All systems with size-controlling rootstocks were irrigated and fertigated using sub-surface drip to maintain a soil moisture between -20 and -60 cbar throughout the growing season. Microsprinklers were used for irrigation and fertigation in the Nema Quad system. The soil at the site is a well-drained Hanford, fine sandy loam. Weeds were controlled by mowing the row middles and applying herbicides to maintain a 1.5m wide weed-free strip down the tree rows. All systems received a light summer pruning and heavy dormant pruning to establish desired structure and improve light interception. Approximately a week before harvest, total canopy light interception using a ACCUPAR LP- 80 meter was measured in each plot. Harvest occurred on two or three separate days, depending on rootstock and cultivar, during the growing season due to variance in fruit maturity, as is common in stone fruit production. Each data tree was harvested individually, total quantity of fruit produced, and total fruit weight data were recorded which enabled calculation of mean fruit weight per tree. Harvest data were collected for growing seasons 2017-2019. A linear model was created in R markdown for each season and cultivar’s harvest. With each linear model, an ANOVA test was conducted using a 95% confidence level and Dunnett’s method adjustments to identify significant differences among the four orchard systems for both scion cultivars. A true significant difference was concluded if the comparison between two systems had a p-value less than 0.05, a t-ratio greater than 1.68 , black plant pots plastic and a confidence level range that did not include 0.During the 2017 harvest season for the June Flame cultivar, trees in all systems produced commercially acceptable mean fruit size, >200g per individual fruit .
The C-6 Quad system produced significantly larger fruit when compared to the Nema Quad system . In the June Flame 2018 harvest, mean individual fruit weights in all systems were again > 200g, . Although the C-9 Quad system produced large enough fruit for fresh market sale, the mean individual fruit weight was significantly less compared to fruit in the Nema Quad system . The harvest season of 2019 for June Flame had individual fruit weight above 200g in all systems . However, it should be noted that the C-6 V and C-9 Quad systems produced significantly smaller fruit compared to the Nema Quad system . In all three seasons for the June Flame cultivar, the C-6 Quad system produced fruit of equal or larger size than the Nema Quad system. In the 2017 harvest season for the August Flame cultivars, all systems with size-controlling rootstocks produced significantly larger fruit than the Nema Quad system . In a few cases, fruit size exceeded 300g per fruit in systems with size controlling rootstocks, >50% larger than the minimum requirement for large sizing in the fresh market . The harvest season of 2018 for August Flame may have been the most productive of all years for both cultivars, all systems exceeded 250g in mean individual fruit weight . The C-6 Quad system had significantly larger fruit than the Nema Quad system while C-9 Quad had significantly smaller fruit . In the 2019 harvest season all systems produced fruit sizes above 200g but were smaller than fruit from previous seasons . Although the C-6 V and C-9 Quad systemsdid not differ significantly from the Nema Quad system, the C-6 Quad system produced significantly smaller fruit compared to the Nema Quad system .During the 2017 harvest season of the June Flame cultivar, the C-6 V and C-9 Quad systems produced significantly fewer fruit per hectare compared to the Nema Quad system.
There was no significant difference between the C-6 Quad and Nema Quad systems . For June Flame in 2018 there were no significant differences in yield per hectare among the C-6 V, C-6 Quad and Nema Quad systems. The C-9 Quad system produced significantly fewer fruit per hectare compared to the Nema Quad system . The 2019 June Flame harvest had close to identical fruit count per hectare between the C-6 Quad and Nema Quad systems .Once again, the C-9 Quad was the only system that produced significantly fewer fruit per hectare compared to the Nema Quad system . In August Flame’s harvest of 2017 there were no significant differences among systems using size-controlling rootstocks and the Nema Quad system for fruit produced per hectare . In the 2018 harvest season for August Flame, the C-9 Quad system produced significantly fewer fruit per hectare compared to the Nema Quad system . Meanwhile the C-6 V and C-6 Quad systems maintained similar fruit counts per hectare as the Nema Quad system. In the 2019 harvest for August Flame the C-6 V system produced significantly fewer fruit per hectare than the Nema Quad system. The C-6 Quad and C-9 Quad systems did not differ significantly for fruit count per hectare compared to the Nema Quad system . It should be noted that during the 2019 harvest season some trees displayed signs of water stress in the field which may have hindered production and skewed results for that season.During the 2017 harvest of the June flame cultivar, there was a significant difference in the slope of the relationship between fruit size and fruit per hectare among the C-6 V and the Nema Quad systems . Data from all systems fit a linear model that had a negative correlation between fruit size and fruit per hectare. Although a negative correlation was visible between fruit size and fruit per hectare in the C-6 V system, its magnitude was not as steep as with other systems in the same season . The following season, 2018, for June Flame there were significant differences in the fruit size vs. fruit per hectare relationship among systems. . Even though no significant differences were detected with the ANOVA analysis, linear models were weak at representing the relationship between fruit size and fruit per hectare and all systems using size-controlling rootstocks had an R-squared value <0.15 .
