Strawberry cultivars and breeding selections growing in tubs in a screenhouse with one genotype per tub.
Strawberry crowns per plant (P = 0.603), crowns plus runners per plant (P = 0.011), runners per plant (P = 0.004), daughters per plant (P < 0.0001), and daughters per runner (P = 0.215) produced from four cultivars (Camarosa, Chandler, Flavorfest, and Keepsake) grown from early August to late October in annual plasticulture and low-tunnel field production systems in Beltsville, MD, USA, in 2020, 2021 (Camarosa unavailable), 2022, and 2023. Asterisks indicate significant differences between production systems for the trait. 95% confidence intervals are shown.
Strawberry crowns per plant (P = 0.070), runners per plant (P < 0.0001), and crowns plus runners per plant (P = 0.002) produced from 10 cultivars grown from early August to late October in an annual plasticulture production system in Beltsville, MD, USA, in 2020, 2021, 2022, and 2023. Cultivar means with different letters indicate mean cultivar differences for runners per plant (white bars) or crowns plus runners per plant (gray bars). 95% confidence intervals are shown.
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Strawberry branch crown development is often associated with yield and berry size. The propagation rate of strawberry cultivars is key to their establishment and longevity in the industry. However, new cultivars often are released with little indication of these traits. This study aimed to characterize cultivars according to production of crowns, runners, and daughter plants, determine if cultivar plant part propagation would vary according to production system, determine if propagation of daughter plants would vary between field environments and production in tubs in a screenhouse, and determine how well subjective scores for vigor and runner production predict objective data from the same plots. Ten cultivars were grown in an annual plasticulture system for 4 years. Four of these cultivars were also grown in a low-tunnel production system in adjacent fields. Three months after planting, immediately after plot-based subjective rating for plant vigor and runner production was performed, the number of crowns, runners, and daughter plants were counted. The same 10 cultivars were planted each year in tubs of potting mix in a screenhouse. In late fall, all plants were dug from each tub and counted. Four cultivars, Camarosa, Chandler, Flavorfest, and Keepsake, were grown in plasticulture and under low tunnels. Those grown under low tunnels produced 37% more runners and 71% more daughters per plant than those grown in annual plasticulture, with no cultivar × system interaction. There was no evidence of differences between cultivars in terms of the number of daughters per runner (estimates of 1.5–2.6). Therefore, the number of runners determined the number of daughters produced. In plasticulture, ‘Camarosa’, ‘Chandler’, ‘Galletta’, and ‘Sweet Charlie’ produced the most runners, and ‘Allstar’, ‘Earliglow’, and ‘Flavorfest’ produced the fewest runners. Plants grown 6 months in tubs in a screenhouse produced between 4- and 13-fold more daughters per mother plant compared to plants grown 3 months in annual plasticulture. Subjective scores for runner production were well-associated with runner counts, but subjective scores for vigor were not well-associated with counts of crowns or crowns plus runners. The wide range of propagation rates, according to both cultivar and propagation methods, should assist nurseries in estimating propagation rates of these cultivars and encourage breeders to measure propagation rates of new cultivars.
The strawberry (Fragaria ×ananassa Duchesne ex Rozier) plant is a small herbaceous perennial with a fibrous root system (Guttridge 1955). Mature plants have a rosette form with a central compressed stem, or crown, at ground level that produces trifoliolate leaves and a terminal inflorescence. The axillary bud at the base of each leaf can remain dormant or produce either a branch crown or a runner with daughter plants, depending on genetics and environmental triggers (Hytönen et al. 2009; Konsin et al. 2001). Branch crown development is important because it is associated with eventual fruit yield (Bartczak et al. 2010). Runner development is important because it is essential for plant propagation.
Strawberry fruit is harvested from the following several diverse production systems: matted row, advanced matted row, annual plasticulture, low tunnels, high tunnels, greenhouses, and multiple developing forms of controlled environment agriculture (Black et al. 2002; Demchak 2009; Lewers et al. 2017b; Samtani et al. 2019; Tang et al. 2020). All growers who produce strawberry fruit need a source of strawberry plants to buy and plant in their fields. Some growers plant “bare-root dormant plants,” and some plant “plug plants” (Weber 2021). Both types of plants are primarily propagated from “daughter plants” formed on runners (Robertson and Wood 1954).
