CHEMICAL THINNING
Chemical thinning is an established and essential practice performed by fruit growers each spring. Because many factors may influence the response to chemical thinners, there are always questions and concerns about chemical thinning as the season approaches. The purpose of this section is to summarize thinning recommendations and to discuss conditions that may influence your chemical thinning program. Consult your state Extension specialist if you have additional questions about thinning. As always, follow the label to assure proper use of thinning chemicals.
Thinning is done to increase fruit size and enhance repeat bloom. The more fruit that are allowed to develop on a tree the smaller each fruit will be. Thinning to a fruit density of less than 25 fruits per inch of limb circumference within 4 weeks of bloom should insure an adequate repeat bloom. Thinning to a fruit density of 13 to 15 fruits per inch of limb circumference will enhance repeat bloom and increase fruit size. It should be noted that these larger fruit are more susceptible to storage problems, so it is critical to follow a calcium management program that includes foliar calcium sprays to maintain optimum storage potential of these larger fruits.
Growers should consider thinning at least all mature bearing trees. Trees that carried a light crop last year will have a heavy bloom and will require extensive thinning this spring. Furthermore, trees with a light bloom will set a higher percentage of flowers than those with a heavy bloom even when thinned with a moderate rate of chemical thinner.
WEATHER FACTORS. Applications made at air temperatures of less than 70oF have usually been ineffective. Optimum temperature is 70-80oF. Above 80oF, the effectiveness of the thinners is increased and lower rates should be used. High humidity also favors chemical absorption. Reduce rates if slow drying conditions prevail during application. Wind during application reduces effectiveness by shortening the time for absorption of chemical into the leaf. Leaves that develop during predominately cool, cloudy weather are more easily penetrated by thinning chemicals. Warm, sunny conditions favor development of a waxy leaf surface that restricts penetration of thinning chemicals. One should consider the average temperature and humidity during the week prior to spraying, when choosing the rate of thinning chemicals to be used.
Trees that have sustained bud damage due to freezing temperatures are more easily thinned, and should probably be given no more than the minimum suggested rate of thinner. Note the condition of the spur leaves and use caution if they appear crinkled and light in color.
Special attention should be given to those low areas that may be frost pockets. In years when blossoms may have been frost damaged or when pollination may have been inadequate due to cold rainy weather during bloom, check the number of seeds in the largest fruits. If the number of healthy developing seeds per fruit averages 3 or fewer, then chemical thinners should be applied at the minimum rate or not at all. Presumably, the longer the interval between a frost and the application of a chemical thinner, the more normal should be the response.
TREE AGE AND VIGOR. Young trees that are growing vigorously are more easily thinned than older, slower growing trees. It is probably better to hand thin very young trees that overset. In this way the leader may be completely defruited to keep it upright, while main laterals are allowed to spread with the weight of some fruit. If no crop is preferred, a combination of NAA and Sevin (carbaryl), at the maximum suggested rates for the cultivar, should remove most of the fruit if applied at petal fall.
Fruit on wood of low vigor due to shading, excessive crop in the previous year, or inadequate nitrogen are especially subject to chemical thinning. Whole trees in a state of low vigor should receive little or no chemical thinner. In healthy trees, the well exposed tops will be most difficult to thin. Presumably, a summer or fall mite infestation which was severe enough to cause foliage bronzing would reduce the vigor of the tree sufficiently to warrant lower rates of chemical thinner the following spring. Following an "off" year of very low crop, thinning is more difficult and the higher dosage is needed.
Thinning response to the various thinning chemicals will vary from farm to farm and among blocks on a specific farm. Growers should develop a thinning history for each block, keeping detailed records of bloom, weather conditions, tree vigor and age, bee activity, thinning chemicals, timing and rates used, and fruit set. This data, collected over several years, will help fine tune a chemical thinning program that reduces the risk associated with both under and over thinning.
WETTING AGENTS. Additives such as Tween 20, X-77, etc., will increase the uptake of thinning chemicals. The rates suggested in this publication are for use without additives or additional pesticides. If a spreader-sticker is used, adjust the rate downward, especially when using NAA or NAD.
CONCENTRATION. Concentrate spraying of thinners is not recommended because of the lack of uniformity in response throughout the tree, and because of the increased chance for error. Rates of Sevin 50% WP in excess of 1 lb./100 gals. do not result in substantially more thinning than the 1 lb. rate, but when using NAA or NAD, thinning (and injury) increases with increasing amount applied.
