Spotlight on apple skin problems

Cracks in the fruit cuticle can form soon after bloom (back), exposing the underlying hypodermal cells to air, stimulating a wound response that results in russet (front).

When apples have bad skin, they’re less appealing to customers, so defective fruit has to be sorted out and diverted to the processing or cull fruit bins. What can you do to send fewer apples that direction?

Dr. Steve McArtney, the Southeastern apple specialist at the Mountain Horticultural Crops Research and Extension Center in Mills River, North Carolina, addressed four fruit skin problems—and their causes and control—in speaking to New York fruit growers at their horticultural meeting in January.

The four problems he addressed are russeting, ­sunburn, poor color, and lenticel breakdown.

Russet

Russeting can have any of several causes, he said—fungi and yeasts on the fruit, insect feeding, frost around bloom time, chemical injury, or rain and high humidity in the two to three weeks after bloom. These can all interact and make russeting more severe.

When working in New Zealand, he found that multiple applications of lime sulfur as a bloom thinner on Gala nearly doubled the incidence of russeting wherever it was used, but in a dry climate, the effects were less severe.

“Russet in Golden Delicious is related to abnormal development of the cuticle, which is the layer of waxy material that covers the epidermal cells in the skin of the fruit,” he said. “Nonrusseted portions of Golden Delicious fruit have either a thicker, more convoluted cuticle or a double layer of cuticle compared to russeted portions.

“Russet in Golden Delicious is caused by a weakness in cuticle formation that leads to microcracks developing in the cuticle; these microcracks expand further with fruit enlargement,” he said.

He found that multiple sprays of gibberellic acid 4+7 (ProVide) applied at seven- to ten-day intervals during the month after bloom reduced the incidence and severity of russeting, and he recommends that regimen at 15 to 20 parts per million on susceptible cultivars. Apogee, which is a gibberellin inhibitor, paradoxically also reduced russeting when tank mixed with the gibberellins, but only in his studies in the southeastern United States. This has not been confirmed elsewhere.

Sunburn

Sunburn browning occurs when the fruit surface temperature reaches 115 to 120 degrees—the exact temperature being variety dependent, he said. Studies by Dr. Larry Schrader at Washington State University have shown that fruit surface temperature can range from 9 to 32 degrees higher than air temperatures, typically 22 degrees higher, McArtney said. So, when air temperatures are above 90 degrees, risk of sunburn browning increases.

In Henderson County, North Carolina, last year, more than half the fruit in some blocks of Fuji developed sunburn, he said. Three sprays of Raynox Plus increased the incidence of burn-free fruit from 43 to 67 percent. Raynox Plus is a combination of carnauba wax and bentonite clay that both absorbs and reflects ultraviolet light.

Other products that work in the same way include ­Surround (kaolin clay) and Purshade (calcium carbonate), he said, but they were not compared with the Raynox Plus treatment.

Red color

The development of red color in the skin of apples is due to formation of anthocyanin pigments and their accumulation in the vacuole of epidermal cells, ­McArtney said. Anthocyanin formation is promoted by high light conditions that favor photosynthesis and cool night temperatures. Direct absorption of light by the fruit itself is required for anthocyanin formation.

Last year, McArtney compared three experimental spray materials and a reflective plastic ground cover on color formation in Cripps Pink apple trees. The spray materials did not increase color. The reflective film (from Global Packaging Solutions) did increase the proportion of apples with more than 80 percent red color to 75 ­percent, compared with 47 percent in control plots, he said.

He put out the 100-inch-wide film in orchard rows four weeks and two weeks before harvest. The short two-week treatment was equally effective compared to the month-long treatment, he said. The film had its greatest effects in vigorous orchards with dense foliage.

Lenticel breakdown

“Lenticel breakdown is a postharvest skin disorder predominantly affecting Gala that can significantly reduce crop value in some years,” he said. “The incidence of lenticel breakdown varies between orchards and years for reasons that are not well understood.”

Studies in Washington State have shown that the incidence of lenticel breakdown in Gala is reduced if fruit are harvested before they are too mature, if foliar calcium sprays are applied during the summer, if the time delay between harvest and cooling is kept to a minimum, if the temperature difference between the fruit and the water dump is minimized, and if soaps and detergents are avoided in the packing line, he said.

McArtney believes that since lenticel breakdown is related to the maturity of the fruit at harvest, postharvest treatments with 1-MCP (SmartFresh) should reduce lenticel breakdown, and his study did show that SmartFresh reduced lenticel breakdown in stored fruit at all harvest dates except initial harvest.

“We will be repeating this study in 2012 to establish if the positive effect of SmartFresh on the incidence of lenticel breakdown in Gala is consistent across years,” he said.