One of the most challenging stages of strawberry cultivar development occurs after the new cultivar is released, when propagation from a few plants to hundreds of thousands of plants must occur for the cultivar to reach the growers that it was intended to benefit. Nurseries need to be able to predict the rate at which they can expect cultivars to produce daughter plants (Kubota and Kozai 2001). This is especially important with new cultivars not previously propagated by a nursery. Some cultivars are released without information about runner and daughter plant production, and the information that is provided can be subjective or merely descriptive (Amyotte et al. 2022; Finn et al. 2018; Lewers et al. 2004; Whitaker et al. 2015, 2017).
When cultivars are released with runner or daughter production data, these data may not agree with the nursery experience (Lewers and Enns 2022; Lewers et al. 2019). This is not surprising because the runner production rate is affected by temperature, soil fertility, daylength, light intensity, light quality, and combinations of these factors (Cárdenas-Navarro et al. 2006; Durner et al. 1984; Janisch et al. 2012; Kim et al. 2010; Morrison et al. 2018; Naznin et al. 2016; Sønsteby and Heide 2007; Zheng et al. 2019). In addition, at different stages of propagation, several nurseries propagate plants using different methods that can affect the propagation rate, such as growing plants in hanging containers with runners hanging over the edges and growing plants in tubs with runners laying on the surface of the potting mix or soil (Zhao et al. 2020). Both strawberry nurseries and breeders are challenged to determine which measures of runner and daughter production are of greatest value to inform propagation rates at different stages of propagation and when using different growing environments.
Therefore, our objectives were to characterize cultivars according to production of crowns, runners, and daughter plants, determine if cultivar plant part propagation would vary according to the field growing system, determine if propagation of daughter plants would vary between field environments and production in tubs in a screenhouse, and determine how well subjective scores for vigor and runner production predict objective data from the same plots.
As part of an established strawberry breeding project, multiple virus-negative cultivars and breeding selections were planted in a screenhouse in late April from 2020 to 2023 (Fig. 1). The 10-m × 27-m screenhouse was 3.6 m high at its peak and arched like a hoophouse. The covering was Xsect Xtra fine-mesh screen (US Global Resources, Seatle, WA, USA). The screen had 0.15-mm × 0.21-mm holes small enough to exclude aphids and whiteflies, which are virus vectors, as well as thrips. The screen allowed rain to pass through, but it reduced air flow to 40% and light transmission to 85%. Light gray fiberglass tubs that were 37 cm wide × 54 cm long × 26 cm deep had six 1-cm drainage holes drilled through the bottoms. The tubs were filled half-way (13 cm deep) with dry Pro-Mix HP high-porosity potting mix (Premier Tech, Rivière-du-Loup, Quebec City, Canada). The tubs and potting mix were placed on heavy-duty plastic grid-benches in the screenhouse. The potting mix was moistened repeatedly with a garden hose and any rain that occurred. Two angled barb stake emitters (Netafim Irrigation, Inc., Fresno, CA, USA) per tub from a centralized irrigation line were placed in the potting mix. In early May, the tubs were planted with one genotype per tub and two or more plants that had spent 7 months in refrigerated storage at 1 °C. Two plants were planted per tub in 2020. In subsequent years, more plants were planted per tub if the genotype produced fewer daughter plants than desired. The plants had been dug from the previous year’s tubs in fall after showing signs of entering dormancy (reduced growth, yellowing and reddening of leaves), trimmed, labeled for return to the screenhouse, placed in labeled clear plastic bags that were closed with a rubber band and placed in a garbage bag, and stored over winter in a cooler set at 1 °C. One week after planting, plants were fertilized weekly with Jack’s Classic All-Purpose 20–20–20 fertilizer (20N–8.8P–16.6K) (J.R. Peters, Inc., Allentown, PA, USA) using a dositron set to deliver 100 ppm nitrogen (N). No fungicides were used.
