Plant growth regulators

Introduction

What is a plant growth regulator?

Plant growth regulators (PGRs) are chemicals used to modify plant growth such as increasing branching, suppressing shoot growth, increasing return bloom, removing excess fruit, or altering fruit maturity. Numerous factors affect PGR performance including how well the chemical is absorbed by the plant, tree vigour and age, dose, timing, cultivar, and weather conditions before, during, and after application.

Plant growth regulators can be grouped into five classes: compounds related to auxins, gibberellins and inhibitors of gibberellin biosynthesis, cytokinins, abscisic acid and compounds affecting the ethylene status. Products that block the biosynthesis of plant hormones are also available (Apogee, Retain)

The five groups of plant growth regulators used in fruit crops include:

Auxins: These are growth promoting substances that contribute to the elongation of shoots, but at high concentrations they can inhibit growth of lateral buds. In addition to being used as plant growth regulators, auxins can also be herbicides (2, 4-D etc.). In apple production napthaleneacetic acid (NAA) is a synthetic auxin that can be used to thin fruit and prevent fruit drop shortly before harvest. For more information on the use of products for thinning see Ontario.ca/apples and find Thinning of Tree Fruit.

Gibberellins: Gibberellins (GA) promote cell elongation, shoot growth, and are involved in regulating dormancy. Promalin®/Perlan® (containing GA4+7 and 6-benzyladenine) have been used to improve fruit size and reduce russetting in apples. ProGibb 40SB and Falgro (containing GA3) are used to delay ripening, improve fruit firmness and extend the harvest period in sweet cherries. Gibberellins are used in tart cherries to manage flowering to avoid over production. Apogee or Kudos 27.5 WDG® (Prohexidione-calcium) inhibits the biosynthesis of gibberellins. Apogee or Kudos 27.5 WDG are used to modify the morphology of trees (apple and cherries) and to control runner production in strawberries.

Cytokinins: Cytokinins promote cell division. Cytokinins are involved in branching and stimulating bud initiation. They are used as fruit thinners (Maxcel® or Cilis Plus® 6-BA) in apples. For more information on the use of these products for thinning see Ontario.ca/apples and find Thinning of Tree Fruit.

Absicisic Acid: Absicsic acid controls the dormancy of buds and seeds, inhibits shoot growth and is involved in regulating water loss from plants.

Ethylene: Ethylene promotes abscission of leaves and fruits, inhibits shoot elongation and inhibits lateral bud development. In apples and cherries, ethylene is involved in the transition of fruit from being physiologically mature to ripe. Ethephon (Ethrel®) is a synthetic compound that releases ethylene upon application. Retain interferes with ethylene biosynthesis and allows fruit to hang on trees longer and lengthens storage life.

There are a variety of plant growth regulators registered for use in fruit crops in Ontario.

Plant growth regulator use in apples

Improving fruit shape

Promalin® and Perlan™

Promalin and Perlan (a combination of 6-Benzyladenine and Gibberellins A4A7) are plant growth regulators used to improve the shape of apples through elongation and development of more prominent calyx lobes on apples that have a natural typiness like Red Delicious, Golden Delicious, Ambrosia and Gala. Perlan is only registered on Red Delicious. For satisfactory results, time the application carefully, follow label directions and ensure complete spray coverage.

Application:

• For optimum results, the first application of Promalin or Perlan should be applied between early king bloom and early petal fall. Applications earlier or later are likely to produce unsatisfactory response.
• For Promalin, apply a single application of 1.2-2.3 L/ha. Alternatively, if a prolonged bloom occurs, apply two split applications of 0.6-1.2 L/ha, first one between early king bloom to early petal fall to side blooms and the second application to follow 3-21 days later (or when the rest of the canopy is in bloom).
• For Perlan, apply a single application of 1.2-2.3 L/ha. If a prolonged bloom occurs, apply 1.2 L/ha first and reapply 5-7 days later.
• High relative humidity and slow drying conditions favour maximum absorption. It is preferable to apply in the morning or evening.
• Optimal temperatures for application are between 24-32°C.

Precautions: If Promalin or Perlan are applied at higher rates or volumes than those recommended on the label or where blooms are weak or frost-injured, fruit thinning may occur. Promalin or Perlan use may also increase the amount of thinning achieved with subsequent blossom thinning sprays. Apples may not respond to Promalin or Perlan if spur vigour is low or the king blossoms have been damaged by frost. Do not apply Promalin or Perlan if rain is expected within 6 hours. Do not apply when air temperatures are lower than 24°C or greater than 32°C.

