APS_JANUARY2024

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20 J ournal of the A merican P omological S ociety Table 4. Mean fruit number and fresh fruit weight/elderberry plants after root 424 pruning and transplanting treatments in 2023. i 425 Fruit no./ Fruit wt./ 426 Treatment plant plant (g) Table 4. Mean fruit number and fresh fruit weight/elderberry plants after root pruning and transplanting treatments in 2023. i

427 428 429 430 431 432

NRP + NT

2281 b

161 b

NRP + T

3497 a

260 a

RP + T

570 c

41 c

i NRP = no root pruning, RP = root pruning, T = transplanting, NT = no transplanting. Means within a column followed by the same letter are not significantly different, P ≤ 0.05. Mean differences are based on a one degree of freedom F-test. 433 434 i NRP = no root pruning, RP = root pruning, T = transplanting, NT = no transplanting. 435 Means within a column followed by the same letter are not significantly different, P ≤ 436 0.05. Mean differences are based on a one degree of freedom F-test. 437

left in their original container. Although NRP + T plants were the most productive, their fruit yield per plant was very low (260 g/ plant) compared with that of plants of a simi lar age in a traditional field planting (2277 g/ plant) (Finn et al. 2008). In a conventional field planting of two-year-old plants with 2,777 plants/ha, an average fruit yield is 6323 kg∙ha -1 . Based on the yield from the NRP + T treatment in this study with pots spaced 90 cm within and between rows (12,345 pots/ha), fruit yield would be 3,210 kg∙ha -1 . High-density production systems gener ally have higher establishment and produc tion costs relative to low-density systems (Fang et al. 2020). Thus, our results suggest that modifications in the container production system would be necessary to increase fruit yield for profitable production of elderberry. For southern highbush blueberry, fruit yield was enhanced as the container size increased from 11.4 to 37.8 L (Motomura et al. 2016). In our study, elderberry fruit yield increased by 38% when the container volume was increased from 8.8- to 14.1-L, indicating that yield might also increase with a pot volume > 14.1-L. Another modification in the container pro duction system that might boost fruit yield would be the use of a different growing sub strate with an enhanced supply of nutrients or a more aggressive fertilization regime with elevated levels of targeted nutrients delivered more frequently via fertigation. To date, few if any, fertilizer or nutrition studies have been

conducted on container or field-grown Ameri can elderberry (Byers 2014). In the present study, roots and shoots were pruned on 4 Apr when some foliar growth had occurred due to warm temperatures beneath the overwintering material. In conventional field plantings of elderberry, annual cane pruning close to the soil surface in succes sive years had similar fruit yields compared with unpruned plants (Thomas et al. 2009). However, annual cane pruning reduced yield when compared with elderberry plants pruned in alternate years. In another study with apple (Young and Werner 1982), shoot pruning with or without root pruning resulted in the subse quent formation of many new roots but result ed in only a slight increase in root dry weight. In our study, shoot pruning was used to adjust each plant to a uniform number of shoots be fore treatments were applied. However, fruit yield may be enhanced in container-grown plants with no shoot pruning for two-year-old plants and prune in alternate years as they ma ture, using suitable pot anchorage. In addition to shoot pruning, the timing of root pruning impacts fruit set and yield (Saure 2007). While the effect of root pruning at vari ous growth stages of elderberry is unknown, Schupp and Ferree (1987) reported that root pruning as late as full bloom did not affect fruit set but reduced fruit size for apple trees planted in soil in the field. In American elder berry, fruit size is usually less important as it is sold in bulk by weight and processed into