Evaluation of Blueberry Types and Cultivars for Early Market Production in Japan Using Unheated Plastic House Culture

Abstract

The purpose of this study was to evaluate the adaptability of blueberry types and cultivars to produce ripe fruit before Japan's rainy season (June and July) in unheated plastic house culture. The flowering and harvesting time, fruit growing interval, and fruit quality parameters of northern and southern highbush and rabbiteye blueberries cultivars were tested. Three- or four-year-old potted plants grown outdoors were brought into an unheated plastic house on February 10, 2008 (experiment 1) and 2009 (experiment 2). Standard watering, fertilizing, and bee pollination methods were used. In experiment 2, the average temperature of air and pot soil in the unheated plastic house was higher than that of the open culture. Flowering time was about 26–40 days earlier, and ripening time was about 7–40 days earlier in an unheated plastic house culture compared to an open culture. This resulted in 80% of all southern highbush and some northern highbush cultivars being almost completely harvested before the rainy season began (the beginning of June). Harvest time for rabbiteye cultivars, however, extended into the rainy season. The fruit growing period (days from 50% flowering time to 50% harvesting time) of all three types of blueberries in the unheated plastic house culture were longer than that in the open culture. Fruit size, soluble solid content, and citric acid content, soluble solid content/citric acid content ratio differed among cultivars. On the whole, soluble solid and citric acid content of all cultivars grown under an unheated plastic house culture were lower than that of fruit grown in an open culture.

Keywords:

• Vaccinium sp.
• off-season production
• pot culture

INTRODUCTION

One of the major problems in producing high quality blueberry fruit in Japan is that the harvest season of many northern highbush and southern highbush blueberry cultivars occurs during the rainy season (June and July). Previously, we evaluated forcing culture (growing in heated plastic houses) of southern highbush blueberry in Japan and determined the proper cultural methods, including the critical date to start heating, length of the heating period, optimal temperatures, pot culture methods, and the appropriate cultivars (Ozeki and Tamada, 2006; Tamada and Ozeki, 2006, 2009; Tamada, 2008).

Blueberry culture in unheated, high tunnels has been studied in many places in the world, including Japan (Tamada, 2008), The Netherlands (Bal, 1997), Portugal (Baptista et al., 2006), Spain (Ciordia et al., 2002, 2006), and the USA (Ogden and Iersel, 2009). Most of these studies reported that the fruit ripening time was much earlier for northern and southern highbush blueberries grown in a high tunnel than the same cultivars grown in an open culture. Because of these results, some Japanese growers are interested in high tunnel culture to produce ripe fruit before the rainy season begins. In unheated plastic houses, cultivars that have low chilling requirements and early ripening characteristics are useful for producing fruit for an early, more profitable market.

The purpose of this study was to evaluate blueberry types and cultivars so that recommendations could be made for unheated plastic house culture in Japan. As in our previous paper (Tamada and Ozeki, 2009), flowering and harvest time, fruit growth interval, yield, and fruit quality parameters were investigated.

MATERIALS AND METHODS

Two experiments were conducted in a plastic house at the Ozeki Blueberry Nursery (Tsuchiura-shi, located about 75 km northeast of Tokyo). Three- or four-year-old plants of three cultivars of northern highbush (NHb), five cultivars of southern highbush (SHb), and three cultivars of rabbiteye (Rb) blueberries were grown in 21-L vinyl pots with peat moss: volcanic soil: chaff, 50:30:20, V:V:V. A total of six plants per cultivar and one plant per pot, were placed randomly in six plots containing all of the cultivars. The plants were transplanted in October of the previous year, and managed under outdoor conditions. The number of shoot and flower clusters per plant or per shoot were not adjusted. Plants were moved into the unheated plastic house on February 10, for the duration of the experiment. The mean outside air temperature and mean air and pot soil temperature in the plastic house during the growing period are shown in Table 1.

TABLE 1 Air Temperature in an Unheated Plastic House and Outdoors, and the Temperature of the Soil in Pots in 2009

			Plastic house air		Pot soil		Outdoors air ^z
Month	Day	Days after Feb 10	Mean temp.	Mean minimum temp. (°C)	Mean temp.	Mean minimum temp. (°C)	Mean air temp. (°C)
Feb.	10–20	1–11	10.8	0.2	11.7	4.2	7.3
	21–28	12–19	12.8	0.5	13.8	6.4	4.8
March	1–10	20–29	13.3	1.7	14.6	7.5	7.1
	11–20	30–39	16.7	4.6	16.7	10.2	10.2
	21–31	40–50	16.1	3.4	16.1	10.3	8.0
April	1–10	51–60	20.0	3.4	19.5	11.8	12.8
	11–20	61–70	20.2	9.9	20.3	14.9	15.4
	21–30	71–80	18.6	5.3	18.4	12.4	14.3
May	1–10	81–90	21.4	10.9	20.7	16.0	18.2
	11–20	91–100	22.5	10.0	22.1	15.4	18.8
	21–31	101–111	22.8	13.7	22.6	18.4	19.5
June	1–10	112–121	23.0	13.3	22.4	17.5	19.4
	11–20	122–131	23.3	15.9	22.8	19.2	19.7
	21–30	132–141	26.0	14.2	25.6	21.6	23.3
July	1–10	142–151	26.0	17.4	25.4	20.8	23.4
	11–15	152–156	28.9	17.7	28.0	21.5	25.3
^zAt Tsuchiura-shi, arranged from Tsuchiura Weather Station's report.