Citation: HortScience 60, 5; 10.21273/HORTSCI18445-25
Other plants dug from the previous year’s tubs were used to generate the plants needed to establish field plantings. Typically, seven plants were estimated to be sufficient to produce the 24 daughter plants needed for four six-plant plots (three replications and a separate observation plot); however, more plants were needed for genotypes that historically produced fewer daughters. These plants were also trimmed, bundled, labeled, bagged, and refrigerated, but they were removed from storage in February, planted in square black plastic pots (11 cm width × 11 cm length × 12.5 cm depth) with one plant per pot, and grown on a greenhouse bench. The plants were fertilized weekly with 100 ppm N using Jack’s Classic All-Purpose 20–20–20 fertilizer (20N–8.8P–16.6K; J.R. Peters, Inc.). In May, these pots were topped with Osmocote Plus 15–9–12 Smart-Release fertilizer (Bloomington Brands LLC, Bloomington, IN, USA) just before they were transferred to suspended slanted 15-cm-wide rain gutters in a screened-off section of the screenhouse. Each plant was placed under a 7.6-L·h−1 drip emitter (01WPC8-B; Netafim, Fresno, CA, USA). A secondary irrigation source was at the top end of each slanted gutter to facilitate bottom watering of the plants; the gutter angle allowed free water flow. Plants were watered daily at noon for 0.5 h. Daughter plants growing from the gutter plants were pegged with the aid of hair pins into STI-1006 six-pack plastic standard inserts with wells 3.8 cm (width) × 3.8 cm (length) × 5.7 cm (depth) (T.O. Plastics, Clearwater, MN, USA). Propagation methods varied each year because of staff shortages and changes. In 2020 and 2021, the daughters were cut from the runners and established under mist in a greenhouse for 10 d, and then they continued growing without misting. In 2022, the daughters were pegged into six-pack containers resting on benches below the gutters in the screenhouse. In 2023, the daughters were pegged into six-packs resting on the edge of the gutters. Six-packs grown in the screenhouse needed very little water while attached to their mother plants and received rainwater, but they were gently hand-watered with a hose if the plants started to wilt slightly.
Plants of multiple cultivars and breeding selections were grown in replicated plots on the North Farm of the Henry A. Wallace Beltsville Agricultural Research Center (US Department of Agriculture, Agricultural Research Service) at Beltsville, MD, USA (lat. 39°01′48.42″N, long. 76°56′07.99″W, 49.4 m elevation) on Downer-Hammonton complex loamy sand and Russet-Christiana complex fine sandy loam soils (US Department of Agriculture, Natural Resources Conservation Service 2023). Lime, compost, and fertilizer were applied to the fields according to soil tests before tilling and shaping beds. Plantings were established annually by randomly applying one of two production systems to one field each of two replicate pairs of adjacent fields: an annual plasticulture system (Black et al. 2002) and a low-tunnel system (Lewers et al. 2020). Both systems used raised beds with trickle irrigation lines under plastic mulch. The annual plasticulture system used black plastic mulch, and the low-tunnel system used white plastic mulch. The low tunnels were covered with Kool Lite Plus 0.152-mm-thick polyethylene film (Klerks Hyplast Inc., Chester, SC, USA). Each replicate production system field had three beds devoted to replicated cultivar plots of six plants per plot. Each cultivar was planted once in each bed, and the assignment of the cultivars to plots in a bed was randomized. Plug plants were planted in six-plant plots in early Aug 2020, 2022, and 2023, and in late Aug 2021 because of insufficient heating in the greenhouse during Feb through Apr 2021. Plants in the low-tunnel field were fertigated weekly with Peters Professional 17–3–17 Peat-Lite Neutral Cal-Mag fertilizer (17N–1.3P–14.1K) (J.R. Peters, Inc.) at 2.2 kg·ha−1 N until mid-September, when fertigation ceased. One week after planting, the plasticulture field was fertigated weekly with the same fertilizer at rates sufficient to deliver 78 kg·ha−1 N by mid-September. One month after planting, Admire-Pro Systemic Protectant imidacloprid systemic insecticide (Admire® Pro; Bayer Crop Science, Research Triangle Park, NC, USA) was applied through chemigation at the labeled rate to prevent damage from root-chewing insects. No fumigants or fungicides were used in field establishment or after planting. Before runners grew to 30 cm long, they were blown away from the middle of the bed with a backpack leaf blower once per week. The sides of the low tunnels were not lowered during this study.
The following seven cultivars were planted all 4 years: ‘Chandler’ (USPP 4,481), ‘Cordial’ (USPP33,636 P2; Lewers and Enns 2022), ‘Earliglow’ (Scott and Draper 1975), ‘Flavorfest’ (Lewers et al. 2017a), ‘Galletta’ (USPP19,763 P2), ‘Keepsake’ (USPP30,578 P2; Lewers et al. 2019), and ‘USDA Lumina’ (USPP36,100 P2). Two additional cultivars, Camarosa (USPP8,708) and Sweet Charlie (USPP8,729), were planted in 3 of the 4 years, and Allstar (Galletta et al. 1981) was planted in 2 of the 4 years. Virus-indexed starts of cultivars developed by breeding programs from the University of California at Davis (‘Camarosa’ and ‘Chandler’), North Carolina State University (‘Galletta’), and University of Florida (‘Sweet Charlie’) were obtained from the Raleigh, NC, USA, Center of the National Clean Plant Network: Berries. All 10 cultivars were planted in the annual plasticulture system, and four of the cultivars, Camarosa, Chandler, Flavorfest, and Keepsake, were also planted in the low-tunnel production system.