Improving Fruit Size

Fruit size can be improved by thinning apples to a desirable crop load through chemical fruitlet thinning and/or hand thinning. The biggest factor for improving fruit size would be early thinning, more information on thinning can be found on the Thinning of Tree Fruit webpage. There are also the following products that can be used additional to thinning fruit to improve fruit size

MaxCel® or Cilis Plus™

MaxCel or Cilis Plus (1.9% or 2.0%, respectively, 6-benzyladenine(6-BA)) improves fruit size by increasing cell division shortly after flowering in the early stages of fruit growth and development.

Application:

• Make 2-4 applications at 10-50 ppm, beginning at petal fall and repeating every 3-10 days. Apply in a spray volume to get complete coverage of leaves and fruit (ie. 1000 L/ha)
• Fruit thinning may occur in some easy-to-thin cultivars and/or if conditions are favourable for thinning. For more information on cultivar specific notes and weather conditions favourable for thinning see the Thinning of Tree Fruit webpage.
• Best results are obtained when maximum temperatures are above 18°C on the day of application and the 2-3 days after application. Avoid spraying when temperatures are above 30°C
• Do not exceed 446 g of 6-BA (22.5 L of MaxCel, 21.3 L of Cilis Plus) per ha for all uses of 6-BA in a growing season.
• Do not apply within 86 days of harvest with MaxCel and within 28 days of harvest with Cilis Plus.

Promalin®

Under favourable weather conditions, Promalin (1.8% 6-Benzyladenine and Gibberellins A4A7) can increase fruit size and weight in most cultivars. For application instructions see Promalin under Improving Fruit Shape.

Reduce russeting in apples

Promalin®

Russeting can occur on susceptible varieties (Golden Delicious) during cool, wet or humid weather at bloom and early stages of fruit development. Promalin can reduce russeting by increasing the epidermal cell density in the skin. Promalin cannot reduce russeting caused by frost damage, disease, herbicide drift, or phytotoxicity.

Application:

• Apply 250-500 mL/ha of Promalin at 7-12 day intervals for a maximum of 4 applications beginning at full bloom to petal fall.
• During conditions that favour russet development (long cold and wet periods during bloom) apply Promalin in shorter intervals and at the higher rate.
• Apply during periods of slow drying conditions to maximize efficacy.

Precautions: Do not apply Promalin when air temperatures are below freezing or greater than 32°C. Rainfall or overhead irrigation within 6 hours after application will often reduce the activity of Promalin. A pH range between 7.0 and 8.5 will provide optimum results.

Modifying tree growth

Promalin®

Feathered nursery trees are important for most high density apple planting systems. Nursery trees should ideally have 10 to 15 feathers per tree to allow for adequate production in the second and third years. Promalin can be used to stimulate lateral bud break and additional branch growth on young trees, resulting in improved branch angles, and providing a better tree structure for early cropping.

Application:

• For nursery stock and non-bearing apples, foliar applications should be made after the trees have reached a terminal height at which lateral branching is desired (eg, 75 cm from the ground).
• For orchard trees, foliar applications are made when new terminal growth is approximately 2.5 to 8 cm long (approximately king bloom to 1 week after petal fall).
• Apply at a rate of 62.5-250 mL Promalin per 10 L of spray solution (125-500 ppm) for nursery and orchard trees.
• Thoroughly soak the area of the tree where branching is desired.
• For optimal results, the pH should be close to 7 and no greater than 8.5.
• A rate of 19 to 38 L of spray mixture applied with a pressurized hand sprayer will treat 200 to 300 non-bearing trees 1 to 4 years old.
• Apply during periods of slow drying conditions to maximize efficacy.
• Promalin can also be mixed with latex paint and applied directly to buds of apples (nursery and orchard). Paint should be applied in the spring when terminal buds have started to swell, but before shoots emerge.
• DO NOT apply Promalin with paint after bud break, as it may result in injury to tender shoot tips and prevent shoot growth from that point.
• The application rate is 100-166 mL Promalin per 500 mL of latex paint. Apply the latex paint mixture with a brush or sponge uniformly covering the bark surface. Apply only to one-year old wood. Notching the bark above the bud with a hacksaw blade prior to treatment will greatly increase efficacy. Make sure to notch wood prior to at least two days of good weather (no rainfall predicted) to allow for adequate healing and to prevent against cankers.

Precautions: Refer to precautions listed above under Reduce russeting in apples.

