Fruit Set and Berry Weight of Four Rabbiteye Blueberry Cultivars Following Exposure to Sub-Freezing Temperatures During Flowering

Abstract

Two spring freezes in Griffin, Georgia during the flowering period in 2008 provided an opportunity to monitor freeze-damage effects on fruit set and fruit weight of four rabbiteye blueberry cultivars under field conditions. Minimum temperatures of −4.2 and −3.5°C occurred on March 9 and March 25, respectively. The estimated dates of 50% open flowers for the cultivars were: Climax, March 23; Alapaha, March 30; Tifblue, April 2, and Ochlockonee, April 10. Initial visual damage estimates (i.e., corolla browning) 3 to 4 days following the last freeze suggested nearly a total crop loss for ‘Climax’ and ‘Alapaha’, and moderate to severe crop loss for ‘Tifblue’ and ‘Ochlockonee’. However, more quantitative estimates of fruit set after counting several thousand flowers and fruit for each cultivar revealed different results than initially anticipated. ‘Climax’ and ‘Tifblue’ did suffer great reductions in fruit set, averaging <8% fruit remaining 6 to 8 weeks after flowering. However, ‘Alapaha’ managed to maintain fruit set of more than 36%, and ‘Ochlockonee’ fruit set was greater than 50%. There were no apparent effects of the freeze on remaining fruit weights. These data indicate that there are differences in fruit set of rabbiteye cultivars following spring freezes during flowering. Hence, growers may want to consider the more cold hardy cultivars in their planting operations if their growing areas are subject to freezes in early to mid-spring.

Keywords:

• Vaccinium ashei
• freeze damage
• fruit set
• flowering time

INTRODUCTION

Freeze damage is a peril faced by blueberry (Vaccinium spp.) growers throughout the Southeastern U.S. and other growing regions, with some fruit loss occurring almost every year. Unlike other fruit crops, such as peaches [Prunus persica (L.) Batsch.], where freeze damage can actually aid in thinning the crop, even mild freeze damage to blueberries often results in significant yield reductions. Cold injury to rabbiteye blueberry (V. ashei Reade) flower buds differs depending on cultivar and the stage of development (Spiers, 1978, 1981; Marshall et al., 2006; NeSmith, 1998). Even temperatures of 0 to −1°C can cause considerable injury to flowers (Spiers, 1978; NeSmith, 1998). Silva (1995) and NeSmith (1998) reported that there were differences in susceptibility of flower parts of rabbiteye blueberries to sub-freezing temperatures. Although these investigations documented injury to flowers, they did not assess post-freeze fruit set. Gupton (1983) purported that rabbiteye blueberry flower injury following sub-freezing temperatures, although not always visible, may prevent successful fertilization and fruit set. Using two older rabbiteye cultivars, NeSmith et al. (1999) demonstrated that fruit set declines rapidly after bee pollination as temperatures fall below freezing. They further documented that when temperatures during later stages of flowering reached −4.5°C, little or no fruit set occurred. The objective of this research was to measure fruit set and berry size of two newer rabbiteye cultivars compared to two older cultivars in response to a late spring freeze under field conditions.

MATERIALS AND METHODS

In 2008, blueberry field plots in Griffin, Georgia reached a minimum temperature on March 9 of −4.2°C, and on March 25 the minimum temperature reached was −3.5°C. Table 1 depicts percentage of open flowers (Stage 6 flowers, Spiers, 1978) for the rabbiteye cultivars Alapaha, Climax, Ochlockonee, and Tifblue at various dates during that time of year. To assess potential freeze damage to subsequent fruit set, several individual shoots per plant were tagged the week of the first freeze, and individual flowers were counted. A total of 400 to 500 flowers per plant were accounted for by this method on 18 plants per cultivar. To determine final fruit set, the number of berries was counted on the tagged shoots 6 to 8 weeks after flowering. For berry size determination, 20 berries per plant were harvested at maturity and weighed individually. When feasible, two determinations of berry size 1 week apart were made. For data analysis, means and standard errors were calculated for all data values for the cultivars.