During the last week of October each year, field plots were rated subjectively for vigor (scale, 0–9) and runner production (scale, 0–5) using half-point scales (Table 1). The vigor rating was based on plant growth and density as well as runner production and was separate from the disease rating. The curved part of a bamboo walking cane was used to push against the foliage from the side to judge resistance and foliage density. The runner rating was closely aligned to the visual assessment of the number of runners growing from each plant (not an actual count), the number of daughter plants from the runners, and whether the daughter plants had multiple runners growing from them.
After the subjective data were collected, runners were removed from the six individual plants in each plot and counted. The number of daughters and crowns from each plant were also counted. For each plant, the number of daughters was divided by the number of runners to obtain an overall value for daughters per runner for that plant. In addition, the number of runners for each plant was added to the number of crowns for that plant. Then, an average for each trait was calculated for the plot (crowns per plant, runners per plant, crowns plus runners per plant, daughters per plant, daughters per runner).
Water to the tubs in the screenhouse was turned off to prevent the pipes from freezing after the plants in the tubs showed signs of dormancy, such as reduced growth and foliage turning an olive green with red edges. Plants were dug from the tubs as described previously, and the number of plants removed from each tub was counted. After several nights with below freezing temperatures, the remaining foliage was removed, and the tubs were moved to a walk-in cooler set at 2 °C and stacked so the top tubs would not rest on the plants. Over the winter, the tubs were dug and all plants were counted. The number of plants dug from the tubs minus the number of mother plants planted was divided by the number of mother plants planted to obtain an average number of daughter plants produced per mother plant.
Analyses of variance models of count values observed for each trait of interest were conducted using generalized linear mixed-effects models that specified a negative binomial distribution and log link function with all mean estimates conditioned on random effects in the model (Stroup et al. 2024). Initially, year was specified as a fixed effect to examine relative cultivar performance specific to the observed years and characterize any observed cultivar × year interactions. Subsequently, year was specified as a random effect using the observed years as a random sample from a population to achieve broad inference for characterizing relative cultivar performance. Means estimates were reported with 95% confidence intervals (CIs) instead of standard errors because, unlike normal distribution, the counts observed for the traits had skewed (i.e., asymmetric) distributions, with smaller variability for counts less than the mean and larger variability for counts greater than the mean. All models were fit using the SAS “proc glimmix” command (SAS version 9.4; SAS Institute Inc., Cary, NC, USA).
An analysis was conducted to determine if there was an effect by production systems on the development of crowns, runners, daughter plants, the sum of crowns and runners per plant, and/or the number of daughters per runner. The analysis used data from four cultivars planted in both production systems each of 4 years, except for Camarosa, which was unavailable in 2021. The model specified cultivar and system as fixed effects. Year and plot in a bed were specified as random effects. To determine if there were any cultivar × year interaction effects, an analysis was conducted using data from seven cultivars planted in the annual plasticulture system all 4 years. The model specified cultivar and year as fixed effects, with plot in a bed specified as a random effect. To characterize differences among all 10 cultivars grown in the annual plasticulture system, another analysis was conducted with cultivar as the only fixed effect and year and plot in a bed specified as random effects. Because the tubs grown in the screenhouse were not replicated within year, the data from the 10 cultivars grown there were analyzed by specifying cultivar as a fixed effect and year (as the only form of replication) as a random effect to determine cultivar differences for the number of daughter plants produced. To determine how well subjective scores predicted counts of plant parts, a linear correlation analysis was conducted using the subjective data and the count data from individual plots.
When four cultivars, namely, Camarosa, Chandler, Flavorfest, and Keepsake, in both plasticulture and low-tunnel production systems were evaluated with year as a random effect, there was no evidence of any cultivar × production system interaction effect for the number of crowns (P = 1.000), number of runners (P = 0.658), number of crowns plus runners (P = 0.753), number of daughters per runner (P = 0.972), or number of daughters (P = 0.611). Therefore, the effects of production system and cultivar could be viewed separately. The four cultivars produced more runners per plant under low tunnels than in annual plasticulture (P = 0.004) (Fig. 2). The mean estimated number of runners per plant under low tunnels was 3.7 (95% CI, 1.4–9.3); however, in annual plasticulture, the mean was 2.7 (95% CI, 1.0–6.8). The increased number of runners under low tunnels was related to an increased number of runners plus crowns under low tunnels (P = 0.011). However, there was no evidence of any difference in the number of crowns per plant grown in the two production systems (P = 0.603).
