Duane Greene1, Jon Clements2 and Jacob Aliengena3
1Stockbridge School of Agriculture, University of Massachusetts, Amherst
2University of Massachusetts Extension
3Center for Agriculture Food and the Environment, University of Massachusetts Cold Spring Orchard
We have been evaluating metamitron as a chemical thinner on apples in Massachusetts since 2010. Evaluations in the early years were from different companies and product formulations. Although the product was evaluated in other countries in the world, it was a steep learning curve in the early years. Since 2015, we have been evaluating different concentrations, times of application, multiple applications and the use with surfactants included in the spray tank using formulations from Adama. Since metamitron (BrevisTM SC) will be available to growers for use in Massachusetts (and New Jersey) in the spring of 2026, we initiated an experiment where metamitron would be evaluated alongside or with other chemical thinners that growers have used and are familiar with their use.
Materials and Methods
In a block of mature Summerland McIntosh/M.9 apple trees, 36 trees were selected. An untreated tree was left between selected trees to act as a buffer from spray drift. At the pink stage of flower development three limbs on each tree were tagged, the limb circumferences were measured and recorded. The number of blossom clusters on each tree were counted and the blossom cluster density calculated. In this experiment we counted flower buds on spur separately from flower buds on one-year-old wood since thinners sometimes act differently on these two types of flower buds. Trees were arranged into six groups (replications) of six trees each based on the blossom cluster density. Weather data from the Cornell NEWA Thinning Model was collected at the orchard and is shown in Table 1 from one day before the first thinner applications were made (May 13) to four days after the last time applications were made (May 27). BreviSmart (https://brevismart.adama.com/) is a thinner prediction model that was developed by Adama for use with metamitron. BreviSmart outputs were checked periodically over the thinning period even though Brevis was applied only once on May 27 (Figure 1). The Cornell Apple Carbohydrate Thinning Model was checked regularly to act as a guide about when to apply thinners and the amount of thinner to be applied (Figure 2).
Details of the thinners, rates and time of application used in this experiment are shown in Table 2. The trees used in this experiment had a TRV dilute spray volume of 100 gal per acre. The airblast sprayer was calibrated to deposit about 70% of the spray volume in the top 2/3 of the trees.A spray containing 10 ppm NAA was applied on May 13 to trees assigned to treatments 2, 5 and 6. Fruit on this date were about 6 mm in diameter. The BreviSmart model indicated on this date that weather conditions were less than good (Yellow) for application (Figure 1). The NEWA carbohydrate model weighted average on May 13 was negative at -22 and the model suggested that the standard rate of thinner to be applied should be reduced by 15% (Table 1). The negative internal carbohydrate conditions prevailed for the next 3 days. The second time of application of thinners was on May 27 when fruit size averaged 15 mm. On this date BreviSmart indicated that weather conditions were Good for metamitron application (Figure 2). The Cornell Carbohydrate Model on the other hand indicated that the carbohydrate balance within the trees was substantially positive on the day of application and for several days following. It suggested
that the strength of the thinner(s) applied should be increased by 30% above normal recommended rates. On May 27 trees designated to receive treatment 3 were sprayed with a thinner containing 10 ppm NAA and carbaryl at a rate of 1 quart/100 gallons. Trees designated to receive treatment 4 were sprayed with 100 ppm MaxCel. Trees designated as treatment 5 and treatment 6, respectively were sprayed with 1.5 and 2.5 pt/acre of Brevis on May 27. At the end of June drop in July all persisting fruit on the tagged limbs were counted and the crop load calculated (Table 1). In 2025 we counted the fruit separately on spurs and those that resulted from set of lateral flowers. At the normal harvest time a 30-apple sample was harvested randomly from each tree in the experiment (Table1). Fruit were taken to the laboratory where they were weighed and the average fruit weight calculated. No phytotoxic effects following Brevis application were noted.
Results and Discussion
Thinning treatments caused only a modest amount of thinning, with only one treatment achieving a significant amount of thinning (Table 2). Application of 10 ppm NAA + carbaryl or NAA 10 ppm + carbaryl applied twice did not cause thinning and the final crop load was similar to that of the control. We often recommend NAA 10 ppm with or without carbaryl to be applied early. The early thinner application (petal fall to 5 mm) can make treated fruit more susceptible to follow up thinner applications. That did not happen this year, due in large part to a lack of a carbon deficit at the time of and following the second thinner application. We followed the NEWA carbohydrate model for over a week looking for a favorable time to make our second thinner application, but cool sunny weather persisted. We finally decided to make the second application when fruit were 15 mm, even with a positive carbon balance since fruit larger than 15 mm become progressively more difficult to thin. The carbon status of the trees at the 15 mm fruit size stage was positive to very positive, thus making it difficult to achieve thinning because of a lack of carbohydrate stress.
It is curious that BreviSmart indicated that application at the 15 mm fruit size was a good time to apply thinners. Trees that received Brevis at the 1.5 or 2.5 pints/ acre rate were not thinned. It is our suggestion that the carbon surplus was too great to make fruit susceptible to thinners. However, MaxCel at 100 ppm plus carbaryl applied to trees that previously received 10 ppm NAA plus carbaryl was the most potent thinner combination in this experiment. This thinner combination reduced crop load down to close to the ideal crop load for these trees (Table 2). Fruit size at harvest provided a very good indicator of the effectiveness of the thinners. Tree sprayed twice with10 ppm NAA plus carbaryl, trees sprayed with MaxCel plus carbaryl and trees that receiving 2.5 pt/acre Brevis had significantly larger fruit at harvest. Based upon observations made in the orchard, the fruit size data appeared to represent more accurately the thinning response in this experiment than the actual fruit set data. Figure 3 depicts typical apples on trees at time of harvest.
In recent years we have been using the two thinning models, BreviSmart and the Cornell NEWA thinning model to provide input for making thinning decisions. The two models do not always agree, which was the case this year. This is not surprising since recommendations are based upon information gathered from two different data sources. This is not necessarily a bad thing since this requires orchardists to evaluate the situation in their trees and draw their own conclusion about what to use, how much to apply and when to make the application(s) in their own orchard setting.
We did not evaluate the time of application in this experiment. Previous use of Brevis has focused primarily on making the first application made at about the 6 mm fruit size stage. We still favor that initial time of application, particularly if there was a negative carbon balance at this time and for a few days following. Application earlier than 6 mm will unlikely be effective since fruit grow is slow and carbon demand will be low as well. As development of Brevis proceeds we hope to evaluate how Brevis fits into thinning programs with thinners having different modes of action, different times and different combinations. We are particularly pleased to have available now a new thinner with presumably a different mode of application. From experience, we have learned that spray thinning programs with different thinners and times of application generally result in a more satisfactory thinning result at the end of the season.
