Annona Species Monograph.pdf - Crops for the Future

Chapter 9. Genetic Improvement
soursop, like the other annonas, in the southern hemisphere under tropical
conditions.
Shading of the generally vigorous annona trees can greatly reduce fruit set.
Therefore, pruning and spacing are cultural practices that need to be adjusted
for enhancing fruit set in any breeding programme. No photoperiod
responses have been reported in annonas, so this factor can safely be ignored.
It is very common to observe trunk and stem breakage of soursop and sugar
apple trees due to winds. Tree shaking may also be partially responsible for
collar-rot by allowing penetration of pathogens, and the fruit skin is easily
damaged by rubbing and exposure to drying winds (Marler et al., 1994).
Control of fruit drop can be improved by windbreaks and under-tree
sprinkling to raise RH above 60% (Nakasone and Paull, 1998).
All Annona species can grow in a wide range of soil types, from sandy soils
to clay loams. However, they prefer rich, well-drained soils, and breeders
have the additional advantage of avoiding root-rot diseases in seedling
populations.
A small population of pollinator insects may limit fruit set of open pollinated
annonas. The morphology and fragrance of flowers suggest that natural
pollination is done by certain species of Coleoptera beetles (Coronel, 1994;
Pinto and Silva, 1996). Low numbers of pollinator insects coupled with slow
anthesis (flower opening) impede insect visits. Consequently, there is often
very low fruit set from natural pollination, varying from 0% in some
cherimoya orchards (Gardiazabal and Cano, 1999) to 26% in some soursop
orchards (Pinto and Ramos, 1999). In contrast, the same authors obtained
26% and 73% fruit set by using hand pollination on cherimoya and soursop,
respectively. Due to protogyny, hand pollination is useful in breeding
programmes, since breeders can select the parents to be crossed and may also
improve a full-sib progeny population more quickly than a half-sib one.
Apart from cherimoya, germplasm banks that contain Annona species are
rare throughout the world (see Chapter 8), which is a limiting factor for
selecting and crossing among elite cultivars. The length of the juvenile period
is also a limiting factor for genetic improvement. Generally, the juvenile
period lasts until the third year. Therefore, if one considers three years of
seedling and fruit evaluations as a minimum requirement, the release of an
F1 cultivar is only possible six years after the cross.
Seedling rootstocks of annonas are generally derived from heterogeneous
open-pollinated plants; hence, it is often difficult to fix specific characters in
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