Continuing the trend from the previous season, in 2019 for June Flame, there were no significant differences in the slope of fruit size vs fruit per hectare relationship for any of the systems . The contrast between the C-6 Quad system and Nema Quad system did have a t.ratio with a greater absolute value than 1.68, however the P.value for the same comparison was still greater than the designated alpha, > 0.05. In this same season the C-6 Quad system had the best fit for the linear model showing a negative correlation between fruit size and fruit per hectare. All other systems fit the model poorly and also did not indicate a clear negative correlation between fruit size and fruit count per hectare . For the August Flame harvest of 2017, data from all systems fit linear models that showed a negative correlation between fruit size and fruit per hectare . Values for the t. ratio between the C-9 Quad and Nema Quad systems were beyond the absolute limit but had a P. value greater than the declared alpha, thus no significant differences were confirmed .For the 2018 harvest of August Flame there were no significant differences in the fruit size vs. fruit per hectare relationships detected among systems . Linear models fit 2018 August Flame data better than other years and showed a clear negative correlation between fruit size and fruit per hectare . In 2019 there was a wide spread of mean fruit sizes per tree in the August Flame data and no significant differences occurred among systems for the relationship between fruit size and fruit per hectare . Although the ANOVA analysis did not indicate differences among systems, linear models indicated a weak negative correlation between fruit size and fruit per hectare with all systems having near horizontal models accompanied by Rsquared values <0.1 .Although R-squared values for the linear models representing the relationship between fruit size and fruit per hectare were identical to those for fruit size and fruit per tree , large plastic pots for plants there were differences detected in the contrast analysis for slopes . Data for the June Flame 2017 and 2018 harvest seasons indicated no significant differences in the relationship for fruit size vs fruit count per tree among any of the systems . In 2019 there was a significant difference in the data for the June Flame cultivar between the C-6 Quad system and the Nema Quad system . In the 2017 harvest data of August Flame there was a significant difference in the fruit size vs. fruit per tree relationship among C-6 V and Nema Quad systems . The difference in 2017 data was visually apparent in the steeper slope indicated in the C-6 V system but that might be a result of the narrow range of fruit loads per tree in that system . No significant differences in the fruit size vs. crop load per tree relationship were detected in the harvest season of 2018, however both, C-6 Quad and C-6 V systems, had t. ratios indicating one may exist, but p-values remained above alpha, therefore a difference was not conclusive . Data for the 2019 harvest of August Flame indicated no significant differences in this relationship between systems, and in fact, the fruit size vs. crop load per tree relationship were most similar among systems in this year compared to other years .
A relationship between light interception and yield was most apparent in the June Flame cultivar with the C-6 Quad and C-9 Quad systems which produced data that fit linear models with the highest R-squared values. The linear model for the Nema Quad system had the steepest slope but not a very strong R-squared value. Data from the C-6 V system had a poor fit with a linear model. Interestingly the systems with data that had a poor fit to the model also had the highest % light interception, often >50% . August Flame cultivars showed a similar pattern for the relationship between amount of light intercepted and yield. Data from the Nema Quad and C-6 V systems had poor fits to the linear models but also had the highest light interception. Data from the C-9 Quad system had a moderate correlation between PAR and yield, fit the model best. The C-6 Quad system is an apparent outlier, having a value of almost 5 Kg/m2 yield with only about 40% light interception, and a very slight negative correlation between the two parameters . Both of the C-6 V systems with the June and August flame cultivars had trends as shown in previous research, higher density systems were able to intercept a higher proportion of light during earlier years because the trees fill their allotted space more quickly, .The mean fruit size for the June Flame cultivar in 2017 was similar among all systems, most likely a result of consistent thinning resulting in the desired crop loads per tree. In 2018 the mean fruit size for June Flame systems was exceptionally large, especially for an early bearing cultivar. Considering that the C-6 Quad and C-6 V systems had some of the largest fruit sizes provides strong evidence that size-controlling rootstocks are not always associated with reductions in fruit size. The C-9 system had poor performance in the trial but, with its success in previous studies and how well systems with the more size controlling rootstocks performed in this trial, it is likely not due to the reduced hydraulic conductance associated with size controlling rootstocks . June Flame systems in 2019 closely mirrored fruit sizes from the previous season, providing more confidence that any reduction in fruit size compared to the Nema Quad system is unlikely a result of size controlling rootstocks. The results from the June Flame cultivar are most promising because there were concerns that the size-controlling rootstocks may have the potential to have negative effects on fruit size in early maturing cultivars. With how quickly early bearing cultivars must set and mature fruit during the spring flush growth, there was concern that reduced hydraulic conductance associated with undeveloped xylem would influence fruit size . However, this trial did not provide evidence that early maturing cultivars on the size-controlling rootstocks produce smaller sized fruit compared to those on more vigorous rootstocks. With the August Flame cultivar, systems using size-controlling rootstocks also were not found to diminish fruit size in this later maturing cultivar. In 2017 all dwarfing systems performed beyond expectations.