Proceedings of The World
Avocado Congress III, 1995 311 - 315
EFFECTS OF IRRIGATION AND
FOLIAR CULTAR® ON FRUIT YIELD AND QUALITY
OF ‘HASS’AVOCADO FRUIT
S.
Vuthapanich
University
of Queensland
Gatton
College,
Lawes,
Australia
Department
of Primary Industries
Queensland
19
Hercules St,
Hamilton,
Australia
A.W.
Whiley
Department
of Primary Industries
Queensland
PO
Box 5083 SCMC
A.
Klieber
University
of Adelaide Waite Campus,
Adelaide
South Australia,
Australia
D.H.
Simons
University
of Queensland
Gatton
College,
Lawes,
Australia
Abstract
The effects of irrigation and foliar Cultar® on fruit yield and quality of 'Hass' avocados growing in a warm, humid subtropical climate were studied during the growing season of 1993/94. Seven-year-old trees were re-watered to field capacity when tensiometer readings reached 20, 40 or 70 kPa, and fruit growth, yield and quality were assessed. Well-watered trees (20 kPa) had twice the yield of the least watered trees (70 kPa) due mainly to higher fruit numbers and more consistent yield between trees. Fruit quality (size, shelf life, disease and disorders, and fruit minerals concentration) was not affected by irrigation regimes.
Seven-year-old 'Hass' trees
were either fertilised with 1 kg tree-1 of urea at panicle
emergence, foliar sprayed with 2.5 g a.i.L-1 Cultar® at mid-bloom, both urea and
Cultar® treatments
applied, or untreated. The urea + Cultar® combination increased fruit size, with a greater
percentage of fruit in the larger size categories. Cultar® alone increased the days
from harvest to maximum ethylene production at 22ºC, and fruit calcium
concentrations.
1. Introduction
Predictability
of fruit quality is important for most markets. Recent studies have identified
considerable variability in avocado fruit quality and postharvest performance (Hofman et al.,
1995). Production factors (e.g. locality, soil type, climate,
cultivar, nutrition and irrigation) have a significant impact on quality and
postharvest performance of fruit (Hofman and Smith, 1994).
Irrigation practices and
Cultar® (paclobutrazol) have been developed to
enhance fruit yield and maturity (based on % dry matter; DM) through water
stress and a reduction in vegetative growth at critical stages of fruit
ontogeny. However there has been relatively little assessment of these
practices on postharvest quality. This paper reports the effects of irrigation
and Cultar® (ICI
Australian Operations Pty Ltd) on 'Hass' avocado fruit yield and quality.
2. Materials and methods
2.1 Irrigation
Seven-year-old 'Hass'
avocado trees growing on clay loam soil (krasnozern) were selected in a
commercial orchard in SE Queensland, latitude 20'S. The climate is described as
warm, humid subtropical with a mean annual rainfall of 1000 mm in a summer/wet,
winter/dry pattern. Trees were irrigated over a cropping cycle to field
capacity using under-tree micro-sprinklers when tensiometer readings at 30, cm
soil depth reached 20, 40 or 70 kPa. Ten uniform trees per row were used, with
each row receiving one of the irrigation treatments.
Fruit dimensions, dry matter
(DM) and flesh mineral concentrations were determined at regular intervals from
six weeks after anthesis, and tree yield, fruit number and size recorded at
maturity. Twenty fruit of average size were harvested from each tree and
immediately dipped in 0.55 mL.L-1 prochloraz for 30 sec. Ten fruit
per tree were ripened at 22T under humidified, ethylene-free air in ventilated
containers, and the other 10 fruit stored at 7ºC for 3 weeks, then ripened at
22ºC as above. Ethylene production was measured by gas chromatography, and days
to eating soft and disease incidence recorded.
2.2. Cultar®
The experimental trees and
orchard site were similar to those described above. The following treatments
were applied to each of five single tree replications, in a randomised block
design:
1. untreated control (standard commercial
practice),
2.
urea, soil applied at the rate of 30% of annual N (1 kg tree-1 ) at
panicle emergence,
3.
