Left: Gala is most susceptible to lenticel breakdo wn. Mild cases resemble chemical burn, and lenticels turn black. Right: In more severe cases, like the Fuji pictured here, flesh under the lenticels sinks, resulting in craters under the skin.
Lenticel breakdown is a skin disorder of apples that appears largely after the fruit has been packed and thus is an expensive problem. The postharvest team at Washington State University in Wenatchee has been researching lenticel breakdown for the past five years. I am sorry to report that there appears to be no single solution. Minimizing this costly disorder will take teamwork, including cooperation from growers, storage operators, and packers.
Gala is most susceptible to lenticel breakdown, but the disorder has also been seen to a much lesser extent on Red Delicious and Fuji, especially after long-term storage (see Figure 1). Mild cases resemble chemical burn, which causes the lenticels to turn black (see Figure 2). In more serious cases, the flesh under the lenticel sinks, creating a crater. (Lenticel breakdown should not be confused with a problem called lenticel blotch pit, which is the drying and browning of the flesh just under the skin, caused by a nutrient imbalance.)
A key to resolving this problem is to determine why fruit from certain orchards develop lenticel breakdown while fruit from other orchards do not. Harvest maturity plays a role. Growers with poor-coloring strains of Gala or Fuji find the fruit downgraded due to lack of red color. Red color is optimized when the nights turn cool, which is not often the case during Gala harvest season in the hotter areas of the state. Thus, the natural reaction is to allow the fruit to remain on the tree to optimize color and size. In our experiments, fruit that is harvested when it is more mature develops more lenticel breakdown than fruit harvested at optimum starch levels. Any gain in profit from increased red color is lost when fruit need to be repacked due to lenticel breakdown.
Although we do not have experimental data, it appears that the interval between harvest and cooling influences the risk of lenticel breakdown. When cooling is delayed after harvest, the risk of lenticel breakdown increases.
Thinking that there might be a connection between mineral content and the risk of lenticel breakdown, last year we examined the mineral content of fruit with and without the disorder. We analyzed tissue from the peel of the apple and found no significant difference in mineral content.
The application of SmartFresh (MCP) has provided mixed results. We have applied SmartFresh to half our storage samples each year for the last three years. In two years, there was no difference in the number of fruit that developed lenticel breakdown, while in one year, the SmartFresh-treated fruit developed more lenticel breakdown than untreated fruit when they were packed after January. These trials were done with a very small number of lots of apples. This year, we hope to further understand the effect of SmartFresh from a trial we are doing with 24 lots of Gala apples.
It appears from our work that the appearance of lenticel breakdown is affected by the length of time the fruit is in storage. Galas packed within three or four months of storage developed less lenticel breakdown than those packed after longer storage. When I tour packing lines in other parts of the world, they report that lenticel breakdown is not a major problem. One of the reasons may be that Galas are packed immediately after harvest and are not held in bins and packed after six or more months. When I get reports from the Washington industry on lenticel breakdown of Red Delicious or Fuji, it usually is after long-term storage of late-picked fruit.
During the packing process, fruit are first dumped into water. In our experiments, when the fruit and water temperatures are close together, there is less lenticel breakdown than when very cold fruit are dumped into hot water.
In other countries, apples do not spend much time in water; they may be dumped into water, but then are moved on belts without brushes. In our trials, we have determined that susceptible fruit held in long-term storage develop more lenticel breakdown when the fruit are presized than when they are not held in water, as on a commit-to-pack line.
The application of soaps and detergents should be avoided on lenticel breakdown-susceptible lots. In other countries, soap is not used, and usually the fruit are not waxed. Soaps and detergents are mostly surfactants, and in our trials some types of surfactants cause injury to susceptible apple tissue. Higher concentrations cause more damage.
Last season, we tested whether there is a connection between postharvest exposure to iron and lenticel breakdown. Peaches exposed to the iron in buckets at harvest or iron in postharvest equipment develop skin damage called inking. Gala apples exposed to very high levels of iron sulfate (500 parts per million) in solution developed more lenticel breakdown than those that had not been exposed.
Treat Gala differently
We continue to work on understanding this disorder, but it has become apparent to me that this industry needs to rethink the way it treats fruit with tender skin. Thus far, Washington has had the greatest economic problem with lenticel breakdown. Gala is grown wherever apples can be grown, including areas of the state that may be too hot for this variety. Harvesting at the proper maturity is one challenge. A second challenge is to reduce the length of time this apple is stored. Gala is essentially a summer apple that is stored long-term because of the huge volume planted. We are using equipment in the packing house designed for a much sturdier apple (Red Delicious) that can stand up to long brush lines, repeated immersion in water, soaps, and drying tunnels. We are also trying to get Gala to conform to our perceived marketing needs by waxing it, which necessitates the use of soaps and surfactants. Perhaps if the ten large marketing organizations banded together and refused to provide Galas that are waxed, we could eliminate much of the lenticel breakdown problem. It is time to rethink how we treat this delicate variety, since in the near future, if not today, we will be packing and storing new varieties that are even more sensitive to postharvest handling challenges.