Proceedings
of Third World Avocado Congress 1995 pp. 294 - 299
FRUIT
SET AND GROWTH PROBLEMS OF 'PINKERTON' AVOCADO IN SOUTH AFRICA
1 Institute for Tropical and
Subtropical Crops (ITSC), Private Bag X1 1208, Nelspruit, 1200, South Africa.
2 Institute for Soil, Climate and
Water, Private Bag X79, Pretoria, 0001, South Africa. Present address: HL &
H Forest Research Division, P 0 Box 1427, White River, 1240, South Africa.
3 ITSC, Private Bag X1 1208,
Nelspruit, 1200, South Africa. Present address: P 0 Box 14, Duivelskloof, 0835,
South Africa.
4 South African Avocado Growers
Association, P 0 Box 866, Tzaneen, 0850, South Africa.
Abstract
Although 'Pinkerton' is known for its consistent heavy yields, the main drawback in South Africa is that it flowers over a very long period (June to December). This results in differences in fruit maturity during picking time, ultimately causing post- harvest quality problems. Fruit size, growth rate and fruit fall of the 'Pinkerton' avocado was monitored for two seasons in different climatic areas.
Results indicate
that the main fruit set period occurred over at least a three month period,
i.e. August, September and October. Data showed that a higher fruit fall can be
expected from the sets that occur during the second half of the flowering
season. Fruit set date had a marked effect on final fruit size. Fruit which set
late (mid- September to mid-October) has a much faster growth rate than early
set fruit (July - August), and on average has the potential to be 40 mm longer
(and thus larger), at the end of the season. Fruit size and growth rate for the
various fruit set periods and for the different tree quadrants are presented,
and coupled to on-site meteorological data.
At both mid-season and
harvesting stages, three trees were harvested in order to determine total fruit
size distribution. Correlations between fruit length, width, mass and volume
are presented. 'Me best correlation was found to be between fruit mass and
volume. These non-destructive measurements can be used to calculate on-tree
fruit sizes and thus predict final fruit size.
Additional keywords: Persea americana, flowering,
fruit, growth.
1. Introduction
Fruit of the avocado are
sometimes used as an evaluation tool for research purposes, as well as for
determining picking dates. However, it is not always certain what the history
of the fruit are, i.e. date of fruit set, fruit growth period, fruit growth
rate, maturity stage and climatic conditions?
These factors are important
parameters that could have an effect on the reproducibility of any research
work undertaken. It has been found in literature that fruit are mostly selected
randomly for research purposes, for instance such as measuring fruit in an
irrigation or fertilization trial. These same factors are also important to the
producer who must decide which fruit are ready for harvesting. Post-harvest
quality problems experienced with 'Pinkerton' fruit motivated this research
project. A study was therefore carried out to investigate indices of fruit
growth.
2. Materials and methods
The trial was
conducted in three different climatic areas, namely Kiepersol (25º05'S;
31º01'E; 800 m; 939 mm), Heidelberg (25º18'S; 30º56'E; 774 m; 755 mm) and
Schagen (25º19'S; 30º45'E; 800 mm; 952 mm). Uniform-sized four to five year old
'Pinkerton' trees grafted on Duke 7 rootstock were selected for data
collection. These trees received standard horticultural practices.
2.1 Fruit set aspects
The tagging
dates for the different sites and different years ranged from early August to end
October. During the 1990/91 season, 100 fruit were selected and measured at
four different tagging dates, which were August 21, September 4, September 19
and October 3. During the 1991/92 season the number of fruit were increased to
over 500 fruit per tagging date. During each monitoring date all tagged fruit
were counted and measured. Data were accumulated on fruit fall, fruit size and
fruit growth rates. These were all related to climatic data.
2.2 Spatial shape
determinations
For this purpose three trees were fully stripped of
all fruit during two sampling periods, i.e. mid-November and at harvesting
(May). Data were accumulated on fruit mass, fruit length, fruit width, fruit
volume (every 5th fruit), seed volume (every 5th fruit - mid-November) and seed
mass (every 5th fruit).
3. Results
3.1 Fruit set and fruit fall
Climate has a definite influence on flowering and fruit set. At Heidelberg these events occurred very late, allowing tagging to only commence in early October (latter part of the flowering season), whereas flowering and fruit set commenced during mid- August at Kiepersol. During the 1991/92 season, unfavourable climatic conditions had the effect that very little fruit formed at the Schagen site.
