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Update On Apple Fruit Thinning and Calcium Applications

As of mid April 1996, apples were late bloom to early fruit set in central counties and mostly mid bloom to petal fall in northern counties. Based on current weather patterns a similar flowering scenario can probably be expected this spring. Proper timing of chemical thinning sprays is crucial for best removal. It is important that full bloom dates are recorded because optimum thinning will occur between 7 to 21 days after full bloom. Proper spray coverage is also essential when applying thinning sprays. For optimum thinning about 80 to 90% of the spray should be directed to the top third of the tree.

There are a number of newer chemicals being evaluated for thinning apples by workers across the U.S. However, it will require several years before some of these materials gain widespread use. Accel is the latest chemical thinner cleared for use on apples, and caution is recommended until you gain experience using this material. This basically means the common thinners you have been using (NAA and Sevin) are readily available and will be the materials of choice. Growers in most apple producing states are still trying to determine which thinning chemicals will perform best for them on newer varieties such as Gala, Braeburn and Fuji. If you haven't evaluated the use of chemical thinners on these varieties it would be wise to apply materials such as Sevin and NAA on small acreage before making widespread applications.

As noted above, NAA (naphthaleneacetic acid) and Sevin (1-Naphthyl N-methylcarbamate) are the thinners recommended for apples at this time. Dr. Dick Unrath conducts one of the top programs in the Southeast on chemical thinning of apples. And in order to provide you with cutting edge information on this topic I am listing below the recommendations Extension Horticulturist Dr. Mike Parker is providing North Carolina apple growers this spring. Naturally, most of these recommendations are based on Dr. Unrath's work and grower experiences. You should base your thinning program on your past experiences using the information provided below for updating and adjustments as required.

In past years we have suggested measuring the lateral fruits rather than the first fruit to develop in each cluster (king bloom). Dr. Unrath has developed a system for using average fruit size (measure all fruits and don't worry about any late flowers that may be present) because king blooms are often killed by freezes or may otherwise fail to set fruit. In using his system he recommends applying thinners such as Sevin and NAA at a 9 to 11 mm (3/8 to 7/16 inch) average fruit diameter for most varieties (use 10 to 12 mm for Golden Delicious). It is best to use at least 100 fruitlets to determine size for thinning. Note that Tennessee still recommends measuring lateral fruits as we have in the past.

If chemical thinning proves inadequate or is not used, fruits should be thinned by hand, leaving no more than one fruit per cluster or spaced every 5 to 6 inches. Hand thinning can be quite costly but when needed can definitely be profitable for the grower.

North Carolina is one if not the only state that has Ethephon cleared (have 24C State label) for use as a thinner on apples. When they use a combined Ethephon plus Sevin thinning spray they apply at an average fruit diameter of 12 to 13 mm but can still apply up to 15 to 18 mm size. It seems Ethephon when combined with Sevin will permit latter application of thinners and still be effective in fruit removal. Alabama doesn't have a label for use of Ethephon as a thinner and we can't recommend it. Ethephon can cause shoot damage and excessive fruit thinning if not used properly.

Accel is the latest thinner to be cleared on apples. It contains 6-BA (cytokinin) and Gibberellins (A4A7). Abbott Labs released this growth regulator in 1994. Timing as with other thinners is critical. Growers who use this material should do their own small trials before making widespread use. Studies conducted in North Carolina indicate Accel must be applied at 7 to 8 mm average fruit size. Later applications at larger fruit sizes have not worked. Note: the label says to spray when king fruitlets are 10 mm diameter. This period usually occurs from 7 to 21 days after full bloom.

The maximum rate of Accel has been increased by Abbott Labs from 20 to 30 gms per acre for thinning apples. North Carolina has increased their suggested rate of Accel to 30 gms per acre for Fuji, Red Delicious spur types and Golden Delicious. Please keep in mind that North Carolina does not recommend Accel as a standard thinner yet (trial use only). North Carolina now has a 24C for using Sevin XLR to thin at petal fall.

Dr. Unrath has conducted some initial thinning studies with Gala and Fuji using a hand sprayer and other forms of application. What he found is that NAA at 5 ppm (using Fruitone N, the sodium salt of naphthaleneacetic acid) provides good thinning of Gala. He has also determined that 2 lbs. of Sevin 50W (1.0 lb. ai.) plus 2 1/2 ppm (one ounce) of NAA (Fruitone N) thins quite well. The ammonium salt of NAA should not be used (registered only as stop drop chemical and causes over thinning). He found these same rates to work well with Jonagold.

