Comparison of Nine Clones of ‘Jewel’ Strawberry in Ontario and Nova Scotia

Abstract

Nine clones of ‘Jewel’ strawberry, all originating from the same source in 1997, and propagated at one location in 2008, were compared in replicated trials planted in Kentville, NS and London, ON in 2009. Six of the clones were from micropropagated plants in Ontario, two were from runner-propagated plants, and the last was from the nursery that had provided the original stock. Data on vegetative and reproductive factors were recorded at both sites in 2009 and 2010. The number of runners, leaf petiole length, and the severity of powdery mildew were recorded 60 days after planting. Plot fullness was rated the first fall and the following spring. In the spring of the second year, plant vigor, phenology, and the number of flower stalks and flowers per stalk were recorded. Fruit yield, whether marketable or unmarketable, and berry counts were recorded twice weekly in the second year. Clonal differences were noted in all the plant measurements. For example, one of the runner-propagated ‘Jewel’ clones, JEEL 9, had the highest number of runners and the longest petioles. Site x clone interactions were significant for spring vigor and spring plotfill ratings and for the number of flower stalks. Differences in flower development were also noted, however, there were no significant differences in harvest date. There was a small but significant difference in berry weight but not in marketable or total yield. The clonal variation described in this study was not of sufficient scope to explain the poor crops that some growers have occasionally reported. Alternative explanations are offered.

Keywords:

• Fragaria × ananassa
• clonal variation
• yield
• berry weight
• plant vigor

INTRODUCTION

‘Jewel’ strawberry (Fragaria × ananassa Duchesne ex Rozier), since its introduction from Cornell University in 1985 (Sanford et al., 1985), has become an important cultivar in eastern Canada and the northeastern United States. Over the past 25 years, many plant nurseries have propagated ‘Jewel’ by runner propagation and tissue culture producing hundreds of millions of plants. Propagation can lead to off-type variants, some of which can affect fruit yield and size, although variants are not frequent (Swartz et al., 1981). Strawberry plants directly from tissue culture usually produce more runners and daughter plants, and these daughter plants often have smaller leaves and crowns than conventionally propagated plants (Damiano, 1980), but this physiological effect is expected to diminish with time ex vitro (Boxus, 1989). The performance of ‘Jewel’ on strawberry farms in eastern Canada has exhibited sufficient variation in recent years to cause concern that somaclonal variation may have arisen in the propagation systems and that some clones may be better performers than others. The experiment described herein is an attempt to measure that variation.

MATERIALS AND METHODS

Nine clones were gathered at the New Liskeard Agricultural Research Station (NLARS) SPUD Unit, New Liskeard, ON in 2008 and then given to a commercial strawberry nursery for one summer of field propagation (Table 1). All of the NLARS clones originated from tissue cultures of the stock plant (JEEL 9) provided by Agriculture and Agri-Food Canada (AAFC)-Kentville, NS in 2002. Previously, this ‘Jewel’ clone had been propagated by runners in a screenhouse at Kentville from plants provided by Nourse Farms Ltd., South Deerfield, MA in 1997. The NLARS clones were initiated into culture either in 2005 or 2006 from runner-derived plants from the 2002 stock plant, and subsequently grown for one or two cycles ex vitro before going to the commercial nursery. The AAFC-Kentville clone supplied in 2008 (JEEL 4) had been runner propagated annually from the original 1997 Nourse Farms plant. An additional clone (JEEL 8) came from Nourse Farms in 2008. This clone was field grown in 2007 but likely came from tissue culture previously. The ‘Jewel’ clone designations, JEEL 1 to JEEL 9, were assigned by the senior author and the field trial managers were unaware of specific clonal origins until after the trial results were gathered.

