Growth Regulator Use In Apples

Chemical Thinning

Fruit thinning is a management practice that reduces yield in the current season but results in increased fruit size and also increased return bloom and yield in the next season. Large fruit size is best obtained with consistent cropload reductions each year through chemical thinning. The use of growth regulating chemicals to thin apple trees is not an exact science and each grower must weigh and evaluate the many factors that affect chemical thinning response in deciding on a thinning program. Although the recommendations in this section are based on research and experience, growers are cautioned that their success with chemical thinning depends on many factors and they should use these recommendations only as a guide.

Weather Factors That Affect Thinning Response

Frost. Frost before application of thinners can greatly increase the amount of thinning obtained from chemical thinners. Frost at bloom can damage fruitlets and reduce seed set, which can result in increased natural drop and greater chemical thinning response. Frost can also damage spur leaves, resulting in greater chemical uptake and thus greater thinning response. Wherever flowers and leaves have been damaged by frost, extreme caution should be used with chemical thinners. Typically, lower rates would be used in such cases.

Sunlight Levels before Application. The amount of sunlight for the 3–5 days preceding application of chemical thinners has an important effect on chemical uptake and response. Intense cloudy weather before application of thinners can result in increased chemical uptake and greater thinning response, due to greater succulence of the leaves and a thin wax cuticle. In addition, intense cloudy weather results in reduced carbohydrate supply for fruit growth and reduced fruit growth rate. This results in increased natural drop and greater chemical thinning response. Growers should reduce the rate of thinner used, if intense cloudy weather precedes application.

Temperature at Time of Application. The uptake of chemical thinners is greater at higher temperatures than at lower temperatures. The optimum is between 70–80° F. Above 80° F, uptake is substantially greater than below 80° F. The time of day applications are made appears to be unimportant. Applications made in the morning or evening when it is cool have a longer drying time on the leaf, resulting in a slow but sustained uptake of chemical, while at higher temperatures during mid-day, drying times are shorter, resulting in a short but rapid uptake of chemical. Thus, the total amount of chemical taken into the plant appears to be very similar regardless of the time of day. Recent research results also indicate that similar thinning is achieved regardless of the time of day applications are made.

Weather After Application. Temperature and sunlight levels for the 5-day period after application of thinners are the predominant weather factors affecting chemical thinning response. The interaction of temperature and sunlight with the chemical thinner creates stress in the tree, which is necessary to make some fruit drop off. Warmer temperatures increase thinning response, while greater light levels decrease thinning response (Fig. 15).

Figure 15. The interaction of temperature and sunlight intensity on thinner action.

Night temperatures are also an important factor to consider. Warm night temperatures greater than 60° F give greater thinning response. With high night temperatures, fruits use up the carbohydrates that were produced during the day at a fast rate, resulting in a deficit of resources for fruit growth and causing the weakest fruits to drop. The greatest thinning can result if warm night temperatures are combined with intense cloudy/warm daytime weather. Under these conditions, the tree produces little reserves during the day and at night the fruits use up all of the reserves produced during the day, making the fruits very susceptible to the stress caused by chemical thinners. Under these conditions, excessive fruit drop can result. The least effective thinning is achieved when bright, warm daytime weather is accompanied by low night temperatures. Under these conditions, the tree produces large amounts of carbohydrates during the day and the fruits use them up at a slow rate during the night. Under these conditions, there is little stress created by chemical thinners and the thinning response is poor. At the time of application of thinnes, growers should critically examine the weather forecast for the upcoming 3–7-day period and adjust rates up or down 50% based on forecasted temperatures and sunlight levels.

Tree Factors That Affect Thinning Response

Pollination. Poor cross-pollination results in low viable seed number per fruit, greater post-bloom fruit drop and greater sensitivity to chemical thinners. In contrast, high seed numbers per fruit result in more difficult-to-thin conditions. In general, if seed numbers are less than 5, thinning rates should be reduced.

Initial Cropload (Fruit Set). A high initial cropload usually results in a relatively high final cropload, regardless of chemical thinning program. Therefore, to achieve a given cropload each year, the initial cropload must be considered when determining the aggressiveness of the thinning program. Growers should use a more aggressive thinning program when initial fruit set is high and a less aggressive thinning program when initial fruit set is lower.