Work by Dr. Unrath on thinning Fuji (really hard to thin) indicates that 20 (one bottle) to 30 grams ai of Accel per acre plus 2 lbs. 50W Sevin per 100 gals. provides good thinning. Please note however, he is using Sevin on a lbs./100 gal. basis and Accel on a per acre basis making careful calibration of gallonage per acre a necessity. Remember, Accel must be applied at an average fruit size of 7 to 8 mm.

Reports being received indicate the wettable powder formulations of Sevin such as 50W may be dropped in favor of XLR formulations. This is mainly because the latter is less destructive to predator mites.

In spraying thinners on these newer varieties, growers should exercise caution and use smaller trial blocks. Once you have checked the thinning activity for a year or two then expand on a larger basis.

The Following Are Trials Being Suggested By Dr. Mike Parker For Newer Varieties and Thinning Agents For 1996 in North Carolina. (This information should prove valuable as you begin using new approaches in your thinning program.)

Cork spot and bitter pit always seem to cause growers problems especially with Delicious and Golden Delicious varieties. The disorders appear related to low levels of calcium and sometimes high levels of nitrogen in the fruit flesh.

Cork spot, characterized by somewhat round dead areas in the flesh, tends to be an orchard problem, whereas bitter pit is mainly a storage type disorder that affects skin and adjacent cells. However, because of variation in symptoms, the distinction between the two disorders isn't always readily evident.

Growers are familiar with calcium sprays used to combat these disorders but this isn't the only answer. Work by George Green of Penn State University indicates a control program for the problem should involve most all cultural practices used in apple production. He feels the best management approach involves use of the five factors listed below:

1. Calcium Sprays - Sprays of calcium chloride or calcium nitrate have been used successfully in controlling cork spot and bitter pit although they seldom completely eliminate it. Calcium chloride is usually the most cost effective material to use and is preferred over calcium nitrate. Work in Alabama indicates where boron is deficient, the addition of both calcium and boron is needed in nutritional sprays.

It is generally best to include calcium in all early cover sprays (at least the first 3 to 5). And where fruit are stored or where bitter pit or cork spot have been especially troublesome, it may be useful to continue adding calcium to all cover sprays throughout the season. Don't apply sprays when temperature is above 80øF or during extended dry periods. During dry periods, skip an application until at least 1/2 inch of rain is received. The most common rate applied is 2 lbs./100 gal. of calcium chloride which usually supplies 2 to 3.5 lbs. of calcium chloride per acre, but varies with gallonage used per acre. In Pennsylvania it has been shown that using 18 to 24 lbs. of calcium chloride per acre per year in cover sprays has provided good control of calcium disorders.

2. Moderate Fruit Density - There is a relationship between size of crop and level of cork spot and bitter pit. Usually the lighter the crop the greater is the problem with calcium disorders. The use of growth regulators to thin heavy crops is highly desirable. Factors which encourage annual cropping such as frost protection, good bee pollination and thinning are necessary to help maintain moderate cropping.

3. Maintaining Moderate Tree Vigor - Excessive tree vigor can utilize calcium that would normally be available to developing fruits. Heavy pruning and excessive use of nitrogen fertilizer can result in too much tree vigor. An inadequate crop load also contributes to excessive vigor. Try to employ cultural practices that provide both moderate cropping as well as tree vigor.

4. Balanced Nutrition - Use soil and leaf analyses in order to maintain an ideal fertility program. Always avoid excessive use of nitrogen, potassium and magnesium and strive to avoid deficient levels of calcium, boron and zinc. Any or all of these conditions can contribute to deficient fruit calcium levels.

5. Soil Conditions - Ideal soil pH for most apple orchards is 6-7. Low soil pH should be corrected (using ag limestone) to reduce availability of aluminum and manganese, which permits development of strong, wide-spread root systems. Be mindful of the magnesium content of the soil and don't use dolomitic limestone unless large amounts of magnesium are needed.

Stress caused by excessive or deficient soil moisture can cause problems with cork spot and bitter pit. Irrigation as well as drainage problems must be properly managed to eliminate this problem.