Citation: HortScience 60, 5; 10.21273/HORTSCI18445-25
Plants grown under low tunnels produced 1.6 (95% CI, 1.3–2.1) crowns per plant and 5.9 (95% CI, 3.6–9.5) crown plus runners. Plants grown in annual plasticulture produced 1.8 (95% CI, 1.4–2.3) crowns per plant and 4.7 (95% CI, 2.9–7.6) crowns plus runners. The increased number of runners under low tunnels was also the main contributor to a higher number of daughters produced by each plant under low tunnels (P < 0.0001) (Fig. 2) because there was no evidence of a difference in the number of daughters per runner between the two production systems (P = 0.215). Under low tunnels, plants produced 2.4 (95% CI, 1.9–3.0) daughters per runner and 8.9 (95% CI, 2.9–28.0) daughters per plant. In annual plasticulture, plants produced 2.0 (95% CI, 1.6–2.6) daughters per runner and 5.2 (1.6, 16.3) daughters per plant for these four cultivars grown in both production systems.
Although there was no evidence of cultivar differences in the number of crowns plus runners per plant (P = 0.189), Flavorfest and Keepsake cultivars produced more crowns per plant [2.8 (95% CI, 2.2–3.6) and 2.2 (95% CI, 1.7–2.9), respectively] than Camarosa and Chandler cultivars [1.3 (95% CI, 0.8–1.9) and 1.1 (95% CI, 0.7–1.6), respectively] (P = 0.000). Conversely, ‘Chandler’ produced more runners [4.5 (95% CI, 1.8–11.5)] than ‘Camarosa’, ‘Flavorfest’, and ‘Keepsake’ [3.1 (95% CI, 1.2–8.0), 2.7 (95% CI, 1.0–6.7), and 2.7 (95% CI, 1.0–7.0), respectively] (P = 0.000). ‘Camarosa’ and ‘Chandler’ also produced slightly more daughters per runner [2.8 (95% CI, 2.1–3.8) and 2.7 (95% CI, 2.0–3.5), respectively] than ‘Flavorfest’ [1.6 (95% CI, 1.1–2.4)], and ‘Keepsake’ [1.9 (95% CI, 1.4–2.6)] was not significantly different from those three cultivars (P = 0.045). As a result of increased runner production and daughters per runner, ‘Camarosa’ and ‘Chandler’ produced more daughters per plant [8.7 (95% CI, 2.2–27.7) and 11.8 (95% CI, 3.7–37.2), respectively] than ‘Flavorfest’ and ‘Keepsake’ [4.2 (95% CI, 1.3–13.3) and 5.0 (95% CI, 1.6–15.9), respectively] (P < 0.0001).
With no interaction effects between cultivars × production systems for the number of crowns, runners, crowns plus runners, daughters, or daughters per runner, cultivar comparisons of these traits were expanded to include six more cultivars that were grown only in the plasticulture system. These six cultivars were Allstar, Cordial, Earliglow, Galletta, Sweet Charlie, and USDA Lumina. Regardless of whether year was considered a fixed effect (with statistical inference made only to the specific year observed) or a random effect (using data from these observed years as samples from the population of years to make a broad statistical inference), there were differences between cultivars in the number of runners [P = 0.001 (fixed); P < 0.0001 (random)], crowns plus runners [P = 0.036 (fixed); P = 0.002 (random)], and number of daughters per plant [P < 0.0001 (fixed); P < 0.0001 (random)]. As with the comparison of four cultivars in two production systems, there was no evidence of cultivar differences with regard to the number of crowns per plant [P = 0.74 (fixed); P = 0.070 (random)]. However, there was also no evidence of cultivar differences for the number of daughters per runner [P = 0.838 (fixed); P = 0.509 (random)] in this expanded comparison of 10 cultivars in only one production system.
