In general, lowbush and highbush varieties range from moderately self-sterile to self-fertile. Almost all rabbiteye varieties are self-sterile and require cross-pollination. All types benefit from a high number of fertilized seeds per berry which improves fruit-set, increases berry size, and speeds ripening. Thus, bee activity is important for trans- porting pollen to receptive stigmas even in self-fertile varieties. Eck (1988) estimates that 60–80% of blueberry flowers must set fruit in order to achieve a commercially viable yield.
Lowbush pollination requirements
Lowbush blueberry clones range from self-sterile to moderately self- fertile (Hall and Aalders, 1961). Cross-pollination generally improves
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fruit-set in V. angustifolium (Wood, 1968). The two main lowbush species V. angustifolium and V. myrtilloidesoften grow together natu- rally in cleared lands. The two occur in about equal proportions in cleared forest, whereas V. angustifoliumpredominates in abandoned farmland. The berries abort when V. angustifoliumis pollinated with pollen from V. myrtilloides. Therefore, one can expect a maximum fruit-set of about 50% in cleared forest where the two species occur.
Fruit-set can be much higher in patches where V. angustifolium pre- dominates. There would be probably fewer pollination problems in lowbush blueberry if plantings could be restricted to two or more good pollen-producing clones of the same species (Hall and Aalders, 1961). With V. angustifolium, over 5% of the plants are male-sterile and 45% produce relatively little pollen (Hall and Aalders, 1961).
Thus, pollen scarcity can be a limiting factor in production, and this necessitates good pollinator activity in order to move the available pollen among plants.
Highbush pollination requirements
Highbush varieties are largely self-fertile (El-Agamy et al., 1979), but cross-pollination sometimes increases number of seeds per berry, fruit-set, fruit size, and speed of ripening. Honey-bee-mediated cross- pollination of the southern highbush variety ‘Sharpblue’ (with
‘Gulfcoast’) increased yield of early-ripening ‘Sharpblue’ fruits by 140%, increased heavy (≥0.75 g) fruits by 13%, and decreased small fruits by 66%, all of which translated to a 43% increase in early- market crop value – about US$2000 acre21(US$5000 ha21) (Lang and Danka, 1991). Cross-pollination did not affect fruit-set. However, fruit- set in ‘Sharpblue’ crossed with V. corymbosum clones ‘O’Neal’ and
‘FL 2–1’ was at least twice as high as in selfed plants; moreover, cross- pollination increased the number of seeds per berry and speed of ripening (Lyrene, 1989). Fruit-set was lowest in selfed ‘Sharpblue’, intermediate in ‘Sharpblue’ and a ‘Sharpblue’ + ‘FL 2–1’ mixture, and highest in ‘Sharpblue’ + ‘FL 2–1’ alone, demonstrating a degree of self non-fruitfulness in ‘Sharpblue’. Congruent results were found by Huang et al. (1997) who demonstrated that crossing ‘Sharpblue’ with
‘O’Neal’ or with ‘Gulfcoast’ reduced the incidence of poorly-devel- oped ovules, reduced incidence of ovule abortion, and increased fruit mass at ripening compared to selfed ‘Sharpblue’.
Gupton and Spiers (1994) showed that varieties perform different- ly as pollen donors, that is have different effects on development of the berry. In crossing experiments with seven southern highbush vari- eties, selfing generally did not affect fruit-set, but it reduced number of seeds per berry, berry weight, and speed of ripening, thus confirm-
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ing earlier studies. Pollens from ‘Georgiagem’ and ‘Cape Fear’ general- ly produced the longest ripening times in the main variety, and
‘O’Neal’ and ‘Gulfcoast’ pollens produced the shortest ripening time.
Pollens from ‘Blue Ridge’, ‘Gulf Coast’, and ‘O’Neal’ produced the heaviest berries in the main variety.
Pollinating southern highbush varieties with pollen from rabbit- eye varieties does not affect fruit-set and speed of ripening, but it reduces number of seeds per berry and berry weight (Gupton and Spiers, 1994).
Solid-block plantings of southern highbush varieties, especially
‘Sharpblue’, are common, but based on these studies growers may benefit from interplanting highbush blueberries with compatible high- bush pollenizers. Speed of ripening, and consequently cross-pollina- tion, are less important in areas where highbush blueberries are grown for processing. Solid-block plantings may give satisfactory results in these cases.
A two-year study in New York established the pollination require- ments of three northern highbush varieties (MacKenzie, 1997). With the variety ‘Bluecrop’, cross-pollination (with ‘Spartan’, ‘Coville’, or
‘Patriot’) conferred little benefit over selfing. For ‘Northland’, the fruit- set was unacceptably poor (≤20%) when flowers were bagged to exclude pollinators, and in 1 year of the trial cross-pollination (with Coville) improved fruit weight, seed number, and speed of ripening over self-pollination or open pollination. In the variety ‘Patriot’, cross- pollination (with ‘Spartan’, ‘Coville’, or ‘Bluecrop’) increased the number of viable seeds compared to open pollination or selfing; cross- pollination (with ‘Spartan’ or ‘Coville’) also increased fruit weight in one year compared to open pollination, and crossing in the second year (with ‘Bluecrop’) improved fruit weight and speed of ripening compared to selfing. The three varieties also differed in the degree of expressed parthenocarpy. ‘Northland’ yielded only two seedless berries while ‘Patriot’ had significantly more seedless berries (58%) than ‘Bluecrop’ (20%).