TABLE 1 The Source and Identity of the Nine ‘Jewel’ Strawberry Clones Included in the Field Plantings Established in 2008 in Ontario and Nova Scotia

Code names	Clone identification	Clone origin
JEEL 1	Clone 05-1 G2	NLARS, ^z initiated into tc in 2005, mericlone 1, multiplied 9 times in vitro, 2 cycles ex vitro
JEEL 2	Clone 05-1 G1	NLARS, initiated into tc in 2005, mericlone 1, multiplied 12 times in vitro, 1 cycle ex vitro
JEEL 3	Clone 06-2 G1	NLARS, initiated into tc in 2006, mericlone 2, multiplied 6 times in vitro, 1 cycle ex vitro
JEEL 4	NS 2008	AAFC-Kentville from the screenhouse in 2008
JEEL 5	Clone 06-3 G1	NLARS, initiated into tc in 2006, mericlone 3, multiplied 10 times in vitro, 1 cycle ex vitro
JEEL 6	Clone 06-4 G1	NLARS, initiated into tc in 2006, mericlone 4, multiplied 6 times in vitro, 1 cycle ex vitro
JEEL 7	Clone 06-5 G1	NLARS, initiated into tc in 2006, mericlone 5, multiplied 7 times in vitro, 1 cycle ex vitro
JEEL 8	Nourse 08	Nourse Farms Ltd. supplied in 2008, field propagated in 2007
JEEL 9	Stock plant 02 NS	AAFC-Kentville from the screenhouse in 2002, original source of tc clones
^zNew Liskeard Agricultural Research Station, SPUD Unit.

Nursery grown plants of the nine ‘Jewel’ clones were dug in Spring 2009 and shipped to Heeman Strawberry Farm, London, ON (43° 02′ N lat., 81° 07′ W long.) and to the Atlantic Food and Horticultural Research Centre, Kentville, NS (45° 04′ N lat., 64° 29′ W long.) to establish field plots. ‘Jewel’ clones were arranged in randomized complete block designs with five replications at both sites. Plants were spaced at 30 cm in-row with 91 cm (36 inches) and 140 cm (55 inches) between rows in ON and NS, respectively. The 2009 planting dates were 11 May in ON and 14 May in NS.

Matted rows were developed in 2009 from 10 plants per plot. In mid-July, about 60 days after planting, the number of runners (>3 cm) was counted and the length of 10 petioles per plot was measured. At the same time, the severity of powdery mildew [Podosphaera aphanis (Wallr)] was scored by examining 10 medium-aged leaflets per plot and rating on a scale of 0–5, where 0: no mildew, 1: 1% to 20%, 2: 21%–40%, 3: 41%–60%, 4: 61%–80%, and 5: 81–100% of the abaxial surface visually colonized. In NS, a second powdery mildew rating was taken on 25 Aug. 2009. In late Oct. 2009, the degree to which the matted rows had filled in (plot fullness) was rated on a scale of 1–5 where 1: 0% to 20%, 2: 21%–40%, 3: 41%–60%, 4: 61%–80%, and 5: 81%–100% of the row filled. This rating was repeated in late May 2010 at the time of early flowering. At this same time, spring vigor was rated on a scale of 1–4 where 1 = all plants weak, with short petioles and small, misshapen or chlorotic leaves, 2 = many weak plants, 3 = a few weak plants, and 4 = all plants vigorous. Flowering phenology was also recorded by examining five crowns per plot and scoring as 1 = flower stalks not emerged, 2 = stalks <5 cm, 3 = stalks >5 cm, 4 = a few primary flowers opened per plot, 5 = half of primary flowers open (∼10% bloom), 6 = 50% bloom, 7 = most flowers set, and 8 = king berries green.

To reduce the plot-end effects, the central 2 m of each plot was harvested in 2010. Fruit was harvested two times per week and sorted into marketable and unmarketable categories. The average fruit weight for the season was a weighted mean based on the mass of marketable fruit from each plot from each harvest, and the yield for each harvest. In ON, all marketable fruit were counted and weighed to calculate the mean, but in NS, a 25-berry sample was used. The average harvest date for the season was a weighted mean based on the dates of harvest and the yield for each date. Data from the randomized block trials were subjected to analysis of variance (GenStat 11.1; VSN Intl. Ltd., Oxford, UK) and when F probabilities were significant, means were separated by least significant difference (LSD, P ≤ 0.05). Data for the two sites are presented as combined means when site × cultivar interactions were insignificant (p > 0.05), and as site-specific means when site × cultivar interactions were significant (p ≤ 0.05).