Fruit Size at Time of Application. Fruitlets are more sensitive to NAA and BA at 10-12mm fruit diameter than at smaller or larger sizes. In warm years, when fruit growth rate is rapid, chemical thinners should be applied slightly before fruits reach 10 mm diameter (8-10 mm). In cool years, when fruit growth rate is slow, the application of chemical thinners should be delayed until fruits are 12-15 mm in diameter. Growers should attempt to time chemical thinner application according to a suitable weather window within the preferred fruit size range.

Sensitivity of the Tree. The internal physiological status of the tree determines its sensitivity to chemical thinners. Growers should use a less aggressive thinning program under conditions when tree carbohydrate supply for the fruitlets is expected to be low, and a more aggressive thinning program when tree carbohydrate supply is expected to be high. The carbohydrate supply available to the fruitlets is a function of temperature and sunlight which affect photosynthesis (supply) and respiration (demand) of fruits and vegetative organs. The carbohydrate supply at the time of thinning is difficult to estimate without the aid of a computer model (developed by Alan Lakso) to calculate carbohydrate supply and demand. In addition the sensitivity of a tree is increased by: 1) heavy croploads the previous year. 2) cloudy weather prior to and after application of chemical thinners. 3) heavy insect and disease damage to foliage during the previous season. 4) severe winter temperatures that damage vascular tissues necessary for the transport of reserves from the root to the top in the spring. 5) warm temperatures in late winter and early spring (Feb. 15–April 15), which cause the tree to use its carbohydrate reserves before bloom.

Chemicals Registered For Thinning In New York

Naphthaleneacetic acid (NAA) is an auxin-type growth regulator that induces fruit thinning at rates from 2.5–15 ppm depending on variety. NAA has some thinning activity from full bloom until fruits are 15 mm in diameter, with the optimum thinning activity when fruit diameter is between 10-12mm fruit size. It is sold as either the Sodium salt (Fruitone-N and Fruitone-L) or the Potassium salt (K-Salt Fruit Fix-200 and Fruit Fix-800). The four formulations give very similar thinning responses if used at the same rate of NAA. NAA stimulates ethylene production in the tree and at high concentrations also inhibits photosynthesis and fruit growth rate for a period of 7–10 days after application. The inhibition of fruit growth rate results in abscission of the weaker fruit on the tree. At early timings such as full bloom or petal fall, there appears to be little negative impact on fruit growth rate from NAA, which results in more modest thinning than at later timings (10-12mm). In some years and with some varieties like Empire, the temporary inhibition of fruit growth caused by NAA results in little gain in final fruit size at harvest. This negative side effect is most common if NAA is applied at rates greater than 10 ppm and at fruit sizes larger than 8 mm. High rates of NAA should be avoided on small fruited varieties. High rates of NAA may also cause pygmy fruit with Delicious and Fuji.

Naphthaleneacetamide (NAD or NAAm) is an amide form of NAA but has much lower activity than NAA. As a consequence, it is a mild but safe thinner that is used at rates from 25–50 ppm. Late timings result in pygmy fruit with Delicious, and are ineffective with other varieties. It is often used at petal fall on early ripening varieties and on certain hard-to-thin varieties such as Macoun.

6-Benzyl Adenine (BA) is a cytokinin-type growth regulator that induces fruit thinning at rates from 35–150 ppm. BA can be used from petal fall to 15 mm fruit size, but the thinning response is poor at either end of that window. The best response is when fruits are 10–12 mm in diameter. It is most effective when temperatures are warm (70°F) for a 3–5-day period after application. It is sold in three formulations as either Maxcel (1.9% BA), or RiteWay (1.9% BA) or Exilis Plus (2.0% BA). The primary advantage of BA is that it results in larger fruit size than with other thinners due to a stimulation of cell division. The primary disadvantage of BA is that it often does not thin adequately by itself. However, when combined with Carbaryl it has performed satisfactorily. In some cases, the use of BA alone has resulted in significant fruit size improvement even though there was little thinning.