Vegetative growth control in apples

Apogee® or Kudos® 27.5 WDG

Apogee or Kudos 27.5 WDG (Prohexadione-calcium) are a plant growth regulators that inhibits the synthesis of gibberellins. Trees treated with Apogee or Kudos 27.5 WDG often have the same number of shoots as untreated trees, but shoots from treated trees are thicker or greater in diameter and are shorter from having compressed internodes. Apogee or Kudos 27.5 WDG do not reduce the number of leaves or fruit size. When used properly in apple orchards, Apogee or Kudos 27.5 WDG can:

• Reduce shoot growth by 20-60% and diminish the time required to dormant prune and/or summer prune.
• Lead to improved fruit colour on red-coloured cultivars.
• Result in a more open tree canopy, which will improve spray coverage.
• Reduce the incidence and severity of fire blight on shoots by reducing growth, making trees less susceptible to shoot infections.

Note: Apogee or Kudos 27.5 WDG have no activity against the fire blight bacteria or blossom blight.

Application:

• Patterns of terminal growth, fruit set and response to Apogee or Kudos 27.5 WDG differs between regions. Therefore, the rate and calendar date of application may vary between regions.
• Make the first application at late bloom to early petal fall when terminal shoots (and/or bourse shoots) are no longer than 2.5-7.5 cm. Accurate timing is critical, an early application (2.5-5 cm) is more effective than a later timing.
• Sufficient leaf area must be available for Apogee or Kudos 27.5 WDG to be absorbed into the leaf.
• Apogee or Kudos 27.5 WDG are non-toxic to bees.
• The application rate is determined primarily by tree size, vigour, and whether protection against shoot blight is an objective. Fire blight infections will be suppressed from this point on. Follow the label to adjust rates for tree-row volume dilute applications. Information on rates for sprays applied at 1, 000 L per ha (dilute) can be found in Table 1 Suggested Apogee or Kudos 27.5 WDG Rates (g product per ha)
• Repeat application(s) should be made at 14-21-day intervals, based on the level of growth control required. Apogee or Kudos 27.5 WDG requires about 14 days to slow growth and it breaks down in the trees within a few weeks. Two applications should provide good control. More treatments may be needed in locations with a long growing season, high vigour trees or trees with no fruit. Do not apply more than 4 applications per season.
• Do not apply more than 5.4 kg of Apogee or Kudos 27.5 WDG per hectare per season.
• Do not apply within 45 days of harvest.
• Apogee or Kudos 27.5 WDG have been used effectively when applied in more concentrated sprays, provided thorough wetting of the canopy is achieved. Low-volume spraying of plant growth regulators and chemical thinners is not recommended.
• Apogee or Kudos 27.5 WDG rates need to be adjusted based on orchard and environmental conditions. For additional information see Table 2 Orchard and Environmental Factors to Adjust Apogee or Kudos 27.5 WDG Rates and Number of Sprays.
• Repeat application(s) should be made at 14-21-day intervals, based on the level of growth control required. Apogee requires about 14 days to slow growth. It breaks down in the trees within a few weeks, so two applications may be necessary for season long growth management.

- = No activity
^tSuggested base rate. Move to next higher or lower level based on factors listed in Table 3-21-C. Orchard and Environmental Factors to Adjust Apogee or Kudos 27.5 WDG Rates and Number of Sprays.
² Vigour is defined as the total amount of shoot growth in a single season, not to be confused with tree-row volume.
³ Spray volume can increase or decrease based on the canopy size (see Sprayers101, A Proposal: Crop-Adapted Spraying (CAS) for Apple Orchards). Spray to runoff. Rates need to be increased when higher water volumes are required for adequate spray coverage.
⁴ Maximum seasonal rate should not exceed a total of 5.4 kg of Apogee or Kudos 27.5 WDG .

Precaution: Do not tank-mix Apogee or Kudos 27.5 WDG with calcium sprays like calcium chloride. In the presence of calcium, Apogee or Kudos 27.5 WDG will precipitate in the tank, clog nozzles and screens, and reduce tree response.

In some instances, Apogee or Kudos 27.5 WDG may increase fruit set and make thinning more difficult. This response is not consistent but is more likely at concentrations above 125 ppm (45 g per 100 L). Apogee or Kudos 27.5 WDG-treated trees may require more aggressive thinning to reduce the crop load to the desired level.