A comparison of cultivars for runner production and crown number revealed that Sweet Charlie produced the most runners, with an estimated mean of 3.9 (95% CI, 1.5–10.3), followed closely by Galletta, USDA Lumina, Chandler, and Camarosa, with estimates of 3.9 (95% CI, 1.5–10.0), 3.8 (95% CI, 1.5–9.7), 3.7 (95% CI, 1.4–9.6), and 2.9 (95% CI, 1.1–7.6) runners per plant, respectively; none of them was significantly different from the others (Fig. 3). The number of crowns per plant ranged from 1.1 to 2.9, with no evidence of cultivar differences [P = 0.74 (fixed); P = 0.070 (random)]. Adding the number of crowns to the number of runners each plant produced identified the same cultivars; the cultivar Sweet Charlie had the most crowns plus runners, with an estimated mean of 6.9 (95% CI, 4.0–11.8), followed closely by cultivars USDA Lumina, Galletta, Chandler, and Camarosa, with estimates of 6.1 (95% CI, 3.7–10.2), 5.5 (95% CI, 3.3–9.2), 5.2 (95% CI, 3.1–8.8), and 4.9 (95% CI, 2.8–8.3) runners per plant, respectively (Fig. 3). ‘Earliglow’ produced the fewest number of runners, with an estimate of 1.2 (95% CI, 0.4–3.3) per plant, although it was not significantly different from ‘Allstar’ and ‘Flavorfest’, with estimates of 1.7 (95% CI, 0.6–4.7) and 2.0 (95% CI, 0.7–5.2) runners per plant, respectively. ‘Cordial’ and ‘Keepsake’ produced runner numbers that were not significantly different from each other at 2.3 (95% CI, 0.9–6.1) and 2.3 (95% CI, 0.9–6.0) runners per plant, respectively; these estimates were also not significantly different from those of ‘Camarosa’, ‘Flavorfest’, and ‘Allstar’. ‘Earliglow’ also produced the fewest number of crowns plus runners, with an estimate of 2.9 (1.7, 5.1), which was not significantly different from the estimates of ‘Keepsake’, ‘Flavorfest’, ‘Cordial’, and ‘Allstar’, which were 4.4 (95% CI, 2.6–7.4), 4.3 (95% CI, 2.6–7.4), 4.0 (95% CI, 2.6–6.9), and 3.3 (95% CI, 1.8–5.9) crowns plus runners, respectively. There was no evidence of any interaction effect between cultivars × years for runners (P = 0.432) or crowns plus runners (P = 0.110).
Citation: HortScience 60, 5; 10.21273/HORTSCI18445-25
Subjective runner scores (Table 1) assigned to the plots were significantly correlated with actual runner counts from plants of the same plots (r = 0.65; P < 0.0001). Subjective vigor scores (Table 1) of the same plots were not closely associated with counts of either crowns (r = 0.38; P < 0.0001) or crowns plus runners (r = 0.39; P < 0.0001).
The number of daughters per runner ranged from 1.5 to 2.6, with no evidence of differences between cultivars [P = 0.838 (fixed); P = 0.509 (random)]. Therefore, the cultivars that produced the most daughters per plant were among those that produced the most runners per plant. ‘Sweet Charlie’ produced the most daughters, followed by ‘Chandler’, ‘Camarosa’, and ‘Galletta’, with no significant differences between them (P < 0.0001) (Table 2). The number of daughters produced by ‘USDA Lumina’, which was also among the cultivars with the most runners, was not significantly different from that of ‘Chandler’, ‘Camarosa’, and ‘Galletta’. The cultivars with the fewest daughters per plant also produced the fewest runners per plant. The number of daughters produced by ‘Earliglow’ was not significantly different from that of ‘Flavorfest’ and ‘Allstar’.
When year was considered as a fixed effect, there was a significant cultivar × year interaction effect for the number of daughters per plant (P = 0.009). ‘Sweet Charlie’ and ‘Camarosa’ were not used in this analysis because they were not planted in the field all 4 years. ‘Chandler’ and ‘Galletta’ were among the top producers of daughter plants 3 of the 4 years, as was ‘USDA Lumina’ (Table 3). ‘Allstar’ was not included in this analysis because it also was not planted all 4 years, but ‘Earliglow’ was among the lowest producers of daughter plants all 4 years, and ‘Flavorfest’ was among the lowest producers 3 of 4 years. The number of daughters per plant was much lower in 2021 and 2023 compared with the number of daughters produced in 2020 and 2022 (Table 3). In 2021, there was insufficient heating in the greenhouse when the plants were being grown for later production of the plug plants for the field; therefore, the field planting was delayed from early August to late August. In 2023, anthracnose crown rot (Colletotrichum siamense) infected plants in the greenhouse that were grown to produce plug plants for the field. Without the use of any fungicides in the greenhouse or field, the surviving plugs that were planted in the field did not grow as well as they might have if they were not infected.
