- Suppress Apple Shoot Growth
- Improve Fruit Shape and Size
- Control Russeting of Golden Delicious
- Promote Side Branching
- Promote Bloom
- Advance Fruit Maturity
- Control Preharvest Drop
Prohexadione calcium (Apogee, Kudos 27.5WDG) is a potent inhibitor of gibberellin biosynthesis. Gibberellins are associated with normal shoot elongation during the growing season. Inhibition of the production of these naturally occurring gibberellins can produce a decrease in shoot growth and overall tree vigor. The control of vegetative growth allows a balance between canopy development and fruit growth and may favorably affect fruit quality.
Prohexadione calcium applications to vigorous apple trees in Washington research trials, however, have not resulted in improved flowering the following year. Similarly, research trials with prohexadione calcium in Washington have not produced beneficial changes in fruit postharvest or storage behavior.
Manufacturers’ labels indicate prohexadione calcium can be used on trees with various levels of vigor, from high to low. However, under Washington growing conditions, prohexadione calcium is recommended ONLY on medium to high vigor trees. Prohexadione calcium has not shown beneficial effects on low vigor trees under Washington conditions. Evidence from other growing regions indicates prohexadione calcium may reduce the incidence of shoot blight infections from the fireblight (Erwinia amylovora) bacterium. Fire blight infection through shoots rarely occurs under Washington conditions, and successful control of fire blight using prohexadione calcium in Washington has not been demonstrated.
Treatment of vigorous shoots with prohexadione calcium does not necessarily result in the stimulation of terminal bud formation. If shoots do not form terminal buds, they have the potential to resume growth later in the season. Research with prohexadione calcium in Washington has shown that two or more applications in the spring can still result in a second growth flush in midsummer, although the vigor of this flush is reduced with a greater number of applications. Once a prohexadione calcium program is started, be sure to maintain an application sequence of every 2-3 weeks per application to sustain the active control over GA biosynthesis in the new shoots. Once a second growth flush has started, this second, midsummer flush is difficult to control with additional applications, especially if there was an interruption in the application sequence earlier in the growing season.
Growth control from a single application of prohexadione calcium lasts only a short time (4 to 6 weeks maximum under most conditions). A minimum of two applications per season is advised under Washington conditions, but more may be needed to maintain season-long control over shoot growth. For best results, the first application should be made early, when newly-forming terminal shoots are no more than about 1 inch in length. The second and any subsequent applications should be made at intervals of 2-3 weeks. Good results have been obtained in Washington using a rate of 6-12 ounces per 100 gallons spray volume (200-300 gallons applied per acre). Growers should carefully follow the growth response to prohexadione calcium in their orchards and make adjustments in both rate and timing as necessary to improve the response. In Washington research trials, properly applied low volume sprays have proven as effective as dilute spray volumes. Three to five applications may be necessary for high vigor trees having a light crop load. Follow label directions for adjuvants and recommendations for mixing and applying prohexadione calcium.
Note: Applications of 12 ounces/100 gallons at pink and repeated within 14 days provide the greatest effect. Being late with the application(s) will reduce the result.
The effectiveness of prohexadione calcium can be reduced if it is applied in water containing high concentrations of calcium salts such as calcium carbonate, typical of “hard” water. It is better to use water free of calcium salts. If using “hard” water, add one pound of high-quality, spray-grade ammonium sulfate for each pound of Apogee or Kudos 27.5WDG used, check spray water pH and adjust to a pH value lower than 7 if spray water is alkaline in pH. CAUTION: This approach may not produce satisfactory growth control if prohexadione calcium is applied in high-calcium water. Do not mix prohexadione calcium with any spray products containing calcium; the efficacy of the prohexadione calcium will likely be reduced.
GA4+7+ BA products (Promalin, Perlan) are labeled to improve apple fruit shape in Washington. These materials can be applied during bloom to elongate apples, particularly strains of Red Delicious. Flat, squatty apples or a lack of "typiness" can be the result of 1) unusually warm or cool weather during bloom or early fruit development, 2) excessive use of ethephon in previous years, and/or 3) use of NAA at a rate of 10 ppm or higher.
Application can be made at any time during the bloom period up to petal fall. However, GA4+7+ BA may be more effective if applied just before full bloom during the balloon stage, when wetting of the entire blossom cluster can be achieved. Where incomplete wetting occurs, use of a nonionic surfactant may improve efficacy.