Apogee or Kudos 27.5 WDG can cause severe cracking on Empire and Stayman cultivars. The cause is unclear, but may be related to environmental conditions. Apogee or Kudos 27.5 WDG may result in decreased yield and marketable yield of Cortland. Despite this, clear benefits of the use of Apogee or Kudos 27.5 WDG on tip-bearing cultivars such as Cortland and Northern Spy have been observed in areas where the shortened internodes of Apogee or Kudos 27.5 WDG-treated trees have produced a more compact tree habit.

Promoting return bloom on trees for the following season

NAA (Fruitone-L®)

NAA (napthaleneacetic acid) is an auxin plant growth regulator that will effectively promote return bloom when used at the thinning stage in trees that are biennial bearing. For biennial bearing trees, a year of full bloom and heavy fruit set would be considered an "on-year" and an "off-year" is where the bloom and fruit set is light. Application of NAA will be the most beneficial in an "on-year" to promote return bloom for the following season in the "off-year".

Application:

• Apply from full boom to 30 days after full bloom at a rate of 39-310 mL/ 1000 L of water (1.2-9.7 ppm).
• Use larger volumes of water under slow drying conditions.
• NAA is considered to be rainfast after 2-3 hours.
• NAA can be mixed with carbaryl (Sevin XLR).
• The optimum conditions to spray NAA are warm, dull, slow drying conditions which often occur in the morning.
• It is best to apply between 21°C and 24°C. This product should not be applied at temperatures below 15.6°C or above 26.7°C.

Precaution: Do not apply during the same growing season as MaxCel or Cilis Plus as this may result in pygmy fruit in prone cultivars (ie. Fuji, Red Delicious, Golden Delicious and Gala).

Enhancing bloom on non-bearing trees

Ethrel®

Ethrel (ethephon) can be used to enhance bloom on non-bearing trees.

Application:

• Apply a foliar spray of Ethrel to non-bearing apple trees, 1 to 2 weeks after bloom (determined by bearing trees in the area).
• On young orchard trees just beginning to initiate a few flowers, delay applications until 3 to 5 weeks after full bloom to avoid over-thinning and misshapen fruit.
• Trees should be large enough and at a stage to bear fruit the year following application.
• For spur type trees, mix 2 L Ethrel in 1,000 L of water and apply as a dilute spray to the point of runoff.
• For non-spur type trees, use 4.25 L of Ethrel in 1,000 L of water. For concentrate application, apply 7 L Ethrel per hectare for spur types or 14 L Ethrel for non-spur types, in 500 L of water.
• Note that this rate may completely remove any fruit from trees, particularly when applied earlier than 4 weeks after full bloom.

Promoting fruit colouring

Ethrel ®

Ethrel (ethephon) can be used to stimulate ripening and improve fruit colour on apples that will be marketed immediately. Apples treated with Ethrel will not store for long and fruit may drop prematurely.

Application:

• To prevent excessive premature drop after the use of Ethrel, apply a stop-drop product such as naphthaleneacetic acid (NAA).
• Two applications are required: one as a tank-mix at the time Ethrel is applied and another 5 days later. Apply the stop-drop at the label rate.
• Spray just the number of trees that can be harvested over a 2-3-day period.
• Depending on cultivar and air temperature, fruit will be ready to harvest 5-10 days after the spray is applied.
• After a spray, check the maturity of the apples on a daily basis for firmness, starch, flavour and colour.
• A few trees sprayed at 3-4-day intervals are easier to manage than a large number sprayed at one time.
• Make sure a market is available for the treated apples before you spray, especially with early cultivars.
• Rates of application depend on cultivar, date of application, tree vigour, temperature, weather conditions and degree of response required.
• Early cultivars like Jerseymac and Paulared, which ripen under warmer conditions, use 0.75-1.5 L of Ethrel per ha with sufficient water to wet the trees thoroughly.
• McIntosh requires 1.5-4.25 L per ha. Use the higher rate early in the season on trees high in nitrogen or on poorly pruned trees.
• Use lower rates on young, well-pruned trees or on trees low in nitrogen.
• Best results are obtained when Ethrel is applied 2-3 weeks before harvesting.
• Good spray coverage is important.

BlushTM

Blush (Prohydrojasmon) is used to increase anthocyanin production and colour development in bi-colour apples like Fuji, Gala, Honeycrisp and McIntosh. It can also help to overcome colour delay in ReTain treated apples. Blush enhances chlorophyll degradation, as well as anthocyanin and carotene accumulation which contribute to a redder apple. Improvement in colour may not be seen if the strain of the variety is known to produce a good colour, or your orchard gets good colour development due to environmental conditions.

