Skin browning on Goldens may have multiple causes

Dr. Nathan Reed of AgroFresh said only a small percentage of fruit treated with MCP developed skin browning. (Photo by Geraldine Warner)

A disorder known as diffuse skin browning (DSB) has been seen on stored Golden Delicious apples in recent seasons. It appears to be worse on fruit treated with MCP (1-methylcyclopropene), and it seems related to how quickly the fruit is cooled when it is put in storage.

“If you cool it quick you will get DSB,” said Dr. Dana Faubion, Washington State University Extension educator. “If you cool it quick with MCP, you will get a marked increase in DSB on Goldens.” But there may be more to it than that.

The amount of diffuse skin browning also seems to depend on where the apples were grown. “So it’s even more complicated than just MCP and storage room temperature or rate of cooling,” Faubion said.

“It’s discouraging how complex some of these problems are.” MCP, a product designed to enhance the storability of apples, is sold under the trade name SmartFresh by AgroFresh, Inc. Dr. Nate Reed, director of postharvest physiology with AgroFresh, said that during the 2003-2004 season, more than a third of Washington’s Golden Delicious crop was treated with MCP, but only 1 percent of the crop was affected by diffuse skin browning.

Though the losses were not significant industrywide, for the growers whose crops were affected it was a serious problem. The disorder was found mainly in apples grown in the Yakima district. Faubion said AgroFresh issued new guidelines for use of MCP on Golden Delicious, and though more than 30 percent of the Golden crop was treated with MCP in 2004, no diffuse skin browning was reported by the industry.

During the 2004-2005 season, Agro­Fresh did research in cooperation with WSU to try to pinpoint under what circumstances the skin browning might develop. A total of 210 bins of fruit were taken from five orchards that had reported the disorder during the 2003 season. Half the fruit was treated with MCP, and both the treated and control fruit were exposed to seven different storage treatments, with the temperature ranging from 31°F to 38°F.

In some cases, ­establishment of the final storage temperature was delayed. In some cases, the MCP application was delayed by two or seven days. In April this year, the fruit was removed from storage, assessed for quality, and run over a commercial packing line, separated by grower lot. In the storage rooms held at the lowest temperature (31°F), an average of 1 to 3 percent of the fruit that had been treated with MCP had diffuse skin browning.

In the control fruit, an average of 0.1 percent had the disorder. In the rooms held at higher temperatures, there was up to 0.1 percent diffuse skin browning, whether the fruit had been treated with MCP or not. Only a very small amount of browning was found in fruit that had been stored for a time at 45°F before it was brought down to the final storage temperature, but Reed said there might be concerns about the effect of the higher initial temperature on fruit firmness.

He noted, however, that the firmness of the fruit in all the treatments was higher when it had been treated with MCP. Overall, the MCP-treated fruit was 1 to 2 pounds firmer than the control fruit. Reed concluded that diffuse skin browning on Golden Delicious apples can be managed by manipulating the storage temperature and pull-down rate (see “Precautions,” page 19).

Faubion said that in the past, storage operators have loaded a room, set the temperature point, and walked away from it. The study shows that diffuse skin browning is influenced not only by the temperature in storage, but how quickly the temperature falls. Fruit seems to be particularly sensitive at the start of the storage period. He questioned whether the finger should be pointed at MCP or the protocol for exporting to Mexico, which requires low-temperature storage as a quarantine treatment.

Dr. Jim Mattheis, plant physiologist with the U.S. Department of Agriculture in Wenatchee, Washington, said MCP does appear to exacerbate diffuse skin browning, particularly when the fruit was treated immediately after being put into low-temperature storage. It also seemed to be influenced by the carbon dioxide level. In fruit not treated with MCP, at 1 percent carbon dioxide there was not much browning, but at 3 percent there was as much as in the MCP-treated fruit.

However, research has shown that peel disorders such as diffuse skin browning are not consistently made worse by high carbon dioxide levels. Carbon dioxide might be involved, he said, but there seem to be other factors, too. As the MCP treatment was delayed, there was less browning, but Mattheis noted that for control of superficial scald, MCP should be applied as soon as possible.

With a delay of more than ten days, the product loses its ability to control scald, particularly on Granny Smith. The benefits of MCP in terms of firmness and acid retention are also reduced by the delay. A rough peel disorder has been seen in Cameo apples, regardless of whether the fruit was treated with MCP or not, though there is some evidence that MCP makes it worse, Mattheis said.

However, he has not been able to show that MCP alone is the culprit. More research needs to be done on the relationship between peel disorders and use of MCP, he suggests. New York Dr. Chris Watkins, horticulturist with Cornell University, New York, said external carbon dioxide injury has always been a problem in McIntosh apples. The epidermal cells turn watery at first. After a few weeks rough depressions develop on the skin of the fruit.

The injury appears to be worsened by MCP. In some cases there’s been a massive difference between treated and untreated fruit, though in others there’s not much difference. Unlike the diffuse skin browning on Washington Golden Delicious, this ­disorder can be totally controlled by DPA (diphenylamine), which Watkins recommends as a “sleep well” strategy.

With a DPA drench, normal CA regimes can be used. However, many storage operators in the eastern United States, particularly the smaller ones, are reluctant to use DPA because of the costs and logistics involved, and there is concern about drenched fruit having more decay. As an alternative to DPA, Watson recommends keeping the carbon dioxide level low—about 0.5 percent—for the first four to six weeks of CA storage, then raising it to 2 percent. A high carbon dioxide level is needed to maintain firmness, ­unless MCP is used.