Carbaryl is a carbamate insecticide that also has moderate thinning action. It is relatively safe and has the added advantage of having good insecticidal properties on leafhoppers and plum curculio. It is relatively rate-insensitive, with similar thinning response from 0.25 lb up to 1.0 lb A.I./100 gallons. One of its best features is that it selectively removes the weaker fruits within the cluster, leaving predominantly one fruit per cluster. Carbaryl has been shown to enhance the effectiveness of NAA or BA when used in a tank mix. Currently it is most commonly used in combination with NAA or BA. Recent research indicates that the major mite predator mite in N.Y. (Typhlodromus pyri) has developed resistance to carbaryl. Thus, Carbaryl can be used in N.Y. without disrupting biological mite control programs. Carbaryl is very toxic to bees and the wettable powder particles of Carbaryl, which are similar in size to pollen grains, can be picked up by bees and carried back to the hive. The liquid formulations of carbaryl are not picked up as easily by bees, so their use is much safer. The liquid formulations have made it possible to apply Carbaryl at a wide range of timings from petal fall to 20 mm fruit size. The liquid formulations of carbaryl have significant amounts of added surfactants and thus have greater thinning activity than the wettable powder formulations. Under cloudy, rainy weather conditions, the liquid formulations may cause fruit skin damage, especially when foliar nutrients or captan are included in the thinning spray. With cloudy, rainy weather, we recommend the wettable powder formulations. With bright sunshine, we recommend the liquid formulations.

Vydate is a broad spectrum carbamate insecticide that also has moderate thinning activity. It is similar to carbaryl in thinning action and is used from 0.25 to 1.0 lb A.I./100 gallons. It is not commonly used in N.Y. for thinning since it is reported to be more toxic to predator mites. However, in pest control programs that do not attempt to conserve predatory mites, Vydate can be useful as both a thinning agent and a broad spectrum insecticide. Like carbaryl, it is usually combined with NAA and BA for greater thinning.

Ethephon (Ethrel) is a growth regulator that stimulates ethylene production by the plant. It can be used to thin apple trees from full bloom to 20 mm fruit size. In N.Y., it has given unpredictable thinning action. In some cases it has defruited the trees. Nevertheless, it does have the advantage that it will thin large fruit (up to 20 mm). Ethrel also has a significant positive effect on return bloom in addition to the thinning effect. Ethrel can also be used as a return bloom enhancer after the thinning period is over. For this use, it is applied at low rates after the window for thinning has passed (usually 4-6 weeks after full bloom).

Chemicals Not Registered For Thinning That Influence Cropload

Lime sulfur is a foliar fungicide that, if used during bloom or during the early post-bloom period at rates of 2.5–3 gal/100 gal, will cause significant thinning. Lime sulfur in combination with oil or fish oil is used increasingly in organic apple production systems. Growers who use lime sulfur should account for the thinning action of this material when they develop their thinning programs.

Ammonium Thiosulfate (ATS) is a foliar nitrogen fertilizer that, if used during bloom at rates of 2–4 gal/100 gal, will cause significant thinning. Growers who use ATS should account for the thinning action of this material when they develop their thinning programs.

Oil or Fish oil are foliar insecticides that also significantly enhance chemical thinner response. Combinations of lime sulfur and oil (2%) or carbaryl and oil (0.25%) or BA and carbaryl and oil (0.25%) give greater thinning than either product alone.

Spray Timings

Chemical thinning can be done at various times depending on the chemical used, beginning with full bloom and ending when fruits have reached 20 mm in diameter. The following four timing windows during the growing season should be considered when applying thinners.

50-80% Bloom. Bloom thinning can be done with caustic thinning chemicals such as ATS or with hormone-type thinners such as NAA. The timing window with caustic thinners is very narrow (1–2 days) since the goal is to allow the king bloom to be pollinated and then apply the chemical to prevent further pollination of other flowers. Thinning response with the caustic blossom thinners is not weather-dependent, but fruit skin injury can occur with high rates and slow drying conditions. Use of NAA at full bloom generally gives a moderate thinning response.

Petal Fall (1 week after full bloom). Thinning at petal fall has the advantage of allowing some assessment of pollination before making the decision about aggressiveness of thinning. As with bloom thinning, the objective is to remove a portion of the crop before competition between fruits reduces fruit size. In addition, after petals have fallen and bee hives have been

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