Managing Fruit Maturity and Abscission
Controlling when fruits mature allows more efficient use of labor and other resources, and prolongs the harvest season. For example, PGR applications could advance fruit maturity in one portion of a block, and delay maturity and improve storability in another.
Hastening Fruit Maturity
Ethephon applications will advance apple maturity by 3 to 5 days under favorable weather conditions. This product will also shorten the storage life of treated fruit, so avoid using it on any apples intended for long-term storage.
Ethephon also improves the color of red-skinned apples. Fruit require cool nighttime temperatures and direct exposure to light for color development, even when ethephon has been applied. Proper training and pruning is critical to allow good light distribution within the canopy. Cultivars and strains that color poorly may not respond adequately to ethephon application. Do not use ethephon on yellow or green-skinned cultivars to advance fruit maturity.
Temperature will influence the activity of ethephon as well as plant sensitivity. Ideally, nighttime temperatures should be between 55 and 65°F, and daytime temperatures between 75 and 85°F. Color development of apple fruit will not be enhanced if nighttime temperatures are above 70°F and daytime highs are above 90°F. However, maturity will still be advanced.
Apply ethephon 15 to 20 days before the anticipated harvest date in a dilute application of 1/3 to 2/3 pint of ethephon plus 10 ppm NAA per 100 gallons of water. The ethephon will promote color development, while the NAA slows abscission and fruit drop. Treat only a small group of trees, no more than can be picked and packed in 3 days. Treating blocks approximately 2 to 3 days apart will allow adequate time for harvest and packing.
Ethephon can also speed the process of fruit abscission (fruit drop), and has been used to synchronize abscission layer formation to aid in mechanical harvesting of tart cherries. As with other fruits, ethephon speeds the ripening and subsequent breakdown of cherry fruit, and may contribute to more rapid softening. However, the fruit must be at or beyond the “straw” color before they will respond to ethephon. Green fruit have not yet developed the physiological ability to respond to ethephon.
As in the case of apples, daytime temperatures above 85°F will result in an over response and in tart cherries can result in additional unwanted side effects such as gummosis. Weak or stressed trees should not receive an application of ethephon as gummosis will be even more severe.
Because daytime temperatures in the Intermountain West routinely exceed 85°F in the weeks leading up to tart cherry harvest, growers often reduce the rate by half, which seems to give the beneficial effect of synchronized fruit abscission but lessens the risk of harmful side effects. Therefore, 1/3 to ½ pint per acre is applied in a dilute spray two to three weeks before harvest. All of the fruit should be at the straw color before ethephon application.
Preventing Premature Fruit Drop
Some apple cultivars, particularly early ones, are susceptible to pre-harvest fruit drop. Most susceptible cultivars respond to a dilute application of NAA. Make the application 7 to 14 days before anticipated harvest at a concentration of 10 to 20 ppm.
Do not apply NAA within 2 days before harvest nor use more than twice per season as a stop-drop treatment. If making a second application of NAA as a stop drop, allow at least 7 days between applications. NAA used on early season cultivars can result in fruit splitting at maturity. Do not exceed 20 ppm concentration and don’t apply as a low volume concentrate spray. NAA will shorten the storage life of treated fruit, so do not apply to any fruit intended for long-term storage. Ethylene inhibitors can be even more effective than NAA for preventing pre-harvest drop (see the following section).
Delaying Fruit Maturity
Applications of gibberellin (GA3, ProGibb) can be used to extend the harvest season of sweet cherries. GA3 applied when fruit is translucent green to straw color at 16 to 48 grams a.i. per acre delays maturity by 5 to 7 days. The result is larger, firmer fruit with bright green stems and a longer storage life. GA3 also slows color and sugar accumulation, resulting in brighter color at harvest but lower soluble solids.
Aminoethoxyvinyl glycine (AVG or ReTain) blocks the formation of ethylene by plants and can be used to delay maturity and to hold fruit on the tree. AVG can be used as a stop drop with the added benefit of firmer fruit at harvest and a longer storage life. AVG has no direct effect on color development, but allowing the fruit to hang on the trees longer will result in larger fruit with more color development.
Timing is critical. Apply at the label rate 4 weeks before anticipated harvest. AVG acts by preventing the natural abscission process from beginning. However, if this process has already started, AVG applications are not effective. Use at least 100 gallons per acre and spray both sides of the row (no alternate row applications) to ensure good coverage.