Application: Make 1-2 applications of Blush, 7-28 days before anticipated harvest at a rate of 190-381 mL/100 L of water (100-200 ppm). If making two applications, apply the first application 21 to 28 days before harvest followed by a second application 7-14 days later. The first application should be made once red colour starts to develop. Results are variable and can be impacted by variety and environmental conditions. Do not apply to trees that are injured or under stress from excessive heat, drought, disease or nutrient deficiency. For best results, apply during slow drying conditions, and avoid application during the hottest part of the day.

Chemical control of preharvest drop of apples

Cultivars differ in their tendency to drop apples before harvest. McIntosh and Honeycrisp are the cultivars of greatest concern in Ontario. Conditions associated with premature fruit drop include:

• trees with high nitrogen content in late summer
• trees that carry a heavy crop load,
• hot days and warm nights just before and during harvest,
• trees deficient in either moisture or the nutrients boron or magnesium,
• trees heavily damaged by pests, 5)trees with fruit growing in clusters that will push off large fruit

NAA

Products such as Fruitone-L ® and Fruit Fix Concentrate contain NAA (naphthaleneacetic acid) and are registered to control drop on apples.

Application:

• Apply NAA as soon as the first undamaged apples begin to drop, at the rate of 10 ppm.
• Timing is important. Do not confuse the normal preharvest drop of sound apples with those that are dropping because of insects, disease or nutritional deficiency.
• NAA is effective for 7-10 days after it is sprayed.
• For stop-drop prevention beyond this, repeat the application of 10 ppm 5-6 days after the initial application.
• It takes 1-2 days for NAA to take effect during warm weather and longer during cool weather.
• It also may take longer for NAA to take effect if applied after the orchard has been spot picked. Apply NAA as soon as the spot pick is finished.
• NAA is only locally systemic and must be applied in a dilute spray to achieve more uniform coverage.
• High volumes of water take longer to dry and allow increased absorption of NAA.
• The optimum conditions for maximum absorption are at or near 21-24°C and high humidity.
• Absorption is less on foliage injured by insects, diseases or frost and at temperatures below 16°C.
• The addition of a non-ionic type spreader sticker like Agral 90 improves absorption of NAA under less than ideal weather conditions.

Precaution: NAA inhibits fruit drop but the fruit continues to mature at an accelerated rate. The higher the concentration used and the greater the number of applications, the greater the ripening effect. One application of 10 ppm has little effect on direct ripening.

Apples treated with NAA for preharvest drop will not keep well in long-term storage, especially McIntosh. Apples treated twice with NAA should be sold immediately. Do not apply NAA closer than 5 days before harvest. Do not apply more than 2 times per season.

ReTain ®

ReTain (Aviglycine hydrochloride) is an effective preharvest drop control agent for apples such as Gala, Honeycrisp, McIntosh, Red Delicious and Northern Spy. It reduces the production of ethylene in maturing apples. ReTain reduces preharvest drop, helps to maintain fruit firmness, may reduce incidence and/or severity of watercore and stem bowl cracking, and may enhance storage potential.

Application:

• Apply Retain 4 weeks before the anticipated harvest date.
• ReTain may delay maturity of apples on the tree and allow a longer harvest window for a particular variety. Although ripening in ReTain-treated fruit is temporarily delayed, fruit harvested at proper maturity may have increased soluble solids, colour, fruit size and firmness, and reduced water core.
• To facilitate harvest on larger acreages, consider leaving some blocks untreated for regular harvest timing, and using ReTain to delay maturity on later blocks.
• Pick fruits treated with ReTain at the same internal maturity, based on starch-iodine tests, as untreated fruits.
• There may be a 7-10-day delay from predicted harvest date of untreated fruit to the start of the harvest of ReTain-treated fruit.
• Use a 333 g pouch (50 g active ingredient) of ReTain per 0.4 ha (1 acre). Use with Xiameter OFX-0309 (organosilicone surfactant) at 0.5-1 L/ 1,000 L of water.
• For a single pick harvest-apply 4 weeks before anticipated harvest, adjusted for seasonal variation. It is better to be a week early than a week late, as late applications will not perform well because ethylene production within the plant may have already begun and this is difficult to suppress once initiated.
• For a multiple pick harvest-apply 1-2 weeks before the anticipated harvest of the first pick. ReTain will not delay the 1st pick but will delay the maturity of the later picks.

