Abstract
We compared the performance of Brussels sprout (Brassica oleracea var. gemmifera) cultivars in New Hampshire and evaluated the effects of topping (apical meristem removal) on marketable yields. A total of 23 cultivars were evaluated in the study, with 8 to 16 cultivars evaluated in any given year. We identified several cultivars that produced moderate to high yields of well-spaced, uniform sprouts that had few Alternaria blight (Alternaria sp.) symptoms, and identified many others, including all red cultivars evaluated, that produced very low yields consistently. In 2013, 2014, and 2015, we used a replicated split-plot experimental design with cultivar as the main plot and topping treatment as the subplot, to evaluate the effects of topping plants. Early and midseason cultivars showed increased yields in response to topping, unless topping was performed too early. Cultivars with sprouts that did not reach marketable size within our growing season generally produced low yields, and topping had no effect on yields. To explore the effects of topping at different dates, we evaluated three cultivars on seven different topping dates plus an untopped control in 2015 and 2017. In addition to reducing stalk height by limiting late-season growth, topping affected marketable yields by affecting the number of sprouts that were either undersized or oversized. The ideal topping date window for minimizing defects and maximizing yields varied slightly for each cultivar, ranging from early to late September.
Brussels sprouts (Brassica oleracea var. gemmifera) are grown for the enlarged axillary buds that form along the main stem or stalk. In the northeastern United States, plants are typically seeded in April or May and harvested from September to November. Harvesting after a frost is recommended because this reduces the sprouts’ bitterness (Fenwick et al. 1983), but freeze damage can occur if plants are exposed to temperatures less than –10 to –15 °C, resulting in a narrow harvest window for growers in cold climates (Kronenberg 1975).
Planting date and cultivar selection affect total yield potential, timing of bud enlargement, and optimal harvest date (Everaarts and Sukkel 1999). The commercial availability of cultivars has changed rapidly in recent years, with some cultivars dropping from the market and several new ones being added. Selection and breeding now takes place primarily in The Netherlands and the United Kingdom, resulting in cultivars that are well adapted to mild climates with long growing seasons (Mou 2022). Published information about performance of newer cultivars in cold climates with short and humid growing seasons, such as those found in the northeastern United States, is limited to two studies: Brown (2013) evaluated six cultivars in one season in Rhode Island, USA, and Ostfeld (2014) evaluated six Brussels sprout cultivars in upstate New York, USA, over 2 years.
One common limitation to the production of Brussels sprouts in the northeastern United States is Alternaria blight or dark leaf spot, caused by Alternaria sp. (Saharan et al. 2015). This pathogen is seedborne and can survive in crop residues (Köhl et al. 2011). Disease severity varies between seasons and regions, in part because environmental conditions greatly affect pathogen development and because isolates are reported to vary widely in virulence (Fatima et al. 2019). Braverman (1977) evaluated resistance to Alternaria brassicicola in Brussels sprout plant introduction lines and the commercial cultivars Jade Cross and Catskill, and identified variation in susceptibility and symptom development. Aside from that report, the scientific literature lacks information about the varying susceptibility to Alternaria blight among Brussels sprout cultivars.
Brussels sprouts may be harvested by the whole stalk or multiple times by stripping the sprouts from the stalk when they reach marketable size. When plants are harvested by the whole stalk (such as for mechanical harvest or for retail sales on-the-stalk), the apical meristem of the plant is often removed to stimulate bud growth, increase sprout uniformity, and increase yields (Jakopic et al. 2016; Metcalf 1954). Removing the apical meristem of the plant (variably referred to as topping, stopping, or decapitation) causes a loss of apical dominance by removing the source of auxin, and redirects energy from vegetative growth to enlarging the already present sprouts (Jones 1972; Thomas 1983).
To increase yield, topping must occur at the right time. Early removal causes terminal sprouts to bolt and limits stalk length, whereas late removal has no effect because plants have stopped growing (Jones 1972). The criteria used to select the topping date varies widely among extension recommendations and research studies, and include dates ranging from late August through September in England and mid-September in Kentucky, USA (Combs and Ernst 2019; Fisher 1974); the number of expanded leaves (Thomas 1983); or a minimum basal sprout diameter (Combs and Ernst 2019; Frappell and Richardson 1969; Murphy 1971). Everaarts (1994), when reviewing the available literature on topping Brussels sprout, concluded that more research was needed to evaluate the effects of topping on modern cultivars, and that the criteria used to choose timing of topping should be based on plant physiological stages in relation to the intended harvest period.