Cultar® ,
sprayed at the rate of 2.5 g a. i. L-1 at full bloom, and
4. treatments 2 and 3 combined.
Fruit measurements, sampling
and harvest procedures were as above.
3. Results
3. 1. Irrigation
Fruit yield was greatest
when trees were more frequently irrigated (20 kPa), due mainly to increased
fruit number (table 1). There was no significant effect of irrigation on
average fruit mass (data not presented). Reduced irrigation increased the
variation in yield between individual trees, with a greater number of trees
having either low or high fruit numbers, but less trees with intermediate fruit
numbers per tree (table 2). Fruit from the most frequently irrigated treatment
(20 kPa) tended to ripen more slowly, but not significantly different to other
treatments (table 1). All other quality parameters measured [days to maximum
ethylene production (DMEP) and eating soft after storage, disease severity,
internal disorders and fruit minerals concentrations] were not affected by
irrigation.
3.2 Cultar®
Fruit from Cultar®-treated
trees had a lower length: diameter ratio (figure 1) due to larger fruit
diameter (data not presented). Total fruit yield per tree was not affected by
treatment, but the use of Cultar® produced greater numbers and yield of fruit which
exceeded 3001 (table 3). Average fruit mass was higher in urea + Cultar® -treated trees only. Cultar® alone reduced fruit % DM, increased flesh
Ca concentrations, and increased DMEP at 220C (table 3), but not following
storage at 7ºC for 3 weeks (data not presented). There was a significant
correlation (r2 = 0.54**) between DMEP and flesh Ca concentrations (data not
presented). No internal disorders were observed.
4. Discussion
Differing effects of irrigation
have been reported for avocado (Bower, 1985; Arpaia,
1989), but irrigation
treatment, soil type, rainfall and other factors can make comparisons
difficult. In this experiment, yield was the only parameter improved by
increased irrigation, however increased variability in yield between trees with
lower irrigation is an important consideration in future investigations. Bower
(1985) noted higher Ca concentrations in fruit from trees irrigated at 55 kPa
than those from trees at 35 and 80 kPa, indicating that irrigation may have
positive effects on fruit quality through fruit Ca. Timing of irrigation can
also be important (Bower, 1985), and further investigations into this aspect
are warranted.
Cultar® has increased avocado yield
in previous studies (Wolstenholme et al., 1990; Kremer-Köhne et al., 1991;
Whiley et al., 1991), but this effect has not always been observed (Cutting and
Bower 1990; Symons and Wolstenholme 1990). Kremer-Köhne et al. (1991) found
that a yield increase in 'Fuerte' only occurred in a low-yield (“off”) year
when an ultra-low volume spray application of Cultar® was used, indicating some of
the factors affecting treatment response. An increase in the percentage of
larger fruit was also found by Symons and Wolstenholme (1990), which is desirable
in reducing the small size problem in 'Hass'.
Increases in fruit flesh Ca
concentration following foliar Cultar® applications have been reported in previous studies
(Cutting and Bower, 1990; A.W. Whiley, pers. comm.). The observed increase in
shelf life may have been because of the reduced maturity (based on % DM;
Vuthapanich et al., 1995) or the higher flesh Ca.
The positive effects of urea
plus Cultar® on
tree yield and fruit size may be due to increased photosynthetic efficiency of
the over-wintered leaves subtending the early-developing fruits (Whiley, 1994)
along with delayed and reduced growth of the renewal spring shoot (Wolstenholme
et al., 1990). This treatment shows potential to increase yield and size, with
little negative impact of other aspects of fruit quality. This research is
being continued for a further season on the same trees to determine consistency
of treatment effects.
Acknowledgments
This
research was supported by the Department of Primary Industries Queensland,
Project 9313 of the Australian Centre for International Agricultural Research,
and the University
of Queensland, Gatton College. We thank Jack Saranah for technical support and
John and Jay Dorrian, Childers for access to their orchard.
References
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