Proceedings
of The World Avocado Congress III, 1995 289 - 293
FRUIT
MATURITY OF THE 'PINKERTON' AVOCADO AS AFFECTED BY FRUIT SET PERIOD
1Institute for Tropical and
Subtropical