Precautions: Do not apply a sun protectant product 3 days before or after a ReTain application. When you mix ReTain with the surfactant, keep agitation to a minimum because Xiameter OFX-0309 has a tendency to foam. To minimize foam, add Xiameter OFX-0309 last, when the tank is full. Compatibility and performance data for ReTain with antifoam products are not available. Do not use surfactants other than Xiameter OFX-0309 because they are not registered with ReTain in Canada. If the rate of ReTain is reduced, maintain the Xiameter rate at a 0.05-0.1% v/v ratio (5-10 L/ 1,000 L water).

Discard any unused spray material at the end of each day.

Do not tank-mix with other products as combination effects and potential interactions between ReTain and NAA or ethephon products have not been thoroughly evaluated.Use of these products on ReTain-treated blocks may negate or reduce the benefits of ReTain.

Do not use overhead irrigation for 8 hours after application.

Adjust spray water volumes based on tree size and spacing. Do not apply to the point of runoff but wet the tree under slow drying conditions.

Use tree-row volume (TRV) reductions or Crop Adapted Spraying cautiously and test on a few rows or small blocks to gain a level of comfort with this technique. For many orchards, 1,000 L water per ha is adequate for size-controlling rootstocks.

For best results, have spray water at a pH between 6 and 8.

To maximize the benefits of using ReTain, segregate treated fruit from untreated fruit in storage to reduce exposure to ethylene.

Only spray trees that are healthy and not under severe drought, insect, disease or nutritional stress. Apply treatments to Gala strains, Golden Delicious types and Honeycrisp with caution. These cultivars appear to be more responsive to ReTain compared to McIntosh. Using ReTain on these sensitive cultivars can significantly impede crop maturity and development.

The harvest window for treated cultivars is shorter but delayed and results in fewer pickings. This can increase harvest efficiency. ReTain is not likely to compensate for the effects of an aggressive calcium spray program to control bitter pit. Calcium sprays can advance apple maturity by accelerating ethylene production in maturing fruit.

HarvistaTM

Harvista (1-Methylcyclopropene or 1-MCP) inhibits sensitivity to ethylene which slows down ripening. The active ingredient, 1-MCP, is also in SmartFresh which is applied post-harvest to apples in a sealed storage room to improve storage performance. Harvista can delay harvest 7-14 days from the anticipated harvest date. It may also reduce preharvest drop, reduce fruit ethylene production, allow for additional time to develop colour and increase size, maintain fruit firmness, delay starch hydrolysis, delay onset or reduce incidence of watercore and enhance storage potential. It can impact or delay red colour development on bi-colour apples.

Application:

• Apply 5.9-17.7 L/ha of product 3-21 days before estimated harvest. Ontario research has shown that applying as close as possible to the estimated harvest date has given better results.
• Use a higher rate for fruit at a more advanced stage of maturity.
• Use a lower rate for bi-coloured apples as Harvista can impact or delay red colour development.
• In-line chemical injector systems are needed to apply the product.
• Xiameter OFX-0309 Fluid Silicone Surfactant may be added to water at 0.05% v/v.

Precaution: Do not apply when temperatures are above 35?C. Do not allow the product to come into contact with copper.

Plant growth regulator use in pears

Modifying tree growth

Promalin®

See Promalin for modifying tree growth under Plant growth regulators for apples

Application:

• For nursery stock and non-bearing pears, foliar applications should be made after the trees have reached a terminal height at which lateral branching is desired (e.g. 75 cm from the ground).
• For non-bearing orchard trees, foliar applications are made when new terminal growth is approximately 2.5-8 cm long (approximately king bloom to 1 week after petal fall).
• Apply at a rate of 125-500 mL Promalin per 10 L of spray solution (250-1000 ppm) for non-bearing nursery and orchard trees.

Precaution: Promalin mixed with latex paint and applied directly to buds is not registered for pears and only registered for apples (nursery and orchard) and sweet cherries (orchard only). Refer to precautions listed under reducing russeting in apples.

Plant growth regulator use in berries

Runner suppression in strawberries

Apogee® or Kudos 27.5 WDG®

Strawberry plants treated with Apogee or Kudos 27.5 WDG (Prohexadione-calcium) have significantly shorter runners than untreated plants. Apogee or Kudos 27.5 WDG arerecommended when runners are no longer needed to increase plant density.

