Agriculture Reference
In-Depth Information
elevations of Mexico (Fig. 7.1). Leaves of the Mexican race are anise-scented
and 60% of the essential oil is the monoterpene estragol (Scora and Bergh,
1992). These races are recognized as subspecies
P. americana
var.
americana
(West Indian), var.
drymifolia
(Mexican) and var.
guatemalenis
(Guatemalan)
and are based upon taxonomic, biochemical, isozyme and molecular data
(Bergh and Ellstrand, 1986; Furnier
et al.
,
1990; Ashworth and Clegg, 2003;
Chen
et al.
, 2009). Ben-Ya'acov
et al.
(1995) have suggested adding a new
race, which they call var.
costaricensis
. Due to the outbreeding nature of these
taxa and human selection and cultivation, there are many interracial hybrids
and some of the principal commercial cultivars are of hybrid origin.
ECOLOGY
Ecological requirements of avocado should be viewed in terms of the areas
of geographical origin. It is generally considered to be a subtropical plant,
except for the tropical West Indian race. Regardless of origin, the avocado has
shown adaptability to a wide range of ecological conditions, from the tropics
to approximate latitudes of 30° N and S. This wide distribution may be due
to the broad genetic dif erences of the three horticultural races. In some high
Andean regions, Mexican avocados grow well under these harsh conditions.
Soil
Avocado is grown in a wide variety of soil types. Deep soils of volcanic origin,
sandy loam soils, calcareous soils and other soil types have supported good
growth. Soil pH may range from 5 to around 7. Since avocado is highly
susceptible to root rots, good drainage is crucial and a high water table
undesirable. Trees show dieback in parts of fi elds when the water table is less
than 1 m.
Avocado has little tolerance for saline conditions. Kadman and Ben-
Ya'acov (1970a) produced various degrees of leaf burn by irrigating an
avocado orchard with water containing 150-170 mg chlorine/l. It has also
been shown that West Indian seedlings are more tolerant to salt than Mexican
seedlings, with Guatemalan having an intermediate tolerance (Table 7.1).
Climate
Rainfall
Most cultivars are sensitive to water stress and to excess moisture caused
by poor drainage. In Hawaii, avocado trees have grown well under annual
rainfall of 3125 mm, largely due to the excellent drainage provided by the