With new cultivars available to commercial growers, and little information publicly available, our objectives were to compare several cultivars of Brussels sprouts in northern New England, USA, and to evaluate the effects of topping and topping date on marketable yields. In addition to marketable yields, we also evaluated cultivar response to Alternaria sp., which can be a common limitation to production.
Materials and methods
Site description
Experiments were conducted in 2013, 2014, 2015, 2017, and 2021 in different fields at the University of New Hampshire Woodman Horticultural Research Farm in Durham, NH, USA (lat. 43°15′N, long. 70°93′W). The site is in US Department of Agriculture hardiness zone 5B (US Department of Agriculture 2012) and with a soil type of Charlton fine sandy loam with 3% to 8% slopes.
Crop management
Soil was amended each year based on soil test results, following regional nutrient recommendations for cruciferous crops [Brassicaceae (Campbell-Nelson et al. 2020)]. Total rates of 130 to 160 lb/acre of nitrogen (N) and 104 to 125 lb/acre of potassium (K) were applied each year, using sulfate of potash–magnesium (0N–0P–18.3K), muriate of potash (0N–0P–49.8K), soybean meal (7N–0.4P–1.7K), Chilean nitrate (16N–0P–0K), and/or calcium ammonium nitrate (27N–0P–0K) in various combinations based on availability and to meet nutrient needs. Fertilizers were applied and incorporated before bed formation. Raised beds measuring ∼10 cm high and ∼60 cm wide were laid in the spring and covered by 0.6-mm embossed, biodegradable black plastic mulch (Bio360; Dubois Agrinovations, Saint-Rémi, Quebec, Canada) for weed control, as is standard practice in the region. Beds were equipped with a single line of drip irrigation tape with 12-inch emitter spacing. Plants were irrigated weekly to provide 1 inch of water per week when natural rainfall did not provide that amount.
In all years, the biological insecticide Bacillus thuringiensis (Dipel DF; Valent BioSciences LLC, Libertyville, IL, USA) was used as needed to manage caterpillars, primarily imported cabbageworm (Pieris rapae), and diamondback moth (Plutella xylostella). Cabbage aphid (Brevicoryne brassicae) populations were present in 2015 and 2021, and in those years, azadirachtin (Aza-Direct; Gowan Co., Yuma, AZ, USA), azadirachtin with pyrethrum (Azera; MGK, Minneapolis, MN, USA), or potassium salts of fatty acids (MPede, Gowan Co.) or insecticidal soap (Safer® Brand Insect Killing Soap; Woodstream Corp., Lititz, PA, USA) were applied to manage cabbage aphids.
Lower leaves (the bottom third of each plant) were trimmed from all plots once in 2013, 2015, and 2017, and twice in 2014, to improve air circulation. Leaves were not trimmed in 2021 because of high levels of leaf drop resulting from moderate to severe Alternaria blight infections in many cultivars.
Cultivar evaluation
A total of 23 cultivars were evaluated, with 8 to 16 cultivars evaluated in any given year. Our goal was to select a wide array of cultivars that were commercially available in our region, based on grower interest and expert recommendations. Over time, cultivars were dropped because they were no longer available or because of very poor performance, and cultivars were added as new ones were released. Cultivars evaluated and seed sources are specified in Table 1. Seeding and transplanting dates and other experimental details are shown in Table 2. In 2013, 2014, and 2015, the experimental design was a replicated split plot including four complete blocks, with cultivar as the main plot and topping treatment as the subplot. During these 3 years, each subplot/experimental unit consisted of six plants. In 2021, we used a randomized complete block design with four blocks, and all plants were topped using criteria informed by experiments conducted in previous years. Each main plot contained 12 plants in a single row, with 18 inches between plants in row and 6 ft between rows.
Brussels sprout cultivars evaluated in 2013–21 in Durham, NH, USA.
Experimental details for Brussels sprout cultivar evaluations conducted from 2013 to 2021 in Durham, NH, USA.
