Annona Species Monograph.pdf - Crops for the Future

Chapter 9. Genetic Improvement
Breeding for pest and disease resistance is another important part of cultivar
development. The soursop cultivar ‘Morada’ has shown less susceptibility to
fruit borer than other cultivars under the environmental conditions of Brazil's
Central Region (Pinto and Silva, 1996). Given the stage of domestication of
the three principal annonas, it is not surprising that this type of resistance is
rare, especially when most annona plantations are monoculture orchards.
Quantitative characteristics, like yield, controlled by numerous major and
minor genes, are more difficult to manipulate than qualitative ones. In
addition, traits whose expression is influenced by environment are also more
difficult to select for than those unaffected by environmental factors. As
Annona breeding programmes have several objectives, it is the responsibility
of the breeder to design cost-effective strategies to select genotypes with the
maximum number of desired traits.
9.4.3 Methods and strategies
Most Annona species and cultivars differ in environmental adaptation,
productivity and fruit quality. Therefore, different conventional methods can
be used in their breeding. According to Fehr (1987a), there are three
requirements for the development of an asexually propagated cultivar: a) a
suitable source of genetic variability; b) evaluation of individuals from the
population; c) asexual multiplication of a new cultivar for commercial use.
Introduction of superior genotypes and/or cultivars to establish a germplasm
collection is, basically, the first requirement of any breeding programme.
This can be complemented by the introduction into the collection of some
wild Annona species with useful genes, mainly for resistance to diseases. All
accessions require comprehensive characterisation and documentation,
followed by evaluation and selection.
Several types of populations can be developed by hybridisation, from which
superior clones are selected. Nonetheless, most of the existing commercial
cherimoya cultivars in Chile and Spain were released after selection and
asexual propagation of open-pollinated progenies (Hermoso and Farré, 1997;
Gardiazabal and Cano, 1999), so hybridisation is not always necessary,
although it can often accelerate a breeding programme. However, breeding
(and selection) in cherimoya has been neglected and only a few new cultivars
have been developed in the past 20 years, due mainly to the lack of breeding
programmes and clear strategies. In contrast, other subtropical and tropical
fruit species, such as mango, have been intensively selected and cloned from
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