Several plant growth regulators are available to control preharvest drop, extend the harvest window and improve storability of pears, but the products do not have exactly the same effect, reports Dr. Yan Wang, postharvest physiologist at Oregon State University.
Postharvest drop is a significant concern both in summer and winter pear varieties.
It results from the expressions of genes and activities of enzymes associated with cell wall degradation in the abscission zone, Wang explained during Washington State University’s Fruit School on Apple and Pear Horticulture in November.
Expression of the genes is triggered when auxin drops below a certain threshold in the abscission zone, which then becomes sensitive to the ripening hormone ethylene.
Three growth regulators are commercially available for controlling drop by maintaining the balance between auxin and ethylene:
—NAA (naphthaleneacetic acid), sold under the brand name K-Salt Fruit Fix, which is a synthetic auxin.
—AVG (aminoethoxyvinylglycine), sold as ReTain, which inhibits ethylene biosynthesis.
—1-MCP (1-methylcyclopropene), sold as Harvista, which inhibits the action of ethylene.
Research has shown that NAA is more effective in controlling drop than either AVG or MCP, but that a combination of NAA and AVG works best, Wang reported. He stressed that this might not be the case for apples, as pears produce much less ethylene while they are on the tree.
However, if not applied at the right time or concentration, auxin-type plant growth regulators can increase ethylene production in the fruit and reduce its storability.
In a trial, pears were treated three weeks before the anticipated harvest date with the label rate of 33 parts per million of NAA. The treatment had no effect on the amount of ethylene in pears kept for four weeks in storage at 30°F. However, NAA applied two weeks before anticipated harvest increased ethylene in the stored pears.
NAA applied at 15 ppm (less than half the label rate) two weeks before harvest had no effect on the fruit ethylene level, but the same rate applied one week before harvest increased ethylene production in the fruit, reducing its storability.
Wang said to avoid this negative effect of NAA, growers should apply a reduced rate of 20 ppm on Bosc or d’Anjou pears and 15 ppm for Bartletts and avoid applying NAA within two weeks of harvest. An NAA application will become effective within two to three days and will control drop for at least two weeks.
Applying AVG with the NAA helps to further reduce fruit drop and counteracts the negative effect of NAA on fruit storability, Wang said.
ReTain and Harvista can also be used to delay maturity and extend the harvest window. Ideally, Bartlett pears should be harvested at between 19 and 17 pounds pressure, d’Anjou at 15 to 13 pounds, Bosc at 15 to 13 pounds and Comice at 13 to 11 pounds.
However, it is sometimes a challenge for growers to harvest pears at the right maturity if fruit is maturing quickly, as it did in 2015, or if they are short of labor.
In a trial, Wang applied ReTain to Bartlett pears one week before anticipated harvest when the fruit firmness was 21 to 20 pounds.
ReTain did not affect fruit maturation until the fruit softened to 19 pounds. It then slowed maturation of the fruit while the pressure was between 19 and 17 pounds, extending the harvest window by about five days. It was less effective if applied two or more weeks before harvest.
Research at the University of California, Davis, showed that Harvista could also delay maturity of Bartlett pears, but results were inconsistent in fruit of different harvest maturity and from year to year.
Wang has also studied the effect of growth regulators on fruit disorders in Bartlett and Starkrimson that result in storage or export losses.
Bartlett is susceptible to a number of storage disorders, including pink end, senescent scald, senescent core breakdown and yellowing. Starkrimson is prone to senescent core breakdown and disorders caused by low oxygen or high carbon dioxide levels.
A preharvest application of ReTain can inhibit the fruit’s ethylene production during storage, reducing senescent disorders and extending the storage life of Bartlett pears, Wang reported.
The closer to harvest it is applied, the more effective it is, but the product has a seven-day preharvest interval. He recommends applying between a half and one pouch (60 to 120 ppm) of ReTain one week before harvest and picking the pears at 19 pounds pressure.
He recommends the same treatment for Starkrimson, though they should be picked at 15 to 14 pounds pressure.
Scientists at the University of California, Davis, have done trials looking at the effect of Harvista on the storability of Bartlett pears.
They found that Harvista had no effect on the color of the stored pears and little effect on firmness, but it did reduce senescence disorders. However, Wang found that the postharvest formulation of 1-MCP, SmartFresh, was more effective at increasing the storability of European pears.
SmartFresh applied after harvest at 300 parts per billion helped maintain the green color of Bartlett pears and reduced senescence disorders while also allowing the pears to recover their capacity to ripen after being stored for four months.
But results with SmartFresh were also inconsistent, Wang said.
Scientists believe that the effects of SmartFresh are influenced by harvest maturity of the fruit, the elevation of the orchard, the time that elapses between harvest and treatment, and the concentration of ethylene in the storage room.
Tests with Starkrimson pears using SmartFresh at 300 ppb showed that after four months in storage at 30°F, the pears did not develop any senescence disorders and had less decay, but they failed to ripen when held for seven days at room temperature.
However, they did ripen if stored for more than two months and held for 14 days at room temperature.
Tests with Bosc showed that SmartFresh-treated fruit retained firmness, sugar and acid levels and green color better than untreated fruit, and sensory panelists preferred the treated pears because of their crunchy and juicy texture.
The treatment also reduced decay caused by bull’s-eye rot, phacidiopycnis rot and cladosporium rot. Wang stressed that MCP is not a fungicide but increases the fruit’s resistance to decay.
In d’Anjou pears, SmartFresh applied at 150 to 200 ppb shut down ethylene synthesis, controlled superficial scald and extended the storage life.
The product must be applied within three weeks of harvest, otherwise it will not control scald.
Wang conducted laboratory trials in which d’Anjou pears were either treated with the antioxidant ethoxyquin or ethoxyquin plus 100 ppb of SmartFresh before being put into low-oxygen storage for eight months.
Pears treated only with ethoxyquin had 6 percent scald, 6 percent speckling and pithy brown core, and 3 percent decay. Pears treated also with SmartFresh had none of those disorders.
However, special steps need to be taken in order for the pears to recover their capacity to ripen after storage. The pears can be stored at a higher temperature than normal and held in controlled-atmosphere storage.
Or, ethylene can be applied with the SmartFresh. Alternatively, the pears can be conditioned with ethylene after storage.
In trials in commercial packing houses in Washington and Oregon, however, SmartFresh applied at a rate of 100 ppb was not enough to control superficial scald or extend the storage life of the fruit. Results were inconsistent, with perfect control of scald in some lots and no response in others.
“A hundred parts per billion works well in the lab, but in big CA storage rooms it’s not high enough,” Wang told the Fruit School. •
– by Geraldine Warner