AVOCADO - Tropical Fruits - page 157

'a' a' soil (crushed lava rock). Generally, a moderate rainfall range between
1250 mm and 1750 mm per annum with good distribution is desirable.
Almost all avocado-growing areas have wet and dry periods, necessitating
some form of supplemental irrigation.
Avocado infl orescences are not damaged by moderate amounts of rain
for short periods, although relatively dry conditions are preferred during
fl owering. Avocado roots are shallow, and prolonged dry conditions during
the critical periods of fl owering and fruit set can cause fl ower and young-fruit
drop. Among the three races, West Indian cultivars are more adapted to high-
summer rains, while the Mexican races possess greater tolerance to water
stress and lower humidity.

Temperature
Humans have attempted to broaden the range of the three races for economic
reasons by cultivation in areas with seasonally adverse conditions. The West
Indian race is best adapted to a humid, warm climate and monsoon rains, with
optimum temperatures around 25-28°C (Table 7.1). Higher temperatures
depress photosynthesis, thus lowering yields. This race is susceptible to frost
and has a foliage tolerance to a minimum temperature of 1.5°C. Mature trees
of Mexican cultivars have been observed to tolerate temperatures as low as
− 4 to − 5°C without damage to the foliage and wood, although fl owers are
damaged. The Guatemalan race is adapted to a cool tropical climate but is less
tolerant of low temperatures than the Mexican race (McKellar et al. , 1992).
Cultivars have shown tolerance to light frost down to −2°C, but fl owers are
damaged by even light frost (Bower, 1981a).
Night temperatures of 15-20°C and day temperatures of ~20°C are the
most suitable temperatures for fl oral development, pollen-tube growth and
embryo development (Table 7.2). Generally, high humidity, exceeding 50%,
is desirable, especially during fl owering and early fruit set. The Mexican-
Guatemalan hybrids, such as 'Fuerte', have shown wider temperature
tolerance to cold than those of the Guatemalan race. Vegetatively, 'Fuerte'

Table 7.2. Effect of growing temperature (day/night) on vegetative growth and
fl owerin g of 'Hass' avocado (after Chaikiattiyos et al. , 1994).

Percentage of terminal shoots

Temperature

(°C)

Panicle Vegetative Dormant

15/10 35 0 65

20/15 20 0 80

25/20 0 100 0

30/25 0 100 0