A small beehive with around 10,000 bees was placed in the plastic house during the flowering period to ensure pollination. Fertilizer (N-P-K: 10-10-10, nitrogen form urea) was applied three times (first week of March, mid April, and mid May, at 10 g per pot at each application time). Watering was done as required based on the appearance of the surface of potting medium. Weeding of pots was done by hand as needed.

Flowering time was evaluated every 3 days, and ripe fruit were harvested every 5 days per plant. Fruit set (percentage) was calculated by dividing the estimated number of florets per plant by total number of harvested fruit. Estimated number of florets was determined by multiplying the total number of flower cluster per plant by the number of florets per plant. The fruit growing interval was considered to be the length from 50% flowering to 50% harvesting time.

Number of fruit and yield (total fresh fruit weight) was recorded for each plant. Fruit were immediately frozen at -20°C for juice analysis. On October 20, frozen fruit were thawed and blended using 20–30 fruits per harvest day. Soluble solid content (percentage) was measured with a hand refractometer and acidity (percentage of citric acid) was measured with a fruit tester.

RESULTS AND DISCUSSION

The results from experiment 1 were limited to blueberry type and cultivar flowering time. This was because a wild masked palm civet (Paguma larvata) broke into the plastic house on May 19 and 20, at the early stages of ripening, and indiscriminately devoured berries and broke many bearing branches from the plants. Consequently, further data collection was impossible. In this article, we only present the results obtained from experiment 2, conducted in 2009.

Plastic House Air and Pot Soil Temperature

Mean air and pot soil temperatures in the unheated plastic house were higher than that outdoors (Table 1). These results were similar to previous reports on blueberry culture in high tunnels (Ogden and Iersel, 2009; Ozeki and Tamada, 2006; Renquist, 2005; Tamada and Ozeki, 2006, 2009). Interestingly, both the mean pot soil temperature and the mean minimum pot soil temperature in the unheated plastic house were higher than the mean room air temperature. The mean minimum air temperature in the unheated plastic house was between 0.2–1.7°C from February 10 to March 10. No low temperature injury to flower buds or florets was observed.

Incidentally, the rainy season in 2009 started at the beginning of June and lasted till the middle of July. The rainfall in the middle of June amounted to 61 mm (twice that of the previous 10 days) and the total hours of sunshine was 30.5 hr (half of the previous 10 days).

Flowering Time

The date at which 50% of the blossoms were open was about 26–40 days earlier (from March 13 to April 2) in the unheated plastic house, compared to open culture (from April 12 to May 5) (Table 2).

TABLE 2 Flowering Time of Three Types of Blueberries Grown in an Unheated Plastic House in 2009

			Flowering time (days after February 10, 2009) ^x
Type ^z	Cultivar	Plant age (year)	20%	50%	80%	Flowering period ^w
NHb	Chandler	3	50 ^v a	56 ^v a	61 ^v a	11
	Hannah's Choice	3	40 ab	46 b	50 b	10
	Nelson ^y	4	40 ab	44 bc	47 c	7
SHb	Emerald	3	29 cd	35 e	38 f	9
	Sapphire	3	31 bcd	37 de	40 ef	9
	Star	3	30 bcd	35 e	38 f	9
	Magnolia ^y	4	33 bcd	37 d	40 ef	7
	Sharpblue ^y	4	26 cd	29 f	38 f	12
Rb	Austin ^y	4	37 bcd	42 c	47 c	9
	Brightwell ^y	4	35 bcd	39 d	45 cd	10
	Tifblue ^y	4	37 bc	42 c	47 c	19
^zNHb: Northern Highbush; SHb: Southern Highbush; Rb: Rabbiteye blueberry.
^ySame plants used in 2008 year test.
^xFlowering day of 20, 50, and 80% in the whole flower cluster per plant.
^wDays from 20% of flowering to 80% flowering.
^vWithin columns, different letters indicate significance at the 5% level by Duncan's range test.

