Fruit Notes

VIRGINIA FRUIT NOTES
January/February - 1998, Vol. 18, No. 2

Richard P. Marini
Extension Horticulturist, Tree Fruits
VPI & SU
Blacksburg, VA 24061-0327

Apple Thinning in 1998
Because apple trees typically produce too many fruit, most orchards require post-bloom chemical thinning to promote return bloom, improve fruit color development and pest control, and sometimes fruit size is improved. Although the size of fruits remaining on the tree is often not increased, average fruit size and packout are usually improved because lateral fruits, that tend to be smaller than king fruit, are preferentially removed by chemical thinners.

Early-season apple growth. In order to understand concepts involved in chemical thinning, one must understand fruit growth. During bloom insects transfer pollen from the anther of one variety to the stigma of another variety. The pollen germinates, grows down the style and fertilizes an ovule that develops into a seed. The rate of pollen tube growth and the number of ovules that are fertilized is influenced by climatic conditions. In general, fertilization is favored by warm temperatures. At petal fall, fruitlets are 4 to 5 mm in diameter. Due to cell division fruitlets will grow about 0.2 to 0.5 mm per day. The rate of fruitlet growth is temperature-dependent and is greatest when day and night temperatures are high. Fruitlets resulting from nonfertilized flowers grow until attaining a diameter of about 9 to 10 mm, and drop about 7 to 10 days after growth cessation. Days from petal fall to growth cessation or days from growth cessation to fruitlet drop also vary with temperature.

Fertilized fruitlets continue to grow for about 50 days after bloom. Some fruitlets are unable to attract carbohydrates and stop growing after attaining a diameter of about 14 to 18 mm and fall during the June Drop. Cell division greatly diminishes at about 50 days after bloom and fruits continue to grow as cells expand. At about 50 days after bloom fruits begin to accumulate starch and cannot be induced to drop with chemical thinners or severe wounding and, for about a 6 week period, the fruits apparently don't have the ability to produce ethylene.

Factors influencing chemical thinners. There are data from several states indicating that chemical thinners work by reducing the amount of carbohydrate available to some fruits. Anything that reduces photosynthesis or translocation of carbohydrate to fruits, or enhances the utilization of carbohydrates (respiration) should theoretically improve the efficacy of thinners.

Light. Light is the energy plants use to produce carbohydrates. Cloud cover or within-tree shade reduce photosynthesis and increase the effectiveness of thinners. Heavy cloud cover for 2 or 3 days, especially when accompanied by warm temperatures, on the day of spraying and for the following 3 days, will increase the amount of thinning.

Drying conditions. Poor drying conditions, such as high humidity, cloud cover and light winds, increase the amount of chemical taken up by leaves and should increase the amount of thinning.

Temperature. Data from Michigan indicate the uptake of NAA is improved at high temperatures. Additionally, respiration and carbohydrate utilization increase as temperature increases. Therefore, thinning should increase at high temperatures.

Fruit size. All chemical thinners except ethephon are effective at average fruit diameters of 4 to 14 mm. My results indicate that for spur type ëDelicious' NAA is most effective from petal fall to about 12 mm, carbaryl and oxamyl are most effective at 10 to 14 mm, and Ethephon is most effective at 18 to 26 mm. Ideal fruit sizes likely differ for other varieties.

Because measuring individual fruits with calipers is so time-consuming, many growers avoid measuring fruit. Therefore, I developed a method for estimating fruit diameter from fruit weight. A mathematical model was developed with data from 3 years, 5 orchards, 4 strains of `Delicious' and 3 strains of `Golden Delicious'. A single model seems to fit both varieties, all orchards and all years very well. The variation in fruit weight explains 97% of the variation in fruit diameter. Below is a protocol for sampling an orchard and a table for estimating fruit diameter from fruit weight.

1. Randomly collect 3 spurs from each of 20 trees per block and place in a paper bag.
2. Do not intentionally avoid spurs with no king fruit or that have only one fruit.
3. Randomly select a spur and remove the 3 largest fruits, retaining the stems, that look as though they will set.
4. Continue until you have 100 fruits.
5. Weigh the 100-fruit sample and compare the total weight of 100 fruits with the average fruit diameter (Table 1).

