In apple orchards, achieving efficient preharvest drop control is essential for maximizing fruit quality and storage potential. To address this concern, three registered materials have been identified for controlling preharvest drop in apples: Naphthaleneacetic acid (NAA- Fruitone N), Aminoethoxyvinylglycine (ReTain), and 1-methylcyclopropene (Harvista). Each material offers unique advantages and requires careful application based on specific apple cultivars and environmental conditions. This article presents detailed recommendations for each material, emphasizing effective strategies for managing preharvest drop this season.

Naphthaleneacetic Acid (NAA- Fruitone N)

NAA provides modest drop control, but its drawback lies in stimulating ethylene production which results in accelerated fruit maturity. Using NAA alone is not recommended due to limited drop control and accelerated ripening. Because NAA has an immediate, it can be applied just before apples start dropping, in a way it can be viewed as a rescue treatment.

The successful application of NAA (Naphthaleneacetic acid) for drop control in apple trees requires careful timing and consideration. Once NAA is applied, it takes approximately 3 days for its activity to take effect, offering about 7 days of drop control. The recommended rates of 10-20 ppm are generally effective, but determining the precise timing of application can be a bit challenging.

To achieve the best results, it’s crucial to strike the right balance. Applying NAA too early may lead to its effects wearing off before the harvest is complete. On the other hand, if NAA is applied too late, a significant number of apples may have already dropped on the ground before NAA starts working. Thus, the ideal time to apply NAA is when a few apples have started to drop naturally. This can be facilitated by gently bumping a few branches and observing if any apples drop. Once this indication is observed, it’s time to proceed with the NAA application.

For more extended stop-drop control, a split application can be employed, with 10 ppm applied at a 7-14 day interval. To maximize the efficacy of NAA, it is best applied in slow drying conditions and warm temperatures, ideally around 70-75°F. Many growers choose to apply their stop-drop sprays early in the morning, taking advantage of the presence of dew on the trees and rising temperatures during the day.

In the case of drop-prone cultivars like McIntosh, if NAA is used, it should be applied when the first sound fruit drops, using a high rate (20ppm). Fruit should be harvested within 10-14 days of application to avoid rapid fruit drop approximately 2 weeks after application. Additionally, NAA-treated fruit should not be stored for prolonged periods and should be marketed before Christmas.

Aminoethoxyvinylglycine (ReTain)

ReTain (AVG) is a plant growth regulator that inhibits ethylene production, resulting in delayed preharvest drop, fruit flesh softening, starch disappearance, and red color formation. Its application offers extended shelf life and improved fruit quality by slowing down essential ripening processes in apples. Timing is crucial, as applying ReTain too early may negatively impact fruit color in cultivars with poor color, but on the other hand if applied too late a significant portion of the crop may become unresponsive to ReTain due to the initiation of autocatalytic ethylene production.

Trends with ReTain

In recent years, two trends have emerged concerning ReTain’s application. First, applying ReTain closer to harvest for Gala and Honeycrisp cultivars can reduce its negative impact on red color development. Secondly, combining ReTain with NAA has proven more effective for drop control, particularly in hot weather conditions.

Recommended Application for Different Apple Cultivars

McIntosh: For this cultivar, a combined application of ReTain (1 pouch) + NAA (10ppm) is suggested three weeks before the expected first harvest, especially in warm weather conditions. If hot weather persists into September, a second application of the same tank mix should be applied two weeks after the first.

Gala: Depending on the strain’s coloring, apply either only ½ pouch/acre of ReTain for older, less colored strains or 1 pouch/acre for newer high-coloring strains. Application should be made 1 to 2 weeks before the expected first harvest. ReTain extends the fruit’s time on the tree, resulting in improved fruit size, color, and reduced stem end cracking.

Honeycrisp: This cultivar exhibits uneven ripening and may experience significant pre-harvest drop in certain years. To address this, a low rate of 1/3 pouch per acre of ReTain is recommended for blocks with drop issues. Apply 1-2 weeks before the expected harvest. ReTain can also be applied in between picks to reduce fruit drop while allowing fruit to continue developing color. Caution should be exercised with Honeycrisp, as ReTain may increase bitter pit incidence during storage.

Late September and October cultivars: ReTain’s negative impact on fruit color development is minimal in late September and October cultivars compared to early September cultivars. Thus, a full pouch/acre of ReTain is recommended for consistent drop reduction and greasiness control. Apply three weeks before the normal harvest date.

Harvista

Harvista offers effective drop control and can be applied closer to harvest (one week or less before anticipated harvest). Unlike ReTain, it does not suppress ethylene production but inhibits its action in the fruit, reducing fruit drop. Harvista’s rapid effect keeps fruit on the tree for more than four weeks, longer than ReTain. However, it also delays red color development. Specialized equipment must be used for Harvista application due to its gas-based active ingredient, 1-methylcyclopropene (MCP). Similar to ReTain, its application on Honeycrisp should be carefully considered, as it may increase bitter pit incidence. Selecting the appropriate preharvest drop control strategy is vital for ensuring optimal fruit quality, prolonged storage, and a successful apple harvest. By following the specific recommendations for each registered material, apple growers can enhance their orchard management and achieve the best results this growing season.

This article was posted in Apples and tagged 1-methylcyclopropene, Amaya Atucha, Aminoethoxyvinylglycine, Apples, Harvista, NAA- Fruitone N, Naphthaleneacetic acid, preharvest drop control, ReTain.