The flowering phenology of blueberry type and cultivars in the unheated plastic house was similar to that of the open culture. It is noteworthy that the time to 50% flowering for southern highbush cultivars grown under a unheated plastic house was the same as that found in the heated plastic house (Tamada and Ozeki, 2006, 2009). Among the cultivars, the time to 50% flowering for ‘Sharpblue’ (SHb) (29 days, March 10) was earlier than other cultivars; and ‘Chandler’ (NHb) was the latest (56 days, April 6). Judging from the flowering time observed in this experiment, the chilling requirement of all blueberry types and cultivars were apparently satisfied by the end of January in most of the central parts of Japan. In this connection, monthly mean air temperature of November, December, and January in Tokyo was 13.0, 8.4, and 5.8°C, respectively (National Astronomy Observatory, 2009).

Harvesting (Ripening) Time

Ripening time for all blueberry types and cultivars, as measured by the date at which 50% of the fruit had been harvested, was 7–40 days earlier under the unheated culture, compared to the open culture, which usually lasts from the first week of June to the third week of July (during the rainy season) (Table 3). However, ripening time in this unheated cultural experiment was late compared to that previously reported in heated greenhouses (Tamada and Ozeki, 2006, 2009). Regardless, our results support previous work suggesting that the ripening time of blueberry fruit can be advanced by increasing temperatures during the fruit growing period (Gough, 1994; Mainland, 2002).

TABLE 3 Harvesting Time and Fruit Growing Interval of Three Types of Blueberries in an Unheated Plastic House in 2009

			Harvesting time (days after February 10, 2009) ^x
Type ^z	Cultivar	Plant age (year)	20%	50%	80%	Fruit growing period ^w
NHb	Chandler	3	130 ^v a	137 ^v b	141 ^v a	81 ^v b
	Hannah's Choice	3	103 c	113 c	117 b	67 cde
	Nelson ^y	4	95 e	109 d	111 bc	65 cde
SHb	Emerald	3	99 d	105 e	113 b	70 cd
	Sapphire	3	91 f	101 f	113 bc	64 de
	Star	3	91 f	101 f	105 c	66 cde
	Magnolia ^y	4	95 e	105 e	106 c	68 cde
	Sharpblue ^y	4	91 f	104 e	106 c	72 c
Rb	Austin ^y	4	123 b	141 b	148 a	99 a
	Brightwell ^y	4	123 b	141 b	148 a	102 a
	Tifblue ^y	4	131 a	144 a	148 a	102 a
^zNHb: Northern Highbush; SHb: Southern Highbush; Rb: Rabbiteye blueberry.
^ySame plants used in 2008 year test.
^xHarvesting day of 20, 50, and 80% in the whole fruit per plant.
^wDays from 20% of flowering to 80% harvesting time.
^vWithin columns, different letters indicate significance at the 5% level by Duncan's range test.

Among the cultivars, the earliest date at which 50% of the fruit had been harvested occurred with ‘Sapphire’ and ‘Star’ (SHb) (May 21, 101 days), while ‘Tifblue’ (Rb) had the latest date (June 30, 141 days).

For most of the southern highbush and the two northern highbush cultivars, 80% of the fruit had been harvested before the start of the rainy season. An exception to this was the northern highbush cultivar ‘Chandler’ (June 30, 141 days) and the three rabbiteye cultivars (July 7, 148 days). These results suggest that one of the objects in this study was met; to finish ripening and harvesting before the rainy season. This was possible under an unheated plastic culture for five southern highbush cultivars and the earliest cultivars of northern highbush grown.

Fruit Growing Period

The fruit growing interval (from 50% flowering time to 50% harvest time) was different among blueberry types and cultivars (Table 3). On the whole, the fruit growing period of southern highbush cultivars was the shortest, and that of rabbiteye cultivars was the longest of all cultivars grown in the unheated greenhouses. However, the fruit growing interval of all cultivars showed a tendency to be longer in the unheated culture than the open culture. ‘Sapphire’ (SHb, 64 days) and ‘Nelson’ (NHb, 65 days) had the shortest fruit growing periods, while ‘Brightwell’ and ‘Tifblue’ (Rb, 102 days) had the longest periods.

Ripening times are generally related to the flowering time of blueberry cultivars, so it is very important to select early cultivars for unheated culture, as in heated culture.

Fruit Set and Yield

Percentage of fruit set is a very important factor influencing yield. As a whole, the average fruit set in this experiment was lower than that of open culture (Table 4). High fruit set usually reflects a high level of self-compatibility or sufficient cross pollination in blueberry culture (Chavez and Lyrene, 2009; Gough, 1994; Sampson and Spiers, 2002; Tamada, 2008). However, it was difficult to compare the fruit set data among blueberry types and cultivars in this study, as the plant age and yield differed and the data was from only one year's experiment.