Although fruit between 20 and 30 mm in length were
tagged, thus limiting pollination and fertilisation problems, a large fruit
fall was still encountered. , At both Schagen and Kiepersol during 1990/91,
greater fruit fall was experienced from fruit setting late, (up to 93%). 717he
same trend was observed during the 1991/92 season for both Heidelberg and
Kiepersol.
3.2 Fruit size
Figure 1
illustrates fruit size differences before harvesting of four different fruit
set periods for Kiepersol during 1990/91. Fruit from the late set periods
became larger than
fruit from early set periods, even though the former had shorter development
time. The same observation was made during the 1991/92 season. Even though
there was a 50 day difference between first and last set, the latest set fruit
still attained a larger size.
Fruit sizes from the
different set periods were also compared on different quadrants of the trees.
On average the western side of the tree produced the largest fruit. Fruit size
distribution on the tree during both mid-November and May Oust before
harvesting) was also determined. At mid-November 68.7% of the fruit were
between 60 and 90 mm in length, with the median being 73.2 mm. During
harvesting (May) 74% of the fruit were between 100 and 130 mm, with the median
at 119.1 mm. This indicates a small improvement in size distribution difference
towards the end of the season.
3.3 Fruit growth rates
The average
fruit growth rate (FGR) for fruit from the late set (1.34 cm3day-1)
during the 1990/1 season is greater than that of the early set (1.00 cm3day-1).
'Me highest FGR was recorded from fruit on the western side of the tree.
Table 1 summarizes average
maximum air temperatures (Max T), average minimum air temperature (Min T),
period applicable, and fruit growth rate (FGR) between first and last set periods
at two sites as well as the relevant fruit growth phases. On average the
maximum air temperature was approximately 1.4ºC greater at Heidelberg while the
minimum was 0. VC higher at Kiepersol. Fruit from Kiepersol showed a higher FGR
than fruit from Heidelberg for both phase I and phase II fruit growth stages.
Even a 2.0ºC higher temperature at Heidelberg during phase I did not induce a
higher FGR.
3.4 Spatial shape
determinations
Zilkah &
Klein (1987) found that the spatial shape of the avocado fruit could be
evaluated by a factor (M), which has been calculated from the ratio of the
fruit volume to circumferential cylinder volume. These authors proposed a
formula (Vf = M x Pi/4 x LD2) to determine volume by measuring
length and diameter. For the purpose of determining the M-value, fruit from the
mid-November and May sampling periods were used.
The average
M-value for 'Pinkerton' under South African conditions differs from small fruit
(0.47) to large fruit (0.62), with an average of 0.545. We also correlated
different fruit parameters of the two sampling periods (Table 2). From these
data an excellent correlation was found between fruit volume and fruit mass,
for both the mid- November (r = 0.975) and the May (r = 0.994) sampling
periods.
4. Discussion
4.1 Fruit fall
and fruit size
The trend where fruit from
later set periods were more prone to fall than fruit from early set periods
indicates the contribution of set period to final fruit number. Even so, the
size and maturity difference of fruit between early and late fruit set periods
are a concern for both researcher and producer/marketer.
to avoid the problem of fruit
being of differing maturities at harvest due to different fruit set periods.
The producer who uses fruit to determine picking dates should also take
cognisance of this problem. This trial has shown that larger fruit can be
produced from a later set period and that such fruit can still be immature. If
the smaller fruit, from early sets, are left to increase in size, they can
actually become over-mature, with the accompanying post harvest quality
problems.
4.2 Fruit growth rate and
climatic aspects
It was found
that higher temperatures at one site does not necessarily result in a faster
fruit growth rate of fruit from that site. It was also found that the average
fruit growth rate of late set fruit is greater than that of early set fruit.
The exact influence of climate on fruit growth, with regard to the different
set periods and the phases of fruit growth, is still not clear. This aspect needs
to be further investigated.
4.3 Spatial shape
determinations
The availability of a formula
with which the producer can determine final fruit mass and possible picking
date, will greatly assist management planning. The advantage of this is that it
is a non-destructive technique.
5. Acknowledgement
Thanks are due to the
participating farmers for supplying trees and fruit for the purpose of this
study, as well as to technical inputs from ITSC personnel. Funding was supplied
by both the Agricultural Research Council and the South African Avocado Growers
Association.
References
Zilkah,
S & Klein, L 1987. Growth kinetics and determination of shape and size of
small and large avocado fruits cultivar Hass on the tree. Scientia Horticulturae
32: 195-202