Agriculture Reference
In-Depth Information
Fig. 9.8  Commercial blue-
berry breeding programme,
with raised beds and drip
irrigation lines
in the first 30 cm of soil (Bryla and Strik 2007 ). The roots are sensitive to excess
water and so in heavy soils, raised beds are used for cultivation, although rabbiteye
blueberries appear to be less sensitive. However, hairless roots require a superfi-
cially humid soil with good drainage, for reasons noted above. This makes likely the
occurrence of hydric stress, to which the plants are also sensitive but generally do
not show immediate symptoms, and also means that soil cultivation close to plants
carries a high risk of causing damage or death to the plants.
Blueberries need a certain number of hours of cold accumulation; the amount
varies depending on the species but is in the range of 200- to 1100 hours at less than
7.2 °C. The Northern highbush cultivars require 800-1200 chilling hours, while
Southern highbush require from 200 to 600 chilling hours (Hancock et al. 2008 ).
Blueberries are considered to have a low nutritional demand compared to many
fruit trees, and optimum growth is normally in soils with acid pH (4.0 to 5.5), where
nutrient availability is reduced (Hanson and Hancock 1996 ; Williamson et al. 2006 ).
Pruning of blueberries is considered essential and has two main functions: to
regulate the fruit load and rejuvenation. The premature aging of orchards is one of
the most common consequences of lack of pruning as a balance is needed between
new shoots emerging from the crown and the canes of different ages needed for
production. Southern highbush plants are also pruned to maintain size (Yarborough
2006).
Levels of self-fertility within cultivars are also a factor. Recent studies have
shown fruit set can decrease by up to 57 % and fruit size by 30 % when comparing the
reaction of different varieties to self- or cross-pollination (Retamales and Hancock
2012 ). It is therefore imperative that breeders test the self-fertility of potential new
releases as they are usually evaluated in small blocks where outcrossing is common.
The fruit is an almost spherical berry (7 to 15 mm in diameter) with a colour
that depends on the variety but can vary from light blue to deep black and which
is covered with an appealing waxy coating (bloom) (Fig. 9.9 ). The fruit contain
20-80 seeds within the endocarp. A feature of the fruit is the bloom scar, which
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