TABLE 4 Estimation of Fruit Set and Yield per Plant of Three Types of Blueberries Grown in an Unheated Plastic House in 2009

					Yield/plant
Type ^z	Cultivar	Plant age (year)	Estimated No. of floret/plant ^x	Estimation of fruit set (%) ^w	No. of fruit	Total fresh fruit wt (g)
NHb	Chandler	3	225	22.9	51.5	109.1
	Hannah's Choice	3	225	19.1	43.0	78.6
	Nelson ^y	4	1,072	19.5	208.8	381.8
SHb	Emerald	3	210	58.2	122.3	222.1
	Sapphire	3	371	36.7	136.3	156.8
	Star	3	208	42.3	88.0	169.4
	Magnolia ^y	4	1,652	26.9	444.5	573.8
	Sharpblue ^y	4	704	44.5	313.5	344.2
Rb	Austin^y	4	1,096	34.4	376.5	532.8
	Brightwell^y	4	1,575	36.9	581.0	819.1
	Tifblue^y	4	1,463	35.4	518.0	650.7
^zNHb: Northern Highbush; SHb: Southern Highbush; Rb: Rabbiteye blueberry.
^ySame plants used in 2008 year test.
^xEstimate based on 20 flowers counted per plant and determined by multiplying the total No. of flowers cluster × No. of floret per plant.
^wCalculated by dividing the estimated total No. of fruit per plant by total No. of harvested fruit per plant.

Fruit Quality Parameters

Fruit size, soluble solids content (SSC), citric acid content (CAC), and pH level of fruit juice were different among cultivars (Table 5). Fruit size and SSC of all cultivars grown in the unheated culture was smaller and lower than that of the open culture. This is a noteworthy result.

TABLE 5 Fruit Quality Parameters of Three Types of Blueberries Grown in an Unheated Plastic House in 2009

Type ^z	Cultivar	Plant age (year)	Mean fresh weight (g) ^x	Soluble solid content (%) ^x	Citric acid content (%)	SSC/CAC ratio
NHb	Chandler	3	2.13 ^w a	9.5 ^w e	0.89 ^w a	10.6
	Hannah's Choice	3	1.78 b	9.6 e	0.47 e	20.4
	Nelson ^y	4	1.89 b	12.1 b	0.77 b	15.7
SHb	Emerald	3	1.83 b	9.5 e	0.65 c	14.6
	Sapphire	3	1.16 d	9.5 e	0.32 f	29.7
	Star	3	1.84 b	10.4 d	0.49 de	21.2
	Magnolia ^y	4	1.29 cd	11.5 c	0.81 b	14.2
	Sharpblue ^y	4	0.95 e	12.6 a	0.60 c	21.0
Rb	Austin ^y	4	1.42 c	12.4 ab	0.42 e	29.5
	Brightwell ^y	4	1.41 c	11.5 c	0.53 d	21.7
	Tifblue ^y	4	1.23 d	12.0 b	0.60 c	13.3
^zNHb: Northern Highbush; SHb: Southern Highbush; Rb: Rabbiteye blueberry.
^ySame plants used in 2008 year test.
^xMean of fruit at three harvest times; 20, 50, and 80%.
^wWithin columns, different letters are significant at 5% level by Duncan's range test.

It is known that fruit size, SSC, and CAC of fruit depends on the cropping load per plant (Tamada, 2008). In this experiment, the number of shoots per plant and flower clusters per plant were not adjusted, which means that no pruning of shoots, flower removal, or fruit thinning was carried out. Therefore, we consider that the cropping load per plant in this experiment was too high and another study of appropriate crop load in pot culture will be needed.

Among the cultivars, fruit size was the largest for ‘Chandler’ (NHb, over 2 g) and smallest for ‘Sharpblue’ (SHb, less than 1 g). Fruit size of many cultivars showed a tendency to decrease with harvest time.

‘Nelson’ (NHb) and ‘Austin’ (Rb) had the highest SSC (over 12%) among the many cultivars, while ‘Chandler’ had the lowest SSC. The CAA was the lowest for ‘Sapphire’ (SHb) and highest for ‘Tifblue’ (Rb) and ‘Chandler’ (NHb). Cultivars with lower CAC had a higher ratio of SSC/CAC. According to a sensory evaluation using three men and three women, fruit of ‘Sapphire’, ‘Star’ (both SHb), and ‘Hannah's Choice’ (NHb) had the best flavor (data not shown).

OBSERVATION

At the end of the harvest in experiment 2 (2009), all potted plants were carried outdoors on July 20 and standard watering and fertilizing procedures were followed. It was noted that shoot growth, flower bud differentiation and development, and leaf defoliation time of plants that were fruited in the plastic house were similar to that of open culture field plants.

In the unheated plastic house, almost no fruit cracking (normally caused by rainfall), insect damage of fruits by Cherry drosophila (Drosophila suzuki Matumura), or bird damage of fruits was observed. Since these problems are prevalent in open culture, plastic house culture represents an attractive alternative.