The number of daughters produced from mother plants of the 10 cultivars established in tubs in the screenhouse were generally in agreement with the number of daughter plants grown in plasticulture in the field, at least in terms of the relative ranking of cultivars (Table 3). For example, Sweet Charlie and Galletta were among cultivars that produced the most daughter plants in both screenhouse tubs and in annual plasticulture, and Earliglow and Flavorfest were among the cultivars that produced the fewest daughters in both the screenhouse and the field. The correlation was different from zero (P = 0.001), but it was not strong (r = 0.36).
This study provides further evidence that genetics and environmental triggers determine whether the axillary bud at the base of each leaf remains dormant or produces either branch crowns or runners (Konsin et al. 2001). With no genotype × environment (cultivar × production system) interaction effect in this study for number of crowns (P = 1.000), number of runners (P = 0.658), or number of crowns plus runners (P = 0.753), the effects of genotype (cultivar) and environment (production system) can be viewed separately. There was a genotypic (cultivar) effect for the number of crowns (P = 0.000) and the number of runners (P = 0.000), with ‘Chandler’ producing more runners and ‘Flavorfest’ and ‘Keepsake’ producing more crowns. However, there was no evidence of any difference between the total number of crowns plus runners (in other words, the number of axillary buds formed and released from dormancy) among the four cultivars (P = 0.189). There was also an environmental (production system) effect for the number of axillary buds (crowns plus runners) (P = 0.011) and the number of runners (P = 0.004) that developed from those axillary buds. However, there was no evidence of additional crown formation under the low tunnels compared with that in plasticulture (P = 0.603). Indeed, there were slightly more crowns produced in plasticulture (1.8; 95% CI, 1.4–2.3) than under low tunnels (1.6; 95% CI, 1.3–2.1).
The four cultivars produced approximately 37% more runners [3.7 (95% CI, 1.4–9.3) vs. 2.7 (95% CI, 1.0–6.8)] and 71% more daughters [8.9 (95% CI, 2.9–28.0) vs. 5.2 (95% CI, 1.6–16.3)] when grown under low tunnels than when grown in annual plasticulture. This finding was opposite of what was found by Orde and Sideman (2019), but there were several differences between these two studies. This study used four once-fruiting cultivars grown on black mulch without a low tunnel instead of white mulch with a low tunnel, and runners were counted after approximately 3 months. Orde and Sideman (2019) used repeat-fruiting cultivars grown on black mulch, with and without low tunnels, and removed and counted runners once per month from as early as June to as late as October. In addition, Orde and Sideman (2019) found that while all cultivars produced fewer runners under their low tunnels, the effect was not significant for all cultivars. By combining the results of these two studies, it may be inferred that their differential findings could be primarily attributable to the use of once-fruiting rather than repeat-fruiting cultivars. However, Durner et al. (1984) found no interaction effect between photoperiod and the type of plant, described as day-neutrals, June bearers, and everbearers. In the current study, although there was no evidence of interaction effects between any evaluated traits of cultivars and production systems, it was limited to four cultivars and the limited environmental differences in these two production systems. Taken together, these studies added to the complexity of predicting daughter plant production rates for nurseries and runner tip producers.
The number of daughters or runner tips that can be produced from a single mother plant is dependent on the number of runners produced. This study supports an understood cultivar effect on those numbers. Although it is impossible to predict these numbers relative to the production environment of a specific nursery or tip producer, when a new cultivar is released, it is important that breeders provide an estimate of the number of runners and daughters that can be expected relative to older cultivars grown in a similar environment. Upon their release, the runner production of ‘Cordial’ and ‘Keepsake’, based on subjective scores, was similar to that of ‘Flavorfest’ and less than that of ‘Chandler’ and ‘Galletta’ (Lewers et al. 2019; Lewers and Enns 2022). Those findings were consistent with the runner count data of this study, which were significantly correlated with this study’s subjective score assignments (r = 0.65; P < 0.0001). Subjective scores for runner production provided in Table 1 could be considered a viable substitute for runner counts when resources are limited, but counts would be ideal.
There was no evidence of differences in the number of daughter plants per runner among the 10 cultivars grown in plasticulture only [P = 0.838 (fixed); P = 0.509 (random)] or between plasticulture and low tunnels (P = 0.215). The range of 10 cultivar means was 1.5 to 2.6 daughter plants produced per runner when grown in plasticulture in approximately 3 months from early August (most years) to the end of October. During 3 months, the mean number of daughters per runner of four cultivars produced in low tunnels was 2.4, but it was 2.0 in plasticulture. Therefore, if growing plants for 3 months for tip production, then it would be reasonable to harvest all runners at once when there are approximately two daughters per runner and both daughters have fresh and viable root initials.