NOTES ON THINNING MATERIALS
CARBARYL insecticide (Sevin) is considered to be the least risky chemical in terms of overthinning. Carbaryl is a selective thinner in that it thins the weakest lateral fruitlets, thus reducing the number of fruits per cluster. It has been shown to be effective in increasing fruit size but has no hormonal effect in increasing return bloom. Apply a single spray of 1/2 to 1 pound of carbaryl 50 WP/100 gals. dilute (0.3 to 0.6 lbs. carbaryl 80) 7 to 28 days after full bloom. The label for carbaryl permits application at up to 2 lbs./100 gals. for insect control, however using rates over 1 lb./100 gals. will not result in increased thinning activity. If increased thinning activity is desired, it can best be achieved by using another chemical thinner in combination with carbaryl. Carbaryl is toxic to bees and to other beneficial insects and this response is dosage dependant. Do not apply during bloom. Restricted entry interval is 12 hours.
NAA. Naphthaleneacetic acid (NAA) is a powerful fruit thinning agent. There is a risk of overthinning, especially when temperatures over 80oF occur following application. The risk of overthinning can be reduced by applying a reduced concentration of NAA in combination with carbaryl. Use of NAA may not always result in increased fruit size, especially if the NAA is applied too late. Use of NAA hormonally stimulates return bloom over and above the increase in return bloom expected from preventing overcropping.
NAA should be applied at concentrations of 2.5 to 20 ppm, depending upon the cultivar to be thinned and whether or not it is used in combination with carbaryl. See Table 22 for specific recommendations. NAA sprays have traditionally been applied when the fruitlets are 10-12mm in diameter, which usually occurs by 14 to 21 days after full bloom. Growers may want to try earlier timings if previous applications of NAA at 10-12mm fruit diameter have not increased fruit size. The rate of fruit growth varies depending upon the weather, therefore fruit measurement is the best method to determine when to apply NAA for consistent results.
NAA can result in foliar injury and excessive thinning when applied at petal fall to summer cultivars. It has no effect on beneficial insects. Do not use NAA and BA (Accel) on Delicious or Fuji in the same season, as pygmy fruit may result. Also, do not use NAA and Promalin on Delicious or Fuji in the same season as this combination also may cause development of pygmy fruits. NAA is not labeled for use in excess of 20 ppm. Restricted entry interval is 24 hours for K-Salt Fruit Fix, 48 hours for Fruitone-N.
NAD. Napthaleneacetimide is a less potent form of NAA. It frequently is used in situations where foliar damage caused by NAA is a problem, especially for summer cultivars. NAD is applied at 25 to 50 ppm at petal fall, or in a post-bloom spray when the fruitlets are 10-12mm in diameter. NAD may be combined with carbaryl, in place of NAA. It is never used on Delicious since small seedless fruit (pygmies) will result and persist until harvest.
Not labeled for use beyond 2 1/2 weeks after full bloom; no more than 50 ppm in a single spray each year. Restricted entry interval is 48 hours.
BENZYLADENINE (BA) is marketed under the trade name Accel. While Accel is not a strong thinner, it can promote increased fruit size and return bloom. To insure adequate thinning, try the following sequence of thinning sprays.
• At petal fall, apply carbaryl.
• When fruit are between 5 and 15mm diameter, apply Accel at 30 grams active ingredient (a.i.) per acre. If fruit set appears heavy, include carbaryl in this second thinning spray.
Warm weather is required for Accel to work effectively. (As a general rule, if the temperature is below 70oF, put the Accel back on the shelf. Temperature of 80-85oF is ideal, resulting in the best thinning result.) Ideally, a 3-day window of appropriate weather is needed if Accel is to work best. Follow weather forecasts and apply Accel when the first ideal or nearly ideal 3-day window within this 5 to 15mm fruit diameter window is expected.
Accel can be tried on a number of cultivars; however, it has worked poorly with Gala. Use the 30 gram a.i. rate except for blocks with a low tree row canopy volume. Do not use Accel and NAA on Delicious or Fuji in the same season as pygmy fruits may result. The restricted entry interval for Accel is 12 hours.
On to Reduction in Russeting of Golden Delicious