Natural fruitlet abscission as related to apple tree carbon balance estimated with the MaluSim model

Abstract

Apple trees produce many more flower clusters than needed for a full crop, but natural early season flower and fruitlet abscission drastically reduce the final fruit number. Natural fruit abscission varies significantly year to year. There have been attempts to try to model apple fruit abscission in the past. However, due to the great complexity of a perennial crop system in a dynamic environment with significant plant manipulations, regulatory processes and controlling environmental variables have been difficult to elucidate. In 1995, a field trial was planted at the New York State Agricultural Experiment Station in Geneva, New York with 3 apple cultivars ('Delicious', 'Gala', and 'McIntosh'). Beginning in 2000 and for 18 years thereafter, we recorded the natural whole-season fruit abscission of untreated trees that received no chemical or hand thinning. We also estimated early season patterns of carbohydrate supply-to-demand each year with a carbon balance model. These data were used to correlate tree carbon balance status and other environmental variables with natural fruit abscission responses. In general terms, natural set, defined as final fruit/flower cluster, of 'Gala' averaged ~1 fruit for each flower cluster (fruit set = 0.9), whereas fewer fruits were set on 'Delicious' and 'McIntosh' (fruit set = 0.7 and 0.6, respectively). Fruit set of 'Gala' was less variable than of 'Delicious' or 'McIntosh', and there was a clear pattern for decreasing fruit set when the number of initial flower clusters per tree increased.Fruit weight was less dependent on fruit number for 'Delicious' and 'McIntosh' than for 'Gala'.Multiple regression models indicated that number of flower clusters per tree and average carbohydrate balance between 0-60 degree days (DD) after bloom and 300-360 DD after bloom were the main significant variables that explained 60-80% of the variability in natural fruit set or final fruit number. For 'Delicious', temperatures of the previous fall also explained a significant amount of variation in final fruit set and final fruit number. For 'Gala', carbon balance from bloom to shortly after petal fall and when fruits were about 18 mm were the two main periods, which were more sensible to carbohydrate deficiency triggering fruit abscission. A later susceptible period was also observed for 'McIntosh', suggesting a larger thinning window for this cultivar.

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

“…This concurs with the observations of the present study (the year 2015 was warmer than 2016). The carbohydrate balance can also play a significant role in apple tree response to fruit abscission; if the carbohydrate supply is abundant it may limit fruit development and abscission (Lordan et al, 2019). Other factors may also explain year-to-year Brevis ® efficacy, including the weather of the previous year, carbohydrate ratios from the previous year, temperature and sunlight from bud break to bloom or post bloom, tree vigor, leaf area, or the sensitivity of the tree itself.…”

“…Other factors may also explain year-to-year Brevis ® efficacy, including the weather of the previous year, carbohydrate ratios from the previous year, temperature and sunlight from bud break to bloom or post bloom, tree vigor, leaf area, or the sensitivity of the tree itself. Lordan et al (2019) reported how these factors can affect natural fruit abscission. The significant interactions between year and cultivars in most of the parameters evaluated can be attributed to the different efficacy between years and cultivars.…”

Pear Flower Cluster Quantification Using RGB Drone Imagery

“…Growers will use the flower cluster estimations to steer pruning and flower thinning, and to get early yield estimations. For example, for yield estimations, knowing the flower clusters per tree instead of per meter will probably lead to higher estimation accuracies since the flower clusters determine the amount of sinks and the carbohydrate balance of its tree, and hence determine the physiological fruit fall of one particular tree [43]. If pear trees show a gradual change in flower clusters according to their position to the neighboring trees individual tree delineation is not necessary.…”

Modelling physiological and environmental factors regulating relative fruit set and final fruit numbers in apple trees

“…For the past 50 years chemical thinning sprays have been the primary method growers use to reduce fruit numbers, but despite over 50 years of experience with chemical thinning, it remains an unpredictable part of apple production with large variations from year to year and within years due to weather variables such as temperature and radiation (Robinson et al, 2017;Robinson and Lakso, 2004;Robinson et al, 2012). There have been many studies that have attempted to understand better the roles of individual factors, with experimental manipulation of cultivar, tree vigor, bloom density, environmental conditions or chemical used (Lakso and Goffinet, 2017;Lordan et al, 2019). Yet, more than 30 years of field trials (Dennis, 2000;Greene, 2002;Greene and Costa, 2012;Greene and Lakso, 2013;Robinson and Lakso, 2011;Williams, 1979) have provided only general guidelines on the effects of weather conditions and timing of application, but have not been able to clarify regulatory processes or provide quantitative rules for prediction of apple chemical thinning response.…”

“…Conditions that lead to low carbohydrate balance are associated with heavy natural fruit drop (Lordan et al, 2019) and easier chemical thinning (Robinson and Lakso, 2011). These include hot temperatures, cloudy, heavy initial set on many weak spurs and stressed trees.…”