For cultivar evaluation experiments, plant topping was performed by removing ∼1 inch of the plant top, including the apical meristem. In 2013 and 2014, all cultivars were topped when basal sprouts first exceeded a 1-inch diameter at the base. In 2015, we used the same criteria, but topped no earlier than mid-September. In 2013–15, using these criteria resulted in different cultivars being topped on different dates (Tables 2 and 3). In 2021, we topped all cultivars in mid-September.
Experimental details for Brussels sprout topping date experiments conducted in 2015 and 2017 in Durham, NH, USA.
Topping experiments
To explore the effects of topping at different dates, we conducted a separate study in 2015 and 2017, evaluating seven different topping dates ranging from 24 Aug to 23 Oct 2015, and from 28 Aug to 11 Oct 2017, respectively, including an untopped control. In 2015, the cultivars Jade Cross E and Nautic were used, and in 2017, Diablo was used. In both years, we used a split-plot design with cultivar as the main plot and topping date as the subplot, with an experimental unit of six plants.
Data collection
Harvest took place in late fall, after plants had experienced several frosts and had stopped growing, before nighttime temperatures fell below 20 °F. Harvest occurred on 6 Nov 2013, between 19 Oct and 18 Nov 2014, 15 Nov 2015, and between 16 and 23 Nov in both 2017 and 2021 (Table 1). Stalks were cut ∼2 inches above the ground for harvest, and height was recorded. For each plot, we estimated the total percentage of sprouts that were too small (diameter, < 0.75 inch), too large (diameter, > 2.5 inches), or showed symptoms of dark leaf spot. All Brussels sprouts of a marketable size (regardless of pest/disease incidence) were removed from each stalk and weighed in a once-over harvest.
Minimum and maximum daily temperature data were collected from a weather station located in Durham, NH, USA (University of New Hampshire 2022) and were used to calculate growing degree days (GDDs) using a base temperature of 50 °F (GDD50) that accumulated between transplant and harvest. This was calculated using the formula GDD50 = (Tmax + Tmin)/2 – 50 °F. When Tmin < 50 °F, GDD = 0.
Data analysis
Data from different years were analyzed separately because of year-to-year variability in cultivars included. Using statistical software (JMP Pro ver. 16; SAS Institute, Cary, NC, USA), analysis of variance was used to evaluate the effects of cultivar (main plot), topping (subplot), and the interaction on yield (weight of marketable sprouts per plant) and the estimated percentage of sprouts that were undersized or oversized. Percent values were arcsine-transformed before analysis to improve normality, and untransformed means are presented. Block was considered a random effect in all models. When an overall F test was significant (P < 0.05), means were compared using Tukey’s honestly significant difference test at the P ≤ 0.05 level. For percentage of sprouts showing Alternaria blight symptoms, all means were compared with a control using Dunnett’s multiple comparisons procedure, and Dunnett’s P values are presented. ‘Jade Cross E’ was selected as a benchmark comparator in all years because it has been reported previously to be susceptible to Alternaria brassicicola (Braverman 1977) and because it was included in all years of our study. In the case of a significant interaction among treatments, results are explained or presented separately for all treatment combinations.
Results
Marketable yield
Total yields of marketable sprouts in a once-over harvest varied by year and cultivar (Table 4). When we analyzed data for the three cultivars that were grown in all 4 years (‘Diablo’, ‘Jade Cross E’, and ‘Nautic’) separately, we found that year, cultivar, and their interaction were all significant (data not shown). As a result, we present the data for each year separately.
Mean marketable yields of several cultivars of Brussels sprout grown in Durham NH, USA, in 2013, 2014, 2015, and 2021.
In general, marketable yields were less in 2013 and 2014 than in 2015 and 2021. The differences among years included weather variations, planting dates (the 2013 experiment was planted 3–4 weeks later than the other experiments), and different cultivars being evaluated. The cumulative number of GDDs between transplant and harvest was comparatively low in 2013 (1790 GDDs) and comparatively high in 2021 (2557–2562 GDDs), with intermediate values in 2014 and 2015 (see Table 2). These differences could explain in part the observed variation in yield.