RESULTS

The plots established well at both sites and by mid-July 2009 plants had produced a mean of 3.1 runners. JEEL 6 was slightly behind in runner development with a mean of 2.6, especially when compared to JEEL 9, the 2002 stock clone, at 4.0 runners (Table 2). By October the plots of JEEL 6 were the only ones showing significantly fewer runner plants as measured by the plot fullness ratings (Table 2). This was more pronounced in NS, where JEEL 6 plot ratings were 24% lower than the site mean, compared to 16% lower in ON. The length of leaf petioles differed among clones with JEEL 1 and JEEL 8 nearly 14 cm shorter than JEEL 9. Strawberry plants directly from tissue culture are known to have enhanced vigor as expressed in increased runner production and, in some studies, longer petioles (Nehra et al., 1994; Swartz et al., 1981) although not all cultivars respond in this way. There was no positive relationship between vigour and temporal proximity to tissue culture among the ‘Jewel’ clones.

TABLE 2 Means Over Two Sites for Plant Measurements Made on Nine Clones of ‘Jewel’ Strawberry Planted in Replicated Trial in Nova Scotia and Ontario

Jewel clones	No. runners ^z	Petiole length ^z (mm)	Plot fullness (1–5) Fall	Plot fullness (1–5) Spring	Spring vigor (1–4)	Phenology ^y
JEEL 1	2.9 cd ^x	134.1 d	4.3 a	3.2 abc	3.1 a	5.4 ab
JEEL 2	2.9 cd	143.8 abc	4.3 a	3.4 abc	3.0 a	5.4 ab
JEEL 3	3.6 ab	143.7 abc	4.6 a	3.4 abc	3.1 a	5.6 a
JEEL 4	3.2 bc	139.0 abcd	4.4 a	3.0 bcd	2.4 cd	5.4 ab
JEEL 5	3.2 bc	147.4 ab	4.6 a	3.5 ab	3.0 a	5.3 abc
JEEL 6	2.6 d	138.3 bcd	3.5 b	2.6 d	2.3 d	5.4 ab
JEEL 7	2.8 cd	143.5 abcd	4.6 a	3.8 a	2.8 abc	5.5 a
JEEL 8	3.1 bcd	134.3 cd	4.2 a	3.3 abc	2.9 ab	5.1 bc
JEEL 9	4.0 a	147.9 a	4.4 a	2.9 cd	2.5 bcd	5.0 c
F-value (Site × Clone)	0.104	0.295	0.280	0.015	0.029	0.297
F-value (Clone)	<0.001	0.026	<0.001	0.005	0.003	0.027
SEM	0.189	3.364	0.162	0.197	0.168	0.123
^zSixty days post planting.
^yPlant floral development: 5 is ½ of primary flowers open (∼10% bloom), 6 is 50% bloom.
^xMean separation within columns by least significant difference test at p ≤ 0.05.

During the establishment year, the severity of powdery mildew was very low by mid-July with mean ratings of below 1 (1%–20%). This disease progressed in NS and on 25 Aug. the mean rating was 3 (41%–60%) but there were no significant clonal differences (data not shown).

‘Jewel’ leaves occasionally have more than three leaflets. The ON plots were examined on 8 July 2009 and on average, there were 0.8 leaves per plot with more than three leaflets. The clones did not differ significantly (P = 0.101) in this trait.

The plot fullness ratings of Spring 2010 were lower than in Fall 2009 at both locations, but the reduction in values was more pronounced in NS where black root rot (Pythium sp. and Rhizoctonia sp. often in concert with nematodes) was prevalent. At both locations, JEEL 6 had the thinnest rows (Table 3). This was also reflected in spring vigor ratings, which placed JEEL 6 as the weakest in NS. In ON, there were no significant differences in vigor ratings (Table 3).