Counts of daughter plants produced in the screenhouse were generally in agreement with those from plants grown in the field. Of greater interest is that the number of daughter plants produced in screenhouse tubs was much greater than the number of daughter plants produced in the plasticulture field, even when the different amounts of growing time are considered. In the screenhouse, plants were established in early May and produced daughter plants 6 months into late October. In the field, plants were established in early August and produced daughters in 3 months, which was approximately half the amount of time. Based on this amount of time, plants in the screenhouse could be expected to produce twice as many plants in the field. Depending on the cultivar, between 4- and 13-fold the number of daughters were produced in the screenhouse compared with the field (Table 3). Several factors could account for this difference. The most obvious is that daughters produced in tubs in the screenhouse are encouraged to root so that they can produce multiple runners. Daughter plant production in a plasticulture field discourages daughter plants from rooting. Other obvious factors include planting in spring because cool weather allows plant growth before the plants start to produce runners, a screen over the screenhouse that reduced light transmission to 85%, and planting in tubs that, along with the screen that reduced airflow to 40%, buffered the plants from desiccation, especially the daughter plant’s young roots. For whatever reason, the higher daughter production rate that occurred in screenhouse tubs indicated that this method may be beneficial for an initial propagation stage to increase plant numbers of a new cultivar. Because of the many diverse strawberry production systems and strawberry propagation systems in place and predicted in future (Samtani et al. 2019; Weber 2021), it may be advisable to count daughters in different growing environments. Counting daughters in environments that allow daughters to root and grow their own daughters may be more informative when predicting how cultivars will perform in tubs for nurseries and in matted rows for growers. Counting daughters in environments that do not allow daughters to root may be more informative when predicting how cultivars will perform in hanging pots or gutters for nurseries or in plasticulture for growers.
Strawberry cultivars and breeding selections growing in tubs in a screenhouse with one genotype per tub.
Strawberry crowns per plant (P = 0.603), crowns plus runners per plant (P = 0.011), runners per plant (P = 0.004), daughters per plant (P < 0.0001), and daughters per runner (P = 0.215) produced from four cultivars (Camarosa, Chandler, Flavorfest, and Keepsake) grown from early August to late October in annual plasticulture and low-tunnel field production systems in Beltsville, MD, USA, in 2020, 2021 (Camarosa unavailable), 2022, and 2023. Asterisks indicate significant differences between production systems for the trait. 95% confidence intervals are shown.
Strawberry crowns per plant (P = 0.070), runners per plant (P < 0.0001), and crowns plus runners per plant (P = 0.002) produced from 10 cultivars grown from early August to late October in an annual plasticulture production system in Beltsville, MD, USA, in 2020, 2021, 2022, and 2023. Cultivar means with different letters indicate mean cultivar differences for runners per plant (white bars) or crowns plus runners per plant (gray bars). 95% confidence intervals are shown.
Contributor Notes
This project was funded by USDA-ARS Project 8042-21220-257-00D and 8042-21220-260-00D. We thank Philip Edmonds, Nhi (Amy) Tran, Turner Smith, and the BARC Research Farm Services for field establishment and maintenance. We also thank the National Clean Plant Network: Berries North Carolina Center for some virus-tested plants, Marvin Pritts, David Fleisher, Amanda Owens, Bill Owens, and Sam Erwin for their helpful comments and advice.
Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the US Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that also may be suitable.
Strawberry cultivars and breeding selections growing in tubs in a screenhouse with one genotype per tub.
Strawberry crowns per plant (P = 0.603), crowns plus runners per plant (P = 0.011), runners per plant (P = 0.004), daughters per plant (P < 0.0001), and daughters per runner (P = 0.215) produced from four cultivars (Camarosa, Chandler, Flavorfest, and Keepsake) grown from early August to late October in annual plasticulture and low-tunnel field production systems in Beltsville, MD, USA, in 2020, 2021 (Camarosa unavailable), 2022, and 2023. Asterisks indicate significant differences between production systems for the trait. 95% confidence intervals are shown.
Strawberry crowns per plant (P = 0.070), runners per plant (P < 0.0001), and crowns plus runners per plant (P = 0.002) produced from 10 cultivars grown from early August to late October in an annual plasticulture production system in Beltsville, MD, USA, in 2020, 2021, 2022, and 2023. Cultivar means with different letters indicate mean cultivar differences for runners per plant (white bars) or crowns plus runners per plant (gray bars). 95% confidence intervals are shown.
