Solutions to a costly problem
Sunburned apples were costing growers in Washington State millions of dollars each year when Larry Schrader became plant physiologist and horticulturist at the Tree Fruit Research and Extension Center at Washington State University in 1995. On average, they were losing a good 10 percent of their crop to sunburn, their number one source of cullage. Sunburn was also affecting growers in Idaho and California.
Schrader found that the amount of light and heat on the fruit were the most important factors. “We have unusually high solar radiation,” he says, “and on top of that, we have high afternoon temperatures. There’s 20 percent more sunlight in our area than in the Midwest, partly because our days are longer, and partly because the air is cleaner.”
Some apple varieties are more susceptible to sunburn than others, Schrader says. He theorizes that varieties may be adapted to the areas where they were developed. For example, Pink Lady originated in Australia, which has the same conditions as Washington, and it’s the most tolerant of the sun. Galas are from New Zealand and Honeycrisp from Minnesota, two areas that have different heat and light conditions. Both varieties are less tolerant.
Kinds of damage
Sunburn browning is the most common type of sunburn in apples. It requires both heat and light: a fruit surface temperature (FST) of 115 to 120 degrees, depending on the variety of apple, and UVB rays. It results in a yellow, bronze or brown spot on the sun-exposed side of the peel, which often doesn’t become visible for a few days after the damage is done.
These apples can still be used for canning, juice and slices, Schrader says, but their value is substantially less than for fresh, whole apples.
Sunburn necrosis requires enough heat for the FST to reach 126 degrees for 10 minutes, but does not require UVB rays. Cells in the peel die, and a dark brown or black spot appears later. Apples with sunburn necrosis are discarded as culls.
“You don’t see a lot in well-managed orchards,” he says. “It’s fairly rare.”
A third kind of damage, photooxidative sunburn, occurs at lower FST and air temperatures and doesn’t seem to need UVB rays either. Visible light is the main cause. This sunburn happens when apples that have been shaded are suddenly exposed to full sunlight, for example when trees are pruned. The leaves that had been shading them are removed and “the fruit hangs there naked,” he says. It can also occur when harvested apples sit in the sun in a bin in the field, on a truck or at the packing shed.
Photooxidative sunburn starts with a white spot that is caused by photo-bleaching, which lasts from three to seven days. Then necrosis begins.
“Necrosis can be really ugly,” Schrader says. “If the necrotic spot gets brittle, the skin can break and insects and other organisms can get in.”
There are other disorders related to heat and light damage as well. For example, watercore causes the inside of the apple to turn light brown, and the core becomes very sweet from accumulation of sugars. Fuji and Honeycrisp apples, both of which sunburn easily, are especially susceptible. While watercore might not be directly related to sunburn, high temperatures seem to exacerbate it, he says.
Apples with Fuji Stain have sunburn before they go into cold storage, but they don’t show any sign of the stain until they’ve been in storage for a period of time. The stain usually appears as a halo around the edges of a sunburned area, and is more likely to occur when the sunburn is severe. The amount of stain increases with the length of time in storage.
The temperature of the peel, the FST, is crucial to the development of sunburn in apples, Schrader says. It can be influenced by several factors. They include the level of light, UVB rays, air temperature, relative humidity, the velocity and direction of the wind, tree vigor, the amount of rainfall and irrigation and how the apples are handled. When any of these conditions are unfavorable, the fruit can become stressed.
“When apples are under stress, it’s more likely the peel temperature will increase,” he says. “A happy tree is less likely to get burned.”
The two most important factors affecting the temperature of the peel are the air temperature and the level of light. The FST can be very different from the air temperature, he says. On most days, it’s at least 20 degrees Fahrenheit higher than the air temperature, and it can be up to 30 degrees higher.
Sunburn almost always appears on the side of the fruit that’s exposed to the sun during the afternoon, he says. As the sun moves from east to west, it heats different spots on the fruit. Each spot stays at a high temperature for about an hour.
“We get maximum FST between 2 and 4 p.m. Apples on the southwest side of the tree, which are fully exposed to sun, are most likely to sunburn.”
Sunburn is becoming a bigger problem as growers plant more dwarf trees in order to get high-density planting, he says. Since dwarf trees have less foliage to shade apples, the chance of sunburn has gone up substantially.
There are a number of ways to keep heat and light from damaging apples. The goal is to prevent the FST from exceeding 113 degrees.
One way is to keep the fruit shaded. “The way growers manage the tree canopy is very important,” Schrader says. They need to prune enough to allow light into the tree, but not so much that they remove too many of the leaves shading the apples. Using shade cloth helps reduce the incidence of sunburn. So does bagging, which involves covering each apple with its own little bag, but the cost is prohibitive for growers of all but very high-value apples, he says.
Another solution is evaporative cooling (EC). Sprinklers spray the trees periodically with water, and as the water droplets evaporate, the surfaces of the apples and the leaves cool by several degrees. Micro-sprinklers that spray mist are the best, he says, because they use less water than pulsating impact sprinklers.
Most orchards with evaporative cooling are divided into three zones and equipped with a timer, Schrader says. Sprinklers spray the first zone for 10 minutes, then the second, then the third, and then return to zone one.
“It takes about 20 minutes for the water to evaporate, then the temperature shoots back up,” he says. “You have to get the timing just right.”
In 1997, Schrader began searching for a substance that could be sprayed onto apples that would prevent them from burning by reflecting light and filtering out UVB rays. Apples produce a protective wax on the cuticle themselves, he says, but it isn’t as thick on the sun-exposed side of the fruit as it is on the sides that are more shaded. He theorizes that either light or heat interferes with the synthesis of the wax, and that apples sunburn because their own wax doesn’t give them enough protection to get them through the season.
He read that the leaves of the Carnauba palm tree, which grows mainly in northern Brazil, produce a wax to prevent themselves from becoming sunburned. He worked on a carnauba wax-based formulation that would augment the natural wax on apples, and in 2003, Raynox became commercially available to growers.
Raynox can reduce sunburn by 50 percent, and it decreases the appearance of Fuji Stain after months of storage, Schrader says. It’s edible and can be sprayed with a standard sprayer. It can’t be washed off by evaporative cooling or rain. Although it stays on all season, it doesn’t stretch as the apples grow, so it should be applied at least three times—preferably four—during the growing season. It’s being used on 30,000 acres in Washington State, and at least 10,000 more around the world, including acreage in Australia, South Africa, Mexico and Chile.
There are other protectants as well, he says. Apple growers who use all these tools can virtually eliminate their losses from sunburn.
The author is a freelance writer based in Altadena, Calif.