Table 1. Apple fruit diameter estimated from apple fruit weight presented in metric and English units.

Wt of a single fruit (g)	Wt of a single fruit (oz)	Wt of 100 fruits (g)	Wt of 100 fruits (oz)	Wt of 100 fruits (lbs)	Mean fruit diam. (mm)
0.15	0.005	15	0.530	0.033	2.67
0.2	0.007	20	0.706	0.044	4.09

0.25	0.009	25	0.883	0.055	5.12
0.3	0.011	30	1.059	0.066	5.92
0.4	0.014	40	1.412	0.088	7.12
0.5	0.018	50	1.765	0.110	8.02
0.7	0.025	70	2.471	0.154	9.33
0.9	0.032	90	3.177	0.199	10.31
1.1	0.039	110	3.883	0.243	11.11
1.3	0.046	130	4.589	0.287	11.79
1.6	0.056	160	5.648	0.353	12.68
2	0.071	200	7.060	0.441	13.70
2.4	0.085	240	8.472	0.530	14.61
2.8	0.099	280	9.884	0.618	15.44
3.3	0.116	330	11.649	0.728	16.42
3.9	0.138	390	13.767	0.860	17.52
4.5	0.159	450	15.885	0.993	18.56
5.2	0.184	520	18.356	1.147	19.74
5.9	0.208	590	20.827	1.302	20.87
6.8	0.240	680	24.004	1.500	22.29
7.7	0.272	770	27.181	1.699	23.68
8.7	0.307	870	30.711	1.919	25.20

Example 1: Suppose the total weight of 100 fruits, with stems, is 1.0 oz. The average fruit diameter is about 5.9 mm.

Example 2: Suppose the total weight of 100 fruits, with stems, is 24 oz. (1.5 lbs.). The average fruit diameter is about 22.3 mm.

Fruit diameter interacts with temperature. As already indicated, the effectiveness of thinners is influenced by both fruit size and temperature for the 2 or 3 days following treatment. However, the 2 factors are not additive. The relative importance of temperature depends on fruit diameter at the time of application. Preliminary analyses of 12 years of data for spur ëDelicious' indicate that the most important factors are average diameter the day of application, and the maximum temperature 2 days after treatment. In general, thinning results for carbaryl are similar regardless of temperature when fruit diameter is less than 8 mm. However, for larger fruit (14 to 16 mm) thinning is better at 70 or 80 than at 50 or 60 . This explains some of the year-to-year variation for thinners, but the total explanation is likely much more complex. Other factors such as light, reserve carbohydrate levels in the tree, and leaf to fruit ratio at the time of thinning are likely also involved. It will take many years of research to fully understand the interrelationships of all these factors.

Assessing Thinning Results

During the past two years we have been studying changes in fruit physiology to try to identify, as soon as possible, characteristics indicating that a fruit will ultimately drop as a result of thinning treatment. Early knowledge of the effectiveness of a thinner is important because one may want to apply a repeat thinning treatment.

Sometimes it takes 3 weeks from the time a thinner is applied until fruits start to drop. By that time it may be two late to apply a second thinner. We have known for a number of years that cessation of fruit growth for 2 consecutive days indicates that that fruit will ultimately drop. Because measuring fruits daily with calipers is expensive, we have been looking for other indications that a fruit is destined to drop. We applied a high rate of Ethrel (3 pints/100 gallons) to induce all fruit on a tree to drop so we could study changes in fruit physiology before they dropped. The first change we observed was a less negative fruit water potential 2 days after treatment, indicating less solute (possibly sugar) in the fruit juice. Three days after application the fruit stopped growing. Four days after application the fruit dry weight and % dry weight failed to change. Cellulase activity became obvious in the abscission zone 4 days after treatment. Cellulase is the enzyme responsible for degradation of cell walls. Six days after application we measured a decrease in fruit respiration and less starch accumulation in treated fruit. Fruit started dropping on the 6th day, but most did not drop until the 8th day after treatment. For less aggressive thinning treatments, the entire sequence of events may be delayed several days.

