made in mid to late June which is 55 to 60 days after full bloom. Fruit diameter is usually 35 to 40 mm by that time.

aaA: Is it necessary to apply more frequently under rainy conditions?

LS: No. RAYNOX^TM is rainfast. In fact, it can be used under evaporative cooling systems in which overhead sprinklers are used daily through the season to decrease the fruit surface temperature (helps protect fruit from sunburn). RAYNOX^TM and evaporative cooling used alone provide only partial protection from sunburn. When combined, RAYNOX^TM plus evaporative cooling can almost completely eliminate sunburn on apples.

aaA: Decisions as to when to apply could be made by simply applying every four weeks, as you've indicated. I suppose if an application were to be due at a time when temperatures were predicted to be cool, a grower may decide to delay until just before sunburning conditions were predicted.

LS: Yes, the label for RAYNOX^TM gives growers some flexibility on dates of application and interval between applications. You are correct that some growers will delay an application if it is cool for a period, and then make an application of RAYNOX^TM before the next "sunburn event" occurs. However, this can be risky when weather conditions change quickly.

aaA: How much do these applications cost per acre?

LS: The recommended rate of application is 2.5 gallons of RAYNOX^TM per acre per application. During 2004, the recommended retail price was $12.60 per gallon in Washington State (or $31.50 per acre per application). The price was higher in other areas because of increased transportation costs. The retail price for 2005 will be increased to offset higher shipping costs and increased cost for raw materials.

aaA: Is there wax residue at harvest?

LS: Yes. Sunburn protection is needed until harvest.

aaA: Is it edible? Is it to be washed off?

LS: It is edible. Food grade ingredients are used in RAYNOX^TM, and they have been approved by FDA for human consumption. Some or all of the residue may be removed by detergents and cleaning agents during the packing operation.

aaA: You have been working on a model to predict the incidence of sunburn based on daily high temperatures, wind, humidity, and other factors. What's the current state of that model?

LS: We have discontinued work on the predictive computer model, as we have a better alternative. We invented a fruit surface temperature sensor ("artificial apple") that simulates a real apple and predicts the fruit surface temperature in a grower's orchard. This sensor is more site specific than the computer model. Growers can use information from this sensor to make management decisions about when to apply RAYNOX^TM and/or when to activate and deactivate their evaporative cooling systems used to protect the apples from sunburn. This sensor permits growers to use their water more efficiently and effectively. A patent is pending on this sensor. Fifty of these sensors have been sold to growers who will Beta test them further during 2005. If the tests are satisfactory, the sensor will be commercially available to other growers in 2006.

In the future, we envision this sensor also being used to control evaporative cooling (EC) systems so that the EC systems are running only when the fruit surface temperature is above a set point. This will save water and energy (for pumping the water), as the EC will be used only when needed rather than being used daily on a timer system.

aaA: Can apples that show no visible sunburn damage when harvested actually have hidden damage that shows up later in storage?

LS: Yes, some of the cultivars (for example, some Red Delicious strains) that become deep red in color at maturity can mask sunburn damage. 'Fuji' apples that have been sunburned often develop another disorder called Fuji stain after 1 to 4 months in cold storage. The incidence of Fuji stain in apples that were severely sunburned is much higher than in apples with slight or no sunburn. So Fuji stain is an example of a heat-induced disorder that does not appear on the apples until they have been in cold storage.

Apples can also be sunburned late in the season and not show the sunburn symptoms until later. We discovered a third type of sunburn that occurs on apples that have not been acclimated to high light. This type of sunburn occurs at much lower temperatures than sunburn necrosis or sunburn browning. It occurs in apples that have been shaded within the canopy and are suddenly exposed to bright sunlight. Even the back side of apples can sunburn easily when suddenly exposed to sunlight. This sunburn may occur when apples are left sitting in an exposed bin after they are harvested. It can take several days before the sunburn symptoms appear.

aaA: Thank you very much for answering our questions! It's clear that you're addressing a central problem that affects apple quality. Growers and consumers alike will benefit greatly from your work. Considering that apples are grown over a very wide territory globally, what simple criteria should any grower use to make an initial assessment as to whether or not sunburn protection deserves attention in his or her district?

LS: The simplest way to make an initial assessment is to examine apples that are fully exposed to sunlight about mid-afternoon. These are the apples that are most likely to be sunburned. If the apples show a yellow, tan, or brown spot on the sun-exposed side, they are sunburned. There are several classes or grades of sunburn ranging from a slight discoloration (yellow) to a dark brown or black spot.

Attachment sent by Dr. Schrader to show the various stages of sunburn.

For more detailed information, see Dr. Schrader’s research report at http://postharvest.tfrec.wsu.edu/PC2003A.pdf.