TABLE 3 Means at Each Site for Traits That Had Significant Site × Clone Interactions for Nine Clones of ‘Jewel’ Strawberry Planted in Replicated Trial in Nova Scotia and Ontario in 2008

	Spring plot fullness (1–5)		Spring vigor (1–4)		Flower stalks (no.)
Jewel clones	ON	NS	ON	NS	ON	NS
JEEL 1	3.5 b ^z	2.9 ab	2.8 a	3.3 a	49 a	98 ab
JEEL 2	3.8 ab	3.0 a	3.2 a	2.8 ab	62 a	90 bcd
JEEL 3	3.5 b	3.3 a	3.2 a	3.0 ab	50 a	113 ab
JEEL 4	3.9 ab	2.0 cd	2.8 a	2.0 c	47 a	62 e
JEEL 5	3.8 ab	3.2 a	2.9 a	3.2 a	45 a	115 a
JEEL 6	3.5 b	1.7 d	2.8 a	1.9 c	47 a	57 e
JEEL 7	4.5 a	3.0 ab	3.1 a	2.5 bc	64 a	90 bc
JEEL 8	4.1 ab	2.5 abc	3.1 a	2.7 ab	51 a	70 cde
JEEL 9	3.5 b	2.2 bcd	2.9 a	2.0 c	46 a	66 de
F-value (Site × Clone)	0.015		0.029		0.005
SEM	0.286		0.240		8.50
^zMean separation within columns by least significant difference test at p ≤ 0.05.

The phenology ratings at the time of flowering indicated that JEEL 9 was slightly slower to develop than many of the other clones (Table 2). This may relate to the high runner numbers and tall canopy of JEEL 9 that may have kept the soil cooler. This later flowering, however, did not lead to a later harvest, since there were no significant differences in mean harvest date (Table 4). The number of flower stalks showed some significant clonal differences (Table 4) but when the sites were separated, the differences were important only in NS (Table 3). The differences in NS could largely be explained by variation in the plot vigor and spring plot-fill ratings, as caused primarily by black root rot. The lowest spring plot-fill and vigor ratings occurred in JEEL 6 and the two runner-propagated clones, JEEL 4 and 9. A strong correlation (r = 0.95) was observed in NS between spring plot-fill and the number of flower stalks. At both locations, ‘Jewel’ flower stalks carried an average of 9 or 10 flowers (Table 4).

TABLE 4 Means Over Two Sites for Reproductive Measurements Made on Nine Clones of ‘Jewel’ Strawberry Planted in Replicated Trial in Nova Scotia and Ontario in 2008

Jewel clones	Flower stalks (no.)	Flowers per stalk	Mkt yield (t/ha)	UnMkt yield (t/ha)	Berry wt. (g)	Mean harvest day
JEEL 1	74 ab ^z	9.1	9.8	2.2 ab	10.3 ab	178.1
JEEL 2	76 ab	9.5	10.5	2.0 abc	10.2 ab	178.1
JEEL 3	82 a	9.1	9.6	2.2 a	10.1 ab	178.3
JEEL 4	55 c	9.3	8.6	1.6 bcd	10.2 ab	177.8
JEEL 5	80 a	9.4	9.7	1.9 abc	10.5 ab	178.0
JEEL 6	52 c	9.6	7.1	1.1 d	9.2 c	177.9
JEEL 7	77 ab	10.4	10.2	1.9 abc	9.8 bc	178.2
JEEL 8	61 bc	9.4	10.1	1.5 cd	10.7 a	178.2
JEEL 9	56 c	9.2	8.3	1.4 cd	10.2 ab	177.6
F-value (Site × Clone)	0.005	0.538	0.341	0.011	0.169	0.086
F-value (Clone)	0.001	0.446	0.149	0.005	0.049	0.720
SEM	6.15	0.391	0.873	0.209	0.2974	0.286
^zMean separation within columns by least significant difference test at p ≤ 0.05.

The yield of marketable fruit was not significantly different among the ‘Jewel’ clones (Table 4) although there were minor differences in unmarketable yield. Total yield, averaged over the two sites, ranged from 8.2 t/ha for JEEL 6 to 12.5 t/ha for JEEL 2 with no significant differences (data not shown). In ON, total yield ranged from 12.7 t/ha for JEEL 6 to 17.2 t/ha for JEEL 2. In NS, total yield ranged from 3.5 t/ha for JEEL 6 to 9.9 t/ha for JEEL 5.