Rabbiteye pollination requirements
Rabbiteye varieties are largely self-sterile and require cross-pollina- tion with a suitable rabbiteye variety (El-Agamy et al., 1979). One exception to this is the variety ‘Centurion’, which is self-fertile at least when grown in North Carolina (P. Lyrene, Univ. Florida, per- sonal communication). But the general rule is that cross-pollination with other rabbiteye varieties improves fruit-set, size, and earliness of ripening. Pollinating rabbiteye with pollen from southern high- bush, on the other hand, reduces fruit-set, number of seeds per
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berry, berry weight, and speed of ripening in rabbiteye (Gupton and Spiers, 1994).
If one selects rabbiteye varieties carefully for orchard interplant- ing, it is possible to provide bloom overlap of compatible varieties for good cross-pollination and thus prolong the harvest season. It is important to select varieties for interplanting that have similar chill hour requirements (that is, winter hours below 45°F (7.2°C)) because these varieties have the most bloom overlap. Table 24.1 describes char- acteristics of some rabbiteye varieties widely used in the southern US.
Information provided in Table 24.1 can help one select rabbiteye varieties for interplanting. For example, an interplanting of
‘Beckyblue’, ‘Bonita’, and ‘Woodard’, having similar chill hours, would promote good bloom overlap and cross-pollination. By inter- planting ‘Climax’, ‘Bluebelle’, and ‘Baldwin’, one could maximize both cross-pollination and length of harvest interval. Many other fac- tors must enter the decision making process, however, including vari- etal differences in flavour and harvesting properties, as well as pollen compatibility. Extension agents and crop advisers are valuable resources for determining the best varietal blend for a particular area.
The planting arrangement of rabbiteye blueberries is important to good cross-pollination (Krewer et al., 1986). The goal is to increase the chance of a bee visiting two or more varieties during the same foraging trip. If the objective is to grow equal numbers of two varieties, they can be planted according to plan 1 (Fig. 24.2). If the goal is two-thirds
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Fig. 24.2.Plan 1 orchard design for encouraging cross-pollination in rabbiteye blueberry. There are equal numbers of two varieties. (Source: Carol Ness.)
of variety A and one-third of variety B, they can be planted according to plan 2 (Fig. 24.3). For three varieties, plan 3 is an appropriate model (Fig. 24.4).
Growers in the southern US perceive that pollination is a limiting factor in rabbiteye blueberry production. One problem is the morphol- ogy of the flower which is not conducive to pollination by honey bees, the most abundant pollinator (Ritzinger and Lyrene, 1999) (see page
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Table 24.1.Rabbiteye blueberry varieties (from Krewer et al., 1986 and 1993).
Variety Chill-hour requirement Spring freeze resistant?
Early-season harvest
Beckyblue 300
Bonita 300
Brightwell 350–400 ✓
Climax 450–550
Premier 550
Woodard 350–400
Mid-season harvest
Bluebell 450–500
Briteblue 400–650
Powderblue 550–650 ✓
Tifblue 550–750 ✓
Late-season harvest
Baldwin 450–500
Centurion 550–650 ✓
Delite 500
Fig. 24.3.Plan 2 calls for two-thirds of variety A and one-third of variety B.
(Source: Carol Ness.)
171). Another is the regular risk of freeze damage during the critical spring pollination window. NeSmith et al. (1999) chilled plants of
‘Brightwell’ and ‘Tifblue’ at 32, 30.2, 26.6, or 23.9°F (0, 21, 23, or 24.5°C) for one hour during flowering and then exposed the plants to pollination by either bumble bees only or to bees plus the growth reg- ulator gibberellic acid (GA3). Fruit-set in the bee pollination treatment dropped sharply in plants chilled to 30.2°F (21°C) or lower, but fruit- set in the bees plus GA3 treated plants did not decline significantly from non-chilled controls until temperatures dropped below 26.6°F (23°C). Thus, GA3 confers some degree of compensation for chill- induced pollination deficiencies. GA3has become widely used in the southern US, but it should not be thought of as a replacement for pol- lination. Compared to hand pollinated plants, fruit weight was lower and time to ripening longer in ‘Beckyblue’ rabbiteye plants treated with GA3only, and fruit weight was maximized when GA3 was used in conjunction with pollination (Cano-Medrano and Darnell, 1998).
Thus, GA3treatment is best thought of as a pollination supplement or compensation for periods of suboptimal pollination conditions.