Plant Growth Regulators for Controlling Apple Pre-harvest Drop
As apples ripen, among many other processes, they begin to increase their production of the ripening hormone ethylene. As ethylene is being produced, it leads to notable changes in apple quality, which include changes in color, texture, and flavor. Furthermore, increased ethylene production also leads to the phenomenon of pre-harvest drop, which refers to the abscission of the fruits from the tree before horticultural maturity. The production of ethylene results in weakened tissue in the abscission zone (the region where the pedicel of the apple is connected to the supporting limb) leading to cell wall breakdown and eventual drop.
In addition to ethylene production, preharvest drop can be increased by events such as droughts, heat, and high winds all of these being able to act as significant sources of stress. Heavy insect infestation and late summer pruning can also increase the severity of fruit drop. It has been reported that different apple cultivars can have different sensitivities to preharvest drop, as early-ripening cultivars (i.e., Honeycrisp, Gala) are more prone to dropping than later-ripening cultivars (i.e., Fuji, Cripps Pink). However, ethylene is considered the primary driver of pre-harvest fruit drop. Therefore, fruit dropping can be counteracted by using different types of plant growth regulators (PGRs) which have different modes of action, such as inhibiting ethylene production or its perception, or directly delaying the formation of the fruit abscission zone. These will be briefly discussed below.
Harvistaᵀᴹ
Harvistaᵀᴹ, whose active ingredient is 1- methylcyclopropane (1-MCP) acts by blocking the apple fruit's receptors for the plant hormone ethylene, thus blocking the fruit from responding to the produced ethylene, slowing down the ripening process and pre-harvest fruit drop. Harvistaᵀᴹ can delay fruit softening, starch disappearance, affect fruit red skin coloration, decrease pre-harvest fruit drop and has also been shown to decrease watercore development. Harvistaᵀᴹ is recommended to be applied within three days prior to anticipated commercial harvest, just before any damaging effects start to take place. Harvistaᵀᴹ should not be used in stressed trees. To apply Harvistaᵀᴹ, a proprietary formulation tank and injector pump needs to be attached to a commercial sprayer (Fig. 1), based on recommendations from its manufacturer, AgroFresh.
ReTain
ReTain, whose active ingredient is aminoethoxyvinylglycine (AVG) uses a mode of action which inhibits the biosynthesis or production of the ripening hormone ethylene. As a result, fruit ripening processes are slowed down, including fruit softening, starch disappearance, and fruit red skin coloration development, in addition to decreasing pre-harvest fruit drop. ReTain has been reported to be able to also delay or reduce watercore development as well as reduce fruit cracking. ReTain will have a poor response on highly stressed trees, so its use is not recommended. In general, in order for ReTain to be effective, it needs to be applied before the natural rise in ethylene production or climacteric peak (which indicates the onset of fruit ripening), and that corresponds to three to four weeks before anticipated commercial harvest. Because different cultivars differ in ethylene production rates, they will also have different responses to ReTain applications. For example, Gala, which normally produces low amounts of ethylene will be very sensitive to ReTain and thus strongly influenced by this PGR, while high ethylene producers such as McIntosh will be less sensitive and thus responsive. Other cultivars such as Honeycrisp, which can be spot-picked and potentially have multiple harvests, can have ReTain applied 7 to 14 days before the expected first harvest date. When applied in this manner, the ReTain is not affecting the maturity of the apples in the first harvest, but rather regulating the maturity of the apples in subsequent harvests. ReTain can be applied using a standard commercial sprayer (Fig. 2) and is manufactured by Valent USA.
NAA
Another PGR that can also be used to reduce fruit pre-harvest drop, whose active ingredient is napthaleneacetic acid (NAA), is commercially sold as Fruitone-L® (AMVAC), Fruitone-N® (AMVAC), Refine 3.5 WSGᵀᴹ, Refine 3.5Lᵀᴹ, PoMaxa. NAA is a synthetic auxin that directly interferes with the enzymes that create the abscission zone that cause fruit to drop during pre-harvest. Although NAA is used to prevent pre-harvest drop, unlike Harvistaᵀᴹ or ReTain, NAA does not delay fruit maturity, but can on the contrary, accelerate it, particularly under temperatures above 85°F. This means that it does not decrease the production or perception of ethylene, but instead it’s mode of action is based on inhibiting the enzymes which work to weaken the cellular composition around the stem, which would normally break the cells apart from one another causing the fruit to drop. NAA has notable effects on increasing the overall ripening process, resulting in increased red skin coloration, flesh softening, and starch disappearance, while, at the same time, allowing the apple to stay on the tree, decreasing fruit drop. As the recommended application time for NAA is very close to the onset of fruit drop, NAA is considered more as a “rescue PGR'', as it is used in order to stop the drop of mature apples due to unforeseen circumstances. Application of NAA requires close monitoring of fruit maturity. NAA sprayed at a rate of 10 ppm will control fruit drop for a period of 7-10 days, but it will take ~3 days for the product to get into the fruit. Therefore, applications that occur too early may not allow for real control of fruit drop, while applications that occur too late might risk dropping a significant amount of the crop before the effect of the PGR takes effect. It is also very important to note that apples treated with NAA will be decreased in their long-term storage potential, as the ripening process is sped up. NAA can be applied using a standard commercial sprayer.
This article appears in August 2022, Volume 13, Issue 5 of the Vegetable and Fruit News.