The three cultivars that were grown in all years (‘Diablo’, ‘Jade Cross E’, and ‘Nautic’) were consistently moderate to high yielding. Other cultivars that were among the highest yielding in each year they were evaluated included ‘Dagan’ (2021), ‘Divino’ (2021), ‘Early Marvel’ (2013, 2015), ‘Marte’ (2021), and ‘Octia’ (2014, 2015). Cultivars that had comparatively low yields in all years they were evaluated included ‘Catskill’, ‘Cryptus’, ‘Falstaff’, ‘Gladius’, ‘Igor’, ‘Nelson’, ‘Redarling’, ‘Redbull’, ‘Roodnerf’, and ‘Rubine’. The cultivars Confidant, Churchill, Gustus, and Hestia performed inconsistently. Notably, all purple cultivars evaluated (‘Falstaff’, ‘Redarling’, ‘Redbull’, and ‘Rubine’) produced extremely low yields in all experiments in which they were evaluated. In general, cultivars that produced low yields did so because they either produced sprouts that were excessively puffy and large (oversized) or because the sprouts did not reach marketable size (< 0.75-inch diameter, undersized).
The majority of cultivars produced uniform and well-spaced sprouts on relatively uniform plants, with a few exceptions. For example, ‘Catskill’ showed a great deal of variability consistently, with some plants developing large sprouts and others having sprouts that remained tiny throughout the growing season. ‘Igor’ had a few plants per plot in which many axillary buds failed to produce a sprout, resulting in an inconsistently filled stalk. Some cultivars—Confidant, Early Marvel, Nelson, Roodnerf, Redbull, and Rubine—frequently produced one or more large lateral shoots near the base of the plant, which resulted in additional stalks that failed to develop mature sprouts, requiring additional trimming. Last, ‘Jade Cross E’ had a unique growth habit among the cultivars evaluated. Although it was consistently high yielding, sprouts grew very close together and the base of the stalk was very thick, making both stalk lopping and sprout removal difficult.
Alternaria blight susceptibility
Because we did not use fungicides, Alternaria blight was always present in our experiments, enabling us to evaluate the percentage of symptoms on a diverse collection of cultivars in several years. In all experiments, the percentage of sprouts showing Alternaria blight symptoms was evaluated at harvest (Table 5). ‘Jade Cross E’ showed a moderate amount of Alternaria blight symptoms in all 4 years, ranging from 21.7% to 50.0% of sprouts showing symptoms at harvest. Using ‘Jade Cross E’ as a benchmark, we found that some cultivars consistently had significantly fewer symptomatic sprouts at harvest (e.g., ‘Diablo’, ‘Nautic’, ‘Roodnerf’), whereas others were not consistently significantly different from ‘Jade Cross E’ (e.g., ‘Gustus’, ‘Churchill’, ‘Octia’). Other cultivars had variable results, with comparatively fewer symptomatic sprouts in some years and higher levels in others. Last, several cultivars were evaluated in only 1 year, and as such, these data should be regarded as preliminary.
Mean percentage of sprouts showing Alternaria blight symptoms at harvest for several cultivars of Brussels sprout grown in Durham NH, USA, in 2013, 2014, 2015, and 2021.
Topping
The effects of topping and topping date on marketable sprout yield were evaluated in 2013, 2014, and 2015, and varied by cultivar (Fig. 1). In 2013, the cultivars that were topped on 10 Sep [64 d post-transplant (DPT)] produced higher yields than untopped plants. Cultivars that were topped later because basal sprouts had not reached a 1-inch diameter until mid-October did not respond. The experiment began 3 weeks earlier in the 2014 growing season, and some cultivars and were topped as early as 4 Aug, when the basal sprouts reached a 1-inch diameter. Those cultivars that were topped earliest (59 or 60 DPT) responded poorly, and marketable yields were reduced compared with untopped plants. For these cultivars, the top sprouts became overgrown and puffy, or, in the worst cases, they expanded to form multiple new stalks at the top of the original stalk. Those topped on 4 Sep (90 DPT) generally had more marketable sprouts than those topped at earlier dates (although these were not always significant). As in 2013, cultivars topped later in the season, on 23 Sep (109 DPT), did not respond to topping. Because early topping was consistently detrimental to yields, we delayed the earliest topping in 2015 until mid-September (14 Sep; 93 DPT), even though some basal sprouts reached marketable size earlier than that. There was a trend of favorable yield responses to early topping in 2015, but most comparisons were not significant, and cultivars topped later in the season typically did not respond, as in other years (Fig. 1).