TABLE 1 Estimated Dates of Flowering for Four Rabbiteye Blueberry Cultivars in Griffin, Georgia during 2008 ^z

	Cultivar
Flowering percentage	Climax	Alapaha	Tifblue	Ochlockonee
Calendar date 2008
5 to 10% open flowers	March 13	March 20	March 24	March 27
30 to 40% open flowers	March 20	March 27	March 31	April 7
70 to 90% open flowers	March 27	April 4	April 7	April 14
^zTemperatures of −4.2 and −3.5°C occurred on March 9 and 25, respectively.

RESULTS

Through visual observation, the March 9 sub-freezing event (−4.2°C) caused slight to moderate damage (i.e., corolla browning) for ‘Climax’ and ‘Alapaha’, and little or no damage on the later flowering cultivars, Tifblue and Ochlockonee. However, the March 25 sub-freezing event (−3.5°C) caused moderate to severe visual damage on all varieties. In fact, initial visual assessment of plants a few days after the March 25 event suggested a near total crop loss for ‘Climax’ and ‘Alapaha’, and a moderate to severe crop loss for ‘Tifblue’ and ‘Ochlockonee’.

Table 2 presents fruit set percentages for the various cultivars 6 to 8 weeks after flowering. The data revealed that although initial assessments were bleak based on floral observations, ‘Alapaha’ and ‘Ochlockonee’ actually had reasonable levels of fruit set. ‘Alapaha’ averaged just over 36% fruit set, and ‘Ochlockonee’ averaged greater than 50% fruit set. The older cultivars, Climax and Tifblue, however, had greatly diminished fruit set, with both cultivars having less than 8% fruit set. As for berry weights, the cultivars seemed to average typical values of those observed in other years. ‘Alapaha’ and ‘Tifblue’ berry weights tend to be smaller than ‘Climax’ and ‘Ochlockonee’.

TABLE 2 Fruit Set and Average Berry Weights of Four Rabbiteye Blueberry Cultivars in Griffin, Georgia during 2008 ^z

	Cultivar
	Climax	Alapaha	Tifblue	Ochlockonee
Fruit set (%)	4.3 ± 1.1	36.2 ± 4.5	7.2 ± 2.4	50.7 ± 3.2
Average berry wt. (g)	1.57 ± 0.12	1.23 ± 0.04	1.24 ± 0.04	1.46 ± 0.05
^zTemperatures of −4.2 and −3.5°C occurred on March 9 and 25, respectively.

‘Climax’ was no doubt the furthest along in development when the two freezing events occurred. This can explain to a degree why this cultivar was so hard hit by the cold damage. However, ‘Alapaha’ was further along in development than ‘Tifblue’ and still managed to set more than 36% of its fruit. ‘Ochlockonee’ likely managed to escape damage due to its later development, but even so, 50% fruit set was very good following these extreme conditions. In a previous study (NeSmith, 1998), it was shown that ‘Climax’ and ‘Tifblue’ flower parts were particularly susceptible to cold injury, with nearly 1°C less cold hardiness than ‘Brightwell’ at stages 5 to 6 of development. Since ‘Alapaha’ has ‘Brightwell’ as one parent, and ‘Ochlockonee’ is a sibling to ‘Brightwell’, this may help explain their improved response to the cold conditions. These two varieties are rather recent releases from the University of Georgia, and it is plausible that their steady performance as noted in the multi-year trials leading to their release was in part due to their ability to set fruit under more adverse conditions (Nesmith et al., 2002, 2003).

Previous studies have documented differences in flower damage of some rabbiteye varieties in response to freezing temperatures (Marshall et al., 2006; Silva, 1995; Spiers, 1978; NeSmith, 1998). However, these reports did not include final effects of cold on fruit set. Ratings of “visible” flower damage can be misleading, as Gupton (1983) reported that, depending on the cultivar, up to 67% of flowers with no visible damage failed to set fruit, even with hand pollination, following cold exposure. He supposed that pistils of the flowers had been damaged, even though not visibly, thus, rendering successful fertilization improbable. Conversely, as this study has indicated, flowers that appeared completely damaged from visual corolla assessment still had the ability to set some fruit. Therefore, one cannot simply visually inspect flowers to assess the potential fruit loss due to freezing.