R. Scott Johnson, Harry Andris, and Kevin Day

University of California Kearney Agricultural Center and UCCE Fresno and Tulare Counties


Chemical thinning of fruit trees to reduce or eliminate hand thinning could be of great economic help to growers. Plums in particular could benefit greatly from the use of a chemical thinning agent. In years with adequate chilling and optimum pollination conditions during bloom, plum set can be very heavy. Since the fruit are born in clusters, much detailed hand thinning is required, resulting in costs of over $1,000 per acre in some cases. With the cost of labor steadily increasing, the need for an effective chemical thinner is even more urgent.

We have been experimenting over the last couple of years with chemical thinning of plums in several different orchards. Some trials have shown reasonable results while others have not worked as well. Our work in 2001 has given us some clues as to why results vary from one orchard to another.

Entry Treatments on Royal Diamond Plums

Entry is a surfactant material currently registered for thinning of various fruit crops. We have experimented with it for many years under its original name of Armothin. Even though it is far from perfect, it is probably the most consistent chemical we have used in recent years. It is applied during bloom and basically "burns" off a certain number of the flowers. In both 2000 and 2001, we applied Entry to a block of Royal Diamond plums at the Kearney Ag Center. Rates of 2% and 3%, each in 100 gallons of water, were applied to 4 reps of 26 trees each. Applications were made at 70 to 80% bloom using an orchard airblast sprayer. An equal number of trees were left unsprayed as the control treatment.

In both 2000 and 2001, the 3% Entry treatment was effective at removing about half of the crop load compared to the unsprayed control (Table 1). For 2000, control trees set about 1300 fruit per tree and Entry reduced this to 750. Theoretically, this reduced the crop load to a commercial level since hand thinning of the controls came out to about the same value. However, a quick hand thinning of this treatment to break up clusters caused a 25% reduction in overall yield. In 2001, fruit set was nearly double the previous year on the control trees. Clearly, Entry was beneficial at reducing this excessive crop load and saving substantial labor costs in hand thinning. Unfortunately, too much fruit was left on the trees in both the control and the Entry treatments, so fruit size was quite small all around. Nevertheless, it should be noted that Entry effectively reduced the fruit load on these Royal Diamond plums and thus could be an effective tool for helping to cut down on labor costs for hand thinning.

Table 1. The effect of Entry (Armothin) on thinning of Royal Diamond plums. Applications were made in 100 gals/acre at 70-80% bloom in both 2000 and 2001.


Number of Fruit/Tree (% of Control)


Before Hand Thinning

After Hand Thinning






Unsprayed Control









2% Entry









3% Entry










Individual Tree Differences

In 2000, it was obvious some trees sprayed with Entry were thinned to a much greater extent than others. Therefore, in 2001, 32 individual trees were tagged and followed through hand thinning and harvest. The total number of fruit per tree before hand thinning ranged from 424 to 1842. We looked at various factors to see if this substantial variability could be accounted for. The primary factor was the nitrogen content of the trees; the lower the nitrogen content, the lower the fruit load. In other words, weaker trees were thinned to a greater extent by the chemical thinning spray. This implies that stronger trees will require a higher rate of Entry to achieve a given level of thinning.

A secondary factor affecting the level of thinning was the percent open blossoms at the time of the spray application. Across this small orchard of about 3 acres, some trees were only at about 40% full bloom while others were closer to 90%. The trees with more open blossoms tended to respond to the chemical thinner to a greater extent. This factor only accounted for a small amount of the total variability but it was still statistically significant. Together, these two factors accounted for almost 50% of the total variability.

Different Approaches to Chemical Thinning

In 2001 we also experimented with some other approaches to chemical thinning. For instance, the 2% Entry treatment in 2000 showed no thinning compared to the unsprayed control. Therefore, in 2001 we followed up the initial 2% spray with a second 2% spray 5 days later. Instead of treating the whole tree, we directed the spray to just the top third of the canopy. These two sprays combined to reduce the fruit load from 2362 to 1531 fruit per tree (Table 1). Thus, multiple sprays may be a more conservative approach to chemical thinning. After the first spray at the beginning of the bloom period, the grower can carefully monitor bee activity and weather conditions to decide if a second spray is warranted.

Directing the spray to certain parts of the canopy gives the grower further flexibility. In 2001 we had a few extra trees where we directed a single spray to just the top third of the canopy. As the fruit developed it was obvious we had achieved some thinning in the top of the tree without affecting the lower fruit very much. Since the lower fruit generally doesnít set as well and also is much faster to hand thin, this might be a strategy to achieve some economic thinning while protecting against over thinning.

Entry Treatments in Other Commercial Orchards

Our experiences with Entry in various commercial plum orchards has been somewhat varied. Several of the trials have given results similar to what we observed in the Royal Diamond block. In other cases there didnít appear to be much of a thinning effect. In particular, there were a couple of Friar orchards we sprayed in 2000 that didnít show significant thinning. It might be that Friar will be a more difficult variety to thin than other plum varieties. However, it is more likely related to the overall health and nitrogen status of these trees. One of the orchards had very healthy trees that always set fruit well even in poor pollination weather. Perhaps in situations like this, it will be necessary to use multiple applications for effective thinning.


In summary, Entry is a chemical thinner that appears to have promise as a blossom thinner in plums. As long as the grower takes into account such factors as the health and vigor of his trees and the stage of bloom during application, and is willing to experiment with multiple applications and directing the spray to certain parts of the canopy, there is reason to expect some level of success.