Mean yield of marketable sprouts for several cultivars of Brussels sprout that were either topped when basal sprouts reached a 1-inch (2.5-cm) diameter or not topped. Experiments were conducted in Durham, NH, USA, from 2013 to 2015. Topping dates for each cultivar are provided. Asterisks denote cultivars for which topped and untopped yields differed significantly (Tukey’s honestly significant difference, P < 0.05); 1 g = 0.0353 oz.
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05170-22
Mean yield of marketable sprouts for several cultivars of Brussels sprout that were either topped when basal sprouts reached a 1-inch (2.5-cm) diameter or not topped. Experiments were conducted in Durham, NH, USA, from 2013 to 2015. Topping dates for each cultivar are provided. Asterisks denote cultivars for which topped and untopped yields differed significantly (Tukey’s honestly significant difference, P < 0.05); 1 g = 0.0353 oz.
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05170-22
Mean yield of marketable sprouts for several cultivars of Brussels sprout that were either topped when basal sprouts reached a 1-inch (2.5-cm) diameter or not topped. Experiments were conducted in Durham, NH, USA, from 2013 to 2015. Topping dates for each cultivar are provided. Asterisks denote cultivars for which topped and untopped yields differed significantly (Tukey’s honestly significant difference, P < 0.05); 1 g = 0.0353 oz.
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05170-22
Our results show that cultivars with sprouts that reach marketable size early in the season, such as ‘Churchill’, ‘Diablo’, ‘Early Marvel’, ‘Gustus’, ‘Jade Cross E’, and ‘Nautic’, had increased yields in response to topping, as long as topping was not performed early enough that new stalks would have time to grow before harvest. Later cultivars that did not produce large sprouts within our growing season, such as ‘Catskill’, ‘Doric’, ‘Falstaff’, ‘Igor’, ‘Redbull’, and ‘Roodnerf’, generally produced low yields, and topping neither increased nor decreased yields.
Topping date and response
To explore the relationship between topping date and marketable sprout yield systematically, we conducted two experiments in which plants were topped at seven dates throughout the growing season. Mean yield, stalk height, and percentage of undersized and oversized sprouts are shown for ‘Jade Cross E’ and ‘Nautic’ (2015), and ‘Diablo’ (2017), all of which were topped at all seven dates or not topped at all (Fig. 2). Photos of ‘Jade Cross E’ and ‘Diablo’ plants topped at all dates are presented in Fig. 3.
Mean yield of marketable sprouts, stalk height, and percentage of undersized and oversized sprouts for Brussels sprout plants topped at different dates. (A–C) Yield of marketable sprouts and stalk height. (D–F) Percentage of undersized and oversized sprouts. The cultivars Jade Cross E (A, D) and Nautic (B, E) were evaluated in 2015, and Diablo (C, F) was evaluated in 2017. For each trait, values marked with the same lowercase letter (for yield and undersized sprouts) or uppercase letter (for height and oversized sprouts) are not significantly different by Tukey’s honestly significant difference test (P ≤ 0.05); 1 g = 0.0353 oz, 1 inch = 2.54 cm.
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05170-22
Mean yield of marketable sprouts, stalk height, and percentage of undersized and oversized sprouts for Brussels sprout plants topped at different dates. (A–C) Yield of marketable sprouts and stalk height. (D–F) Percentage of undersized and oversized sprouts. The cultivars Jade Cross E (A, D) and Nautic (B, E) were evaluated in 2015, and Diablo (C, F) was evaluated in 2017. For each trait, values marked with the same lowercase letter (for yield and undersized sprouts) or uppercase letter (for height and oversized sprouts) are not significantly different by Tukey’s honestly significant difference test (P ≤ 0.05); 1 g = 0.0353 oz, 1 inch = 2.54 cm.