Application:

• Apply prior to the beginning of runner initiation.
• Make subsequent applications at 14-21-day intervals, up to a maximum of three applications per season, if required.
• In day-neutral production systems, plant as early as possible in spring, and apply Apogee or Kudos 27.5 WDG after the first flower trusses are removed but before runners develop.
• In June-bearing varieties grown on plastic mulch, use Apogeeor Kudos 27.5 WDG after planting in the year before harvest.
• Avoid use of Apogee or Kudos 27.5 WDG when buds or blooms are present. Apogee can reduce flower truss length and make harvest more difficult.
• Do not apply later than 21 days prior to harvest.
• The effects of Apogee or Kudos 27.5 WDG last for two to three weeks after application. Afterward, the plant produces gibberellin normally and typical growth continues.
• Use 45 g/100 L water (Do not exceed 135 g/ha of Apogee or Kudos 27.5 WDG ).
• Use adequate water to insure thorough coverage.
• Always tank-mix Apogee or Kudos 27.5 WDG with Agral 90 at a rate of 50 mL/100 L of water. Agral 90 improves uptake of Apogee or Kudos 27.5 WDG into the plant.

Precaution: Apogee or Kudos 27.5 WDG are relatively new tools for Ontario growers. Test Apogee or Kudos 27.5 WDG on a small scale and leave untreated checks in order to evaluate the effectiveness on different varieties and production systems.

Always tank-mix Apogee or Kudos 27.5 WDG with an equal amount of ammonium sulphate by weight. Ammonium sulphate (AMS) can improve efficacy as Apogee or Kudos 27.5 WDG are impacted by hard water. Use a high-quality, greenhouse grade AMS to avoid plugging of nozzles. Do not tank-mix Apogee with calcium sprays like calcium chloride, which will reduce Apogee or Kudos 27.5 WDG effectiveness.

Plant growth regulator use in sweet cherry

Fruit quality

ProGibb® 40 SG and Falgro®

ProGibb 40 SG and Falgro (Gibberellic acid) delays fruit ripening 4-5 days and thus extends the picking period, which may delay the susceptibility to rain cracks. The treatment also increases fruit size, firmness and resistance to postharvest disorders.

Application:

• Dissolve 20 Falgrotablets in 1,000 L of water to produce a 20 ppm solution and apply as a foliar spray to runoff.
• Apply 21 days before normal harvest when the fruit is at the straw-coloured stage.
• Harvest when fruits are at the desired shade of red but not within 21 days of application.

Vegetative growth control

Apogee® or Kudos 27.5 WDG®

Apogee or Kudos 27.5 WDG (Prohexidione-calcium) can be used to reduce terminal growth in sweet cherries. Reduction in terminal growth should help reduce the time required to dormant prune as well as open up the tree canopy, leading to improved spray coverage and reduced disease pressure. For sweet cherries, growers can expect a significant reduction in terminal shoot growth - ranging from 20-50% in a given season by using Apogee or Kudos 27.5 WDG at the proper timing and concentration.

Application:

• Terminal shoot growth in Ontario proceeds rapidly usually during the first 6 weeks of the season.
• Since 2 weeks are required for Apogee or Kudos 27.5 WDG to slow growth effectively, it is essential to make the first application when terminal shoots are no longer than 2.5-5 cm. This typically coincides with late bloom, when sufficient leaf area has developed for Apogee or Kudos 27.5 WDG to be translocated absorbed into the leaf.
• Apogee or Kudos 27.5 WDG reportedly have no detrimental effects on bees, so the first application can be made before bees are removed from the orchard.
• Apply 45 g/100 L of dilute spray (which equates to 125 ppm or 125 mg/L of active ingredient) and just 'spray' dilute (up to 3000 L/ha). A repeat application must be made 14-21 days later.
• Sprays are limited to a maximum of 2 sprays per season and a total of 2.7 kg of formulated product per ha per season.
• Based on the 125 ppm rate and a tree-row volume dilute rate of 2,000 L/ha, 1.8 kg of product per hectare (728 g/acre) will be required.
• Calcium sprays should not be tank-mixed with Apogee or Kudos 27.5 WDG .

Precautions: A spray adjuvant (Agral 90, LI-700) should be included to improve plant uptake of Apogee or Kudos 27.5 WDG . In addition, where a high calcium or magnesium water source (hard water) is used, it is important to include an equal amount of ammonium sulphate (AMS) fertilizer by weight with Apogee or Kudos 27.5 WDG . Effects of Apogee or Kudos 27.5 WDG on fruit set, fruit size and yield are inconsistent based on results reported in the literature. Several studies have reported a resurgence in late-season growth in some situations when marked suppression of growth was achieved early in the season. Furthermore, some resurgence in growth the year following application has been observed. Therefore, fine-tuning of treatment timings and concentrations may need to be considered.