There were small but significant (P = 0.049) differences in berry weight with JEEL 6 at the low end of the range (Table 4). As with yield, JEEL 6 had the smallest berries—9.3 g in ON and 9.2 g in NS.

DISCUSSION

The clones of ‘Jewel’ examined in this study were very similar, as would be expected since they all were generated by asexual propagation, either by runners or tissue (meristem) culture, from a single source—‘Jewel’ from Nourse Farms Ltd. obtained in 1997. Tissue culture is known to produce variants, such as white streak, but these can be rogued out of the propagation stream (Swartz et al., 1981). Tissue culture can also affect plant vigor as expressed by the number of runner plants, branch crowns, and the petiole length and size of the leaf blades (Boxus, 1989). These vigor-related traits are temporary; they revert to normal values after one or two seasons of runner propagation in the field (Cameron and Hancock, 1986; Marcotrigiano et al., 1984). The ‘Jewel’ clones used in this experiment were at least two propagation cycles ex vitro, including the nursery year, so no lingering physiological effects would be expected. The clonal differences measured in this study, such as the reduced runnering in the tissue-cultured JEEL 6 clone and the longer petioles in the runner-propagated JEEL 9 clone, could be due to genetically-based somaclonal variation. Further study would be required to verify this. There are alternative explanations, however.

One possible explanation is physiological—that variability arose in the nursery, which led to JEEL 6 having plants that were weaker perhaps due to lower nutrient status. The purpose of the nursery year in this study was to improve the uniformity of the plants, since they were coming from different locations or had different lengths of time from tissue culture but conceivably the location in the nursery, such as the edge of the field or proximity to sprinkler heads, could lead to variation in nutrient status or amount of carbohydrate reserves. Similarly, increased nutrient status could lead to longer petioles, as was seen with JEEL 9. Interestingly, the two runner-propagated clones (JEEL 4 and 9) originally from the same stock plant in 2002, were significantly different in runner number and flowering phenology.

Another possible explanation is that the clones expressed variability due to differential infection by pathogens, such as viruses. Strawberry plants are susceptible to many viruses with the potential to diminish vigor, often without visually diagnostic symptoms (Converse, 1987). It is these and other pathogens that make necessary the use of plant certification programs involving pathogen indexing, meristem culture, soil fumigation, and the use of insecticides to control insect vectors. Notwithstanding these practices, infection occasionally can occur, which could lead to an increase in variability in field trials if some entries were infected and others not. Poorly developed disease resistance in micropropagated propagules has been reported in a number of crops including strawberries (George, 1993). This susceptibility generally disappears as the crop ages.

‘Jewel’ is a desirable cultivar due to its fruit quality, its harvest season, and its potential productivity. Unfortunately, it is susceptible to root- and crown-damaging diseases, such as red stele root rot (Phytophthora fragariae Hickman var. fragariae), Verticillium wilt (Verticillium spp.), and black root rot that can cause a field to fail (Particka and Hancock, 2005; Sanford et al., 1985). These diseases are not easy to diagnose unless samples are taken during a short time span, often before the symptoms become severe. Crop failure or a reduced yield may often be pathological in origin rather than due to a variant clone.

Many studies have found no differences in fruit production or quality between propagules from micropropagated and conventionally propagated strawberry plants (Beech et al., 1988; Boxus, 1998; Cameron et al., 1989; López-Aranda et al., 1994). The ‘Jewel’ clonal variation described in this article, was measurable, but not of sufficient impact to significantly reduce yields. This gives increased confidence in the propagation system, although trials to verify clonal fidelity to the original type cultivar are warranted from time to time. It would be helpful in such trials if control plants were available from a long-term cryopreserved source. Several techniques for cryopreservation of strawberry meristems have been developed (Hirai et al., 1998; Kartha et al., 1980; Navatel and Capron, 1997).

ACKNOWLEDGMENTS

We thank Sherry Fillmore and Brad Walker for their statistical advice and data analysis, Nourse Farms Ltd. (South Deerfield, MA) for supplying plant material, Strawberry Tyme Farms Inc. for field propagating all the clones, and Heeman Strawberry Farm for hosting the Ontario site. Atlantic Food and Horticulture Research Centre contribution no. 2387.