Effect of different application rates of metamitron as fruitlet chemical thinner on thinning efficacy and fluorescence inhibition in Gala and Fuji apple

Abstract

Crop thinning is an important and difficult agricultural practice. Knowing the effect of the application dose of a product is a crucial element of any thinning program. The aims of this study were to investigate the effect of different metamitron doses on Gala and Fuji apples applied at fruit king diameters of between 8 and 10 mm and to determine fluorescence inhibition at the different application rates. Trials were conducted over two seasons from 2015 to 2016 in apple orchards in Lleida (Spain). Photosynthesis inhibition caused by metamitron was also analysed and measured, using chlorophyll fluorescence and biexponential pharmacokinetic models. Under the trial conditions, the application of metamitron reduced final fruit set, number of fruits per tree and crop load depending on the application rate. A dose effect was observed in all yield parameters. Moreover, when metamitron showed high efficacy, there was an improvement in fruit weight, coloration and diameter. The estimated parameters A, α and B using a biexponential equation were related with final fruit set, however the period of inhibition has to be finished before prediction can be made of metamitron efficacy in the year. The fluorescence analysis showed a dose effect, with metamitron dose increasing inhibition. Additionally, the same result was also observed in the area under curve analysis, with metamitron dose reducing the area and inhibition increasing. In all yield parameters, the fluorescence and area under curve analyses showed differences between cultivars, with the inhibition caused by metamitron higher in Gala than in Fuji. Moreover, differences between years were observed. 2015 was warmer than 2016, and the higher temperatures increased the thinning efficacy of metamitron.

Evaluation of chemical fruit thinning efficiency using Brevis® (Metamitron) on apple trees (‘Gala’) under Spanish conditions

“…However, in the other trials, where the climatology was not favorable for the application of a chemical thinner, there was no observed effect of the single applications, concurring with earlier observations of Byers (2003). Average fruit weight, diameter and coloration increased with the Brevis-induced thinning effect, which concurs with the observations of Brunner ( 2014), Gonzalez et al (2019c) , Maas and Meland (2016) and McArtney et al (1996). They reported a negative linear relationship between the number of fruits and their average weight, color and diameter, which increased significantly in the treatments in which the chemical thinner reduced the number of fruits per tree.…”

Multi-year investigation on the rate, timing, and use of surfactant for thinning apples with post-bloom applications of metamitron

“…Temperature effects on dark respiration would therefore be a primary factor in determining the magnitude of this sink within developing fruitlets (Jackson 2003;Lakso et al 2006). Indeed, Gonzalez et al (2019Gonzalez et al ( , 2020 determined that the degree of fruit abscission with MET is highly dependent on night temperature, rate of application and cultivar, and suggests these are all explanations for the high variability that can exist in thinning efficacy among different trials. Gonzalez et al (2020) observed that when average night temperature exceeds 14 °C the day of and 4 d after a single application of MET, thinning efficacy is approximately 10%-25% higher in comparison with average night temperatures below 14 °C.…”

“…Application of a PSII inhibitor such as MET to apple trees can result in a transient carbohydrate stress that may persist for several days Brunner 2014;Stern 2015;Rosa 2016;Gonzalez et al 2019) and increase the sensitivity of the fruit to a chemical thinner. In addition to this direct effect, if MET is applied in combination with a chemical thinner that has been formulated with a wetting agent, then the resulting carbohydrate stress may result in aggressive thinning compared with a chemical thinner not formulated with a wetting agent.…”

Metamitron and Shade Effects on Leaf Physiology and Thinning Efficacy of Malus × domestica Borkh

“…In 2017, initial fruit set was low resulting in a higher difficulty to create a negative carbohydrate balance and finally, low thinning efficacies, even in MET + SN. However, in 2018, initial fruit set was very high and higher night-time temperature during the 5 nights before application (in Girona) and some cloudy days during the 3 days after (in the tree locations) might have potentiated the reduction in photoassimilates needed for fruit growth [82]. This might have contributed to some over-thinning with MET + SN treatment, which promoted abscission rates of 58% in Sint-Truiden and above 70% in Girona, responsible by a 58% cut in yield in the latter.…”