Growth regulators for tart cherries

Fruit ripening

Ethrel®

Ethephon (Ethrel) can be used in tart cherry orchards to promote fruit loosening, uniform maturity and to facilitate mechanical harvest.

Application:

• Apply Ethrel at a rate of 2.75 L/ha (applied in approx. 2,400 L of water per ha).
• Apply when fruit are enlarging rapidly, with the grass-green colour beginning to turn yellow or developing a tinge of red. This generally coincides with 7-14 days before anticipated harvest. This rate helps to loosen fruit to facilitate mechanical harvesting.
• Consult the product manufacturer for further information.
• Application of Ethrel in concentrate sprays (i.e., 1,000 L of water per ha or less) achieves the same level of loosening as dilute applications. Uniform coverage is important.
• Tank-mixing: There is little or no data regarding tank-mixing Ethrel. Do not tank-mix with foliar nutrients or compounds such as fruit-cracking inhibitors, etc. While no problems have been reported by growers for tank-mixing Ethrel with the fungicides and insecticides commonly used at this time, it is possible these materials may act as a buffer to the Ethrel and thereby alter activity.

Precaution: Trees respond more quickly to Ethrel applications in higher temperatures. For this reason, apply Ethrel only in the temperature range of 18-30ºC. Tree vigour also affects the effectiveness of Ethrel treatment. Do not spray trees that have low vigour or are severely stressed by drought, disease or winter injury, which is indicated by gumming on the trunk and scaffold limbs. Treat only trees that are vigorous and in good health.

Moderating early production

ProGibb® 40 SB and Falgro®

ProGibb 40 SB and Falgro (Gibberellic acid) are applied in the fourth year to moderate early production. This allows flowering in year 5 at a reasonable level rather than allowing heavy bloom and production. Overproduction can significantly reduce growth in future years.
For mature Montmorency tart cherry trees infected with cherry yellows virus, apply GA annually. This helps maintain and extend high fruiting capacity and reduces occurrence of blind nodes through the stimulation of lateral shoots and spurs.

Application:

• Apply about 3 weeks after full bloom, from shuck fall to 2 weeks after shuck fall.
• Use concentrations of 10-15 ppm (15 ppm is most common). Use lower rates on more vigorous trees. Rates can vary depending on the age and vigour of the tree.
• Gibberellic acid should not be applied to stressed trees.
• Apply the product as a fine mist.
• Maintain a neutral pH, below 8.
• Apply during slow drying conditions.
• For specific application instructions, which include rate and use of spreading agents, consult the product label.

Precaution: Do not harvest within 21 days of application.

Vegetative growth control

Apogee® or Kudos 27.5 WDG®

Apogee or Kudos 27.5 WDG (Prohexidione-calcium) can be used to reduce terminal growth in tart cherries. Reduction in terminal growth should help reduce the time required to dormant prune, as well as open up the tree canopy leading to improved spray coverage and reduced disease pressure. Limited research is available on the response of Montmorency tart cherries to Apogee or Kudos 27.5 WDG .

Application:

• Terminal shoot growth in Ontario proceeds rapidly usually during the first 6 weeks of the season. Since 2 weeks are required for Apogee or Kudos 27.5 WDG to slow growth effectively, it is essential to make the first application when terminal shoots are no longer than 2.5-5 cm. This typically coincides with late bloom, when sufficient leaf area has developed for Apogee or Kudos 27.5 WDG to be translocated into the leaf.
• It reportedly has no detrimental effects on bees, so the first application can be made before bees are removed from the orchard.
• Apply Apogee or Kudos 27.5 WDG at 1,350 g/ha when new terminal growth is 5-20 cm.
• If required, make a second application 14-21 days later (a maximum of 2 applications per season are permitted).

Precautions: Do not harvest within 20 days of application. Calcium sprays should not be tank-mixed with Apogee or Kudos 27.5 WDG . Refer to precautions listed under vegetative control of tart cherries.

Conclusion

Plant growth regulators play an important role in the production of high quality trees and fruit. Read the product label and follow all safety precautions. For additional information on the timing and use of plant growth regulators refer to the crop calendars in OMAFRA Publication 360 Guide to Fruit Production.

This information was written by Kathryn Carter and Erica Pate, Fruit Specialists, OMAFRA; and Amanda Green, Tree Fruit Specialist, OMAFRA.

Thanks to Dr. John Cline, University of Guelph, Simcoe, Leslie Huffman and Margaret Appleby (formerly of OMAFRA), and Dr. Julia Reekie, Agriculture and Agri-Food Canada-Kentville for their collaboration.