Caution: Fruit thinning can result if GA4+7+BA is applied to young trees just coming into full bearing. Do not apply more than once per season. Research trials in Washington have documented occassional reductions in fruit diameter from applications of GA4+7+BA products; growers concerned about fruit size should use these products with caution.
GA4+7 (ProVide, Novagib) can help improve fruit finish in years when russet conditions are present. Physiological russeting is associated with climatic factors such as precipitation, high humidity, and cool temperatures in the early stages of fruit development. Apply ProVide in two to four consecutive sprays of 10-13 fl. oz./100 gallons (dilute basis), beginning at petal fall and continuing at 7- to 10-day intervals. Novagib should be applied as 20–26 fl oz per 100 gallons per acre at petal fall, repeating this treatment at 7–10 day intervals, applying a total of 52–80 fl oz per acre. Use of a wetting agent is not recommended. Four sprays of GA4+7 are normally recommended, but two sprays are often sufficient.
Caution: Do not exceed 40 oz of ProVide or 80 oz of Novagib per acre per season. Avoid application to weak or very young trees. Use of GA4+7 at the higher rate should be accompanied by an aggressive chemical thinning program to avoid reductions in return bloom.
Young trees of some apple varieties can be slow to develop side branches and fruiting spurs. As a result, they become leggy and difficult to bring into heavy fruiting. This is particularly a problem with trees on vigorous rootstocks in deep, fertile soils. Materials containing the cytokinin 6-benzyladenine (BA) have been demonstrated to improve branching and reduce the incidence of blind wood.
To promote lateral bud break, apply GA4+7 + BA products (Promalin, Perlan) as a foliar spray or spot-apply with a brush using a latex paint mixture. Low concentrations of foliar applied product (0.25-1 pt/5 gallons spray solution) should be timed when there are 1 to 3 inches of new terminal growth. For the latex application, high concentrations (3.2–5.3 fl. oz. product/pint latex paint) are applied to one-year-old wood in the spring when terminal buds begin to swell but before green tissue emerges. The latex paint should not contain mildewcides or other chemicals potentially harmful to trees. Please consult product labels for more detailed guidelines.
Similarly, pure BA products (MaxCel, Exilis Plus, Exilis 9.5SC) can also promote branching in young trees. Make 3-4 foliar applications of 250-500 ppm BA at 7-10 day intervals to non-bearing trees when shoot growth is at least 30 inches. Results are best when sprays are made in cool morning temperatures and targeted toward shoot tips.
Alternatively, BA products may be applied at high concentrations (5000-7000 ppm) when mixed with latex paint and applied to one-year-old wood in the spring when terminal buds begin to swell but before green tissue emerges. The latex paint should not contain mildewcides or other chemicals potentially harmful to trees. Please consult product labels for more detailed guidelines.
The response to any of these products will depend on growing conditions, rootstocks, variety, and strain. The degree of growth response to treatment with any of these products will be directly related to tree vigor. More dwarfing rootstocks and spur-type scions will produce a smaller growth response than more vigorous trees. Do not apply these products on low vigor trees or trees under stress from such factors as drought, low fertility, or winter injury.
Caution: Do not apply any of these products when air temperatures are lower than 40°F or greater than 90°F.
Young trees that are slow to bear or mature trees that produce only a limited number of flowers in off years may be helped by applications of ethephon (Motivate). Delay ethephon application until at least 5–6 weeks after bloom (after the beginning of June drop) to avoid excessive fruit thinning. NAA products (K-Salt Fruit Fix 200, K-Salt Fruit Fix 800, Fruitone, PoMaxa, Refine) may similarly be applied as a single application at 3–5 ppm five to six weeks after bloom to induce flowering the following year. If results are unsatisfactory after the first year, 1–2 applications may be required the next year at 7–10 day intervals to stimulate flowering.
Biennial or alternate bearing can be problematic in a number of apple cultivars, particularly Golden Delicious, Honeycrisp, Cameo, and Fuji. In an "on" year, trees in biennial cycles set heavy crops which generally produce high numbers of small fruit, often with poor color and eating quality; in the "off" year, flowering and fruit set are typically very low, resulting in small yields of large fruit that can be prone to physiological disorders suchs as bitter pit. Effective pruning and chemical thinning are crucial to mitigating biennial bearing patterns, but strategic use of plant growth regulators may also help promote consistent annual cropping.
Ethephon may be applied 5-6 weeks after bloom in the heavy crop year to improve flowering the next season. NAA may also be applied in single or multiple applications at 3–5 ppm starting five to six weeks after bloom to induce flowering the following year. Even though these spray programs may be popular in some sectors of the apple industry, growers should be advised that ethephon and NAA have rarely increased return bloom in several years of WA research trials.