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05170-22
Mean yield of marketable sprouts, stalk height, and percentage of undersized and oversized sprouts for Brussels sprout plants topped at different dates. (A–C) Yield of marketable sprouts and stalk height. (D–F) Percentage of undersized and oversized sprouts. The cultivars Jade Cross E (A, D) and Nautic (B, E) were evaluated in 2015, and Diablo (C, F) was evaluated in 2017. For each trait, values marked with the same lowercase letter (for yield and undersized sprouts) or uppercase letter (for height and oversized sprouts) are not significantly different by Tukey’s honestly significant difference test (P ≤ 0.05); 1 g = 0.0353 oz, 1 inch = 2.54 cm.
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05170-22
Photos of Brussels sprout plants topped at different dates. (A) Shown, arranged from left to right and top to bottom, are ‘Jade Cross E’ plants grown in 2015, topped on 24 Aug, 4 Sep, 11 Sep, 17 Sep, 24 Sep, 2 Oct, and 23 Oct; and untopped plants. (B) Shown, arranged from left to right and top to bottom, are ‘Diablo’ plants grown in 2017 topped on 29 Aug, 4 Sep, 11 Sep, 18 Sep, 26 Sep, 3 Oct, and 11 Oct; and untopped plants. Photos were taken on 15 Nov 2015 (A) and 16 Nov 2017 (B).
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05170-22
Photos of Brussels sprout plants topped at different dates. (A) Shown, arranged from left to right and top to bottom, are ‘Jade Cross E’ plants grown in 2015, topped on 24 Aug, 4 Sep, 11 Sep, 17 Sep, 24 Sep, 2 Oct, and 23 Oct; and untopped plants. (B) Shown, arranged from left to right and top to bottom, are ‘Diablo’ plants grown in 2017 topped on 29 Aug, 4 Sep, 11 Sep, 18 Sep, 26 Sep, 3 Oct, and 11 Oct; and untopped plants. Photos were taken on 15 Nov 2015 (A) and 16 Nov 2017 (B).
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05170-22
Photos of Brussels sprout plants topped at different dates. (A) Shown, arranged from left to right and top to bottom, are ‘Jade Cross E’ plants grown in 2015, topped on 24 Aug, 4 Sep, 11 Sep, 17 Sep, 24 Sep, 2 Oct, and 23 Oct; and untopped plants. (B) Shown, arranged from left to right and top to bottom, are ‘Diablo’ plants grown in 2017 topped on 29 Aug, 4 Sep, 11 Sep, 18 Sep, 26 Sep, 3 Oct, and 11 Oct; and untopped plants. Photos were taken on 15 Nov 2015 (A) and 16 Nov 2017 (B).
Citation: HortTechnology 33, 2; 10.21273/HORTTECH05170-22
For all three cultivars, stalk height was reduced by topping early (Fig. 2A–C). Stalk height was significantly shorter when plants were topped on 11 Sep or earlier, than if they were topped on 17 Sep or later. In general, marketable yields of sprouts were maximized at intermediate topping dates, between 4 Sep and 3 Oct. For ‘Jade Cross E’, yields were maximized at early September topping dates, whereas for ‘Nautic’, yields were highest for mid-September topping dates, and ‘Diablo’ yields were highest for later September topping dates. We did observe slight dips in yield on the 11 Sep topping dates for ‘Jade Cross E’ in 2015 and for ‘Diablo’ in 2017; the reasons for this are not clear.
In addition to reducing stalk height, topping affected marketable yields by affecting the number of sprouts that were either undersized or oversized. As shown in Fig. 2D–F, the percentage of oversized sprouts was minimized by topping later, and the percentage of undersized sprouts was minimized by topping earlier. Topping at intermediate dates also increased stalk uniformity (Fig. 3). The ideal topping date window for minimizing defects varied slightly for each cultivar: 4 to 24 Sep (1641–1994 GDDs) for ‘Jade Cross E’, 17 to 24 Sep (1900–1994 GDDs) for ‘Nautic’, and 11 Sep to 3 Oct (1657–2018 GDDs) for Diablo.