At this point it seems that fruits stop growing several days after application because cell division ceases due to a lack of carbohydrates. The subsequent synthesis of cellulase is ultimately responsible for fruit drop. Fruit diameter is the only factor that we have identified that can be measured in commercial orchards because it is easy to measure, requires no expensive equipment, and is nondestructive. So the question is "how do I measure fruit to evaluate a thinning treatment?" One must realize that certain fruits are most prone to drop and some will stop growing sooner than others. Compared to the side fruits, the king fruit is the largest in the cluster, will continue growing longest, and take longer to drop after growth cessation. So don't expect all fruits to stop growing or to drop on the same day. Fortunately, all fruit of a certain type tend to stop growing at about the same time. Although some of the details on sample size will require another two years to work out, below is a suggested method for using fruit diameter to evaluate the effect of a thinner.

1. During pink, tag a representative limb and remove, with tweezers, all stigmas from the king blooms on 10 flower clusters. Flag these clusters so you can measure the king fruits each day to determine when the non-fertilized fruits stop growing. These fruits represent the first period of fruit drop. When these king fruits have all dropped you will know that most of the fruit still on the tree have set and this is your first opportunity to really evaluate fruit set.
2. To evaluate fruit growth, flag a fruiting spur on each of four sides on each of four trees representative of those in a block (total of 16 spurs/block). If trees in different sections of a block look different it may be a good idea to measure fruits on additional trees that represent the different types of trees. On each flagged spur, tag the king fruit and the largest side fruit. We use small tags with strings, purchased at an office supply store. The string is tied to the fruit pedicel and each tag is numbered with a pencil. On each fruit make a small mark with a magic marker, about half way between the calyx and the stem so calipers can be used to repeatedly measure the fruit at the same spot. Measure each fruit to the nearest millimeter (mm) 2 days before treatment, the day of treatment, and at 2-day intervals thereafter. The number of days to fruit growth cessation will increase with increasing fruit diameter and decrease with increasing temperatures. Usually growth cessations begins 4 to 8 days after treatment. Fruits that stop growing within 3 days after treatment were destined to drop naturally and growth cessation is not the result of a thinning treatment. Fruits that fail to grow during a 2-day period, while most fruits continue to grow, are destined to drop. Sometimes we experience very low temperatures (daily highs of about 45 degrees and nighttime minimums of about 35 degrees) during this time. All fruits grow very little under such conditions, so you may have to delay judgement until the next measurement date.

By measuring fruit every other day one can accurately assess the results of a thinning treatment 6 to 8 days after treatment. If one uses fruit drop to assess the effects of a thinning treatment, it will take 14 to 21 days after treatment.

Based on my observations in Virginia, growers who consistently do the best job of fruit thinning are those who carefully observe what is going on in the orchard during and after bloom and measure fruit. They leave some non-thinned control trees for comparison so they know how much thinning their treatments actually did. They often make more than one thinning application on a block and they keep good records of what they did for future reference.

Maintain Appropriate Concentration

New chemical labels are based on the amount of active ingredient (AI) per acre rather than AI per 100 gallons. This change has led to some confusion, especially as we move to tree row volume (TRV) spraying. We have too little good data indicating how much AI is needed per acre in varying volumes of water with different tree sizes. Some materials must be applied at a certain concentration (the amount of AI/gallon of water), to obtain good thinning results.

Last year I applied thinners to some small trees and got very little thinning. Based on the tree size and row spacing the TRV was about 30%, so a dilute spray would require applying 120 gallons/acre. Because I was using a handgun, I was concerned that 120 gallons per acre was too little to get good coverage, so I actually sprayed 153 gallons/acre. I wanted to apply 2 pints of Vydate/acre. But I reduced the amount of chemical to 30% or 10 oz/acre in 153 gallons. This gives a concentration of 150 ppm of active ingredient. I got essentially no thinning from this treatment. In the past I have found the 600 ppm is required for thinning equivalent to Sevin. In order to have 600 ppm active ingredient I should have used 2.5 pints of Vydate in 150.

I am still not sure how to adjust water volume and AI of growth regulators for different sized trees. At this point, I think it may be best to make a solution of the appropriate concentration and adjust the amount of water per acre to apply a dilute spray based on tree row volume.