Agriculture Reference
In-Depth Information
Breeding
Intensive avocado breeding has been conducted over many years in the USA,
Israel, South Africa and other countries. However, until recently, cultivars in
the avocado-growing countries of the world originated as chance seedlings
(Bergh, 1976).
Breeding methods may be by self-pollination or by hybridizing two selected
parental cultivars, the choice depending upon objectives of the breeding
programme. The avocado is highly heterozygous, because of signifi cant cross-
pollination between complementary cultivars, and signifi cant variability
can be obtained among self-pollinated progenies (Lavi
et al.
,
1993b; Violi
et al.
, 2009). Selfi ng can be achieved by the use of cages with pollinator bees
enclosed within the tree or by using isolated trees of a desired parent cultivar
(Bergh, 1976). Outcrossing is reported to be nil where trees of a cultivar
are separated from other genetic lines by 100 m or more. Variations in daily
weather conditions during the fl owering period may disrupt the diurnal
synchrony of the fl owers, allowing self-pollination.
Cross-pollination is necessary when two or more desirable traits are to be
combined or when traits intermediate between two cultivars are desirable, but
this has proved dii cult with avocado. The major cultivars in subtropical areas
are hybrids of the Guatemalan and Mexican races, as the Mexican race is the
source of cold-hardiness. In tropical regions, hybrids of the Guatemalan and
West Indian races prevail.
Selection and evaluation
Breeding objectives may be divided into traits that are universally desired
and those which are regionally specifi c. Tree characteristics sought after in
all growing areas are ease in propagation, vigour, precocity, spreading, short
fruit-maturation period, heavy and regular bearing, and wide adaptability
with resistance to disease and insect pests. Traits specifi c to regions are cold
or heat tolerance and salinity tolerance. Other qualities, such as dwarfi ng or
semi-dwarfi ng, genetic uniformity, freedom from sun-blotch disease, caused by
a viroid, and drought resistance, are also constantly evaluated (Bergh, 1976).
Universal fruit characteristics are resistance to diseases, pests and
blemishes, long tree storage, uniform ripening, minimum fi bres and long shelf-
life (Currier, 1992). Traits that dif er due to specifi c climatic conditions or
market preferences are fruit size and shape, skin colour, oil content and chill
tolerance. Market preference is usually based upon consumer familiarity with
certain traits. Fruit size is a good example. Consumers in California generally
prefer a size range of 200-300 g, while in most Latin American countries
larger fruits are preferred.