Discussion
We identified several Brussels sprout cultivars that produced moderate to high yields of well-spaced, uniform sprouts with few Alternaria blight symptoms. ‘Diablo’ and ‘Nautic’ performed well consistently on all metrics in all years. ‘Confidant’, ‘Dagan’, ‘Divino’, ‘Early Marvel’, ‘Hestia’, and ‘Marte’ produced high-quality sprouts and had high yields in at least 1 year, and thus have potential for commercial producers. Ostfeld (2014) found that ‘Gustus’, ‘Diablo’, and ‘Nautic’ were all acceptable using stalk size, plant growth characteristics, and number of sprouts, along with other variables, consistent with our results. In Rhode Island, USA, Brown (2013) found that ‘Doric’ was one of the best cultivars of the six evaluated because large sprouts developed and stalks did not lodge, whereas we observed comparatively low yields for this cultivar. This may have been a result of the earlier planting date and longer growing season used by Brown (2013) compared with our work.
Of the purple-red cultivars we evaluated, the most uniform was ‘Redarling’, but even it failed to produce reasonable yields in our relatively long 2021 growing season. Consistent with the results of Brown (2013) and Ostfeld (2014), who found ‘Falstaff’ and ‘Roodnerf’ yields to be low, we could not recommend any of the purple-red cultivars for commercial producers in our region. It is important to note that cultural practices such as fertility and irrigation management may greatly affect marketable yields, and although we used standard commercial practices for the region, cultivars might perform quite differently under different management regimes.
Although Braverman (1977) did identify variation in susceptibility to Alternaria brassicicola among several Brussels sprout accessions, and Brown (2013) observed cultivar differences in susceptibility to Alternaria blight, the literature otherwise lacks information, suggesting that cultivars possess resistance to this common pathogen. Brown (2013) described ‘Nautic’ as highly susceptible to Alternaria blight in comparison with ‘Falstaff’, ‘Roodnerf’, and ‘Doric’. We found that all of these cultivars, including ‘Nautic’, had significantly fewer symptomatic sprouts than ‘Jade Cross E’, a very susceptible cultivar, in all of the years they were evaluated. Although we observed variability from year to year, and some cultivars had inconsistent results, others showed consistently moderate levels of Alternaria blight, and others remained comparatively free of symptoms. We cannot rule out the possibility that the pathogen was seedborne on the seeds we planted for our experiments; the cultivars that showed the most symptoms may have been infested at planting. For future studies, hot-water treatment of all seeds could help to eliminate this possibility to establish clearly that symptoms reflect cultivar resistance (Cardoso et al. 2020; Köhl et al. 2011). However, regardless of the source of disease in our experiments, the results we report are likely to reflect the real experiences that growers would face when growing Brussels sprouts under similar conditions, and our results may help them avoid cultivars that are likely to show symptoms of Alternaria blight.
Our results support earlier work that has shown that topping has the potential to increase marketable yields, increase stalk uniformity, and minimize the proportion of sprouts that were under- or oversized (Everaarts 1994). Although the ideal dates will almost certainly vary depending on management and weather variables, these results provide clear evidence that 1) topping affects sprout defects, marketable yields, and stalk height; and that 2) there are drawbacks associated with topping too early or too late. As Jones (1972) reported, we found that topping too early, even if the basal sprouts had already reached marketable size, was detrimental. For late-maturing cultivars, topping often had no effect, but was not detrimental. As a result, we would recommend that producers top plants during the month of September in our region, especially for cultivars with basal sprouts that have reached marketable size by that time. Specifically, our studies generally showed a benefit from topping when plants had received 1641 and 2018 GDD50 after transplant when the total growing season represented less than 2300 GDD50. The degree to which ideal topping time depends on the GDDs that plants have received, compared with the number of GDDs remaining in the season, remains unclear. During the years that this work was conducted, we observed that very few GDD50 accumulated after 15 Oct. It is likely that in warmer climates, where plants continue to grow later into the fall, optimal topping dates could be later. A more generalizable model would likely need to take into account the expected seasonal GDD accumulation as well as cultivar-specific growth rate and requirements.
In conclusion, our study describes the performance of modern Brussels sprout cultivars in the northeastern United States, and we presented information about differences between cultivars in development of Alternaria blight symptoms—a common limitation to production of this crop. This facilitates research-based recommendations for growers in the region. Until now, this information has been lacking in the peer-reviewed literature. We also provide information about several modern cultivars’ response to topping at different dates, and provide evidence that topping during the month of September may increase yields of some cultivars.
Units
References cited
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