Caution: Applications of ethephon may reduce fruit size. Early-season applications of ethephon before the start of June drop may cause excessive thinning. Use of ethephon on weak trees can produce excessive thinning, excessive flowering the following season, and stunting of growth.
To promote more color by advancing fruit maturity, ethephon (Motivate) can be applied 7 to 21 days before expected harvest, depending on cultivar and season of fruit maturity. Follow label instructions carefully. Applications to advance maturity 3 to 5 days can result in smaller fruit size and shorten the storage life of fruit not harvested at proper maturity. Ethephon may not promote color when warm weather persists late in the season. Ethephon may not improve color on poor-coloring varieties and standard strains; it is less effective on interior, shaded fruit. Caution: Ethephon promotes abscission and fruit drop. Use in combination with a preharvest stop-drop spray. Ethephon is not effective for color change on Golden Delicious or advancing maturity of Granny Smith.
NAA (K-Salt Fruit Fix 200, K-Salt Fruit Fix 800, Fruitone, PoMaxa, Refine) may be used to control preharvest drop of apples. NAA does not actually re-tighten the pedicel (fruit stem) after application, but retards the development of the abscission layer between the pedicel and the spur. Experimental evidence shows that NAA sprays are best applied alone and are more effective at dilute concentrations. Application timing of NAA products to control preharvest drop of apples is critical. Generally, NAA should be applied 7 to 14 days prior to planned harvest, but no closer than 2 to 5 days before harvest.
NAA becomes effective for reducing fruit drop 3 to 4 days following application and has an effective period of 2 weeks. NAA has been applied as a stop-drop for apples by aircraft in those cases where it is not possible or desirable to make ground-based applications. By aircraft, the rate used is 0.25 to 0.5 pint of NAA 800 per acre. See manufacturer’s label for specific recommendations as products may differ.
NAA does not completely suppress fruit ethylene production; NAA-treated fruit may show evidence of changes in skin color and/or flesh softening during the interval between application and harvest, even though the typical climacteric ripening response may not be observed and fruit drop is reduced. Growers should frequently monitor both fruit maturation and fruit loosening following NAA application. Careful attention to these possible changes can help growers take advantage of reduced fruit drop while minimizing the risk of losses at harvest and/or of problems after storage.
ReTain. ReTain (aminoethoxyvinylglycine, AVG) is an inhibitor of ethylene biosynthesis in fruit tissues and can be used to adjust harvest timing and control fruit drop. It is registered for use on both apples and pears. Inhibition of ethylene biosynthesis in apples delays maturation and permits fruit to remain on the trees longer for better color and greater size without adverse effects on storage life. For pear growers, ReTain may help maintain fruit firmness for 7–10 days. The preharvest interval (PHI) for ReTain has been set at 7 days before harvest. The manufacturer recommends that ReTain be applied once 4 weeks before the anticipated beginning of normal harvest for that season based on appropriate maturity indices of untreated fruit. If fruit will be harvested using a multiple-pick schedule, ReTain should be applied once at 1-2 weeks before the start of normal harvest of untreated fruit. The recommended application rate for ReTain is 50 grams active ingredient per acre (one 0.73-lb. pouch per acre). Variety-specific rates have not been determined. If weather conditions are not favorable for ReTain application, it is suggested that the product be applied slightly earlier to avoid problems with PHI. Apply ReTain with a registered organosilicone surfactant. Tank-mixes with NAA or ethephon are discouraged because these products may counteract the ethylene inhibition produced by ReTain. Tank mixes with Biobit, DiPel, or XenTari biological insecticides are permitted.
For optimum response, apply ReTain during periods of slow drying conditions to enhance uptake. ReTain should be applied in a sufficient amount of water to ensure thorough wetting of the fruit, but not to runoff. Generally, 100 gallons per acre is adequate for most modern Washington orchards. Adjust water volumes based on tree size, spacing and canopy density. Do not use overhead irrigation or cooling systems for at least 8 hours following a ReTain application.
To minimize foaming of spray mixture, fill spray tank with half the amount of water needed for the final spray volume, add ReTain (in its soluble packaging) and continue to fill tank. Add the surfactant just prior to filling the tank. Minimize agitation of the mixture. Use approved surfactants at a concentration of between 0.05% and 0.1% v/v (0.4-0.8 pint/100 gallons maximum). Compatibility and performance data with anti-foaming agents are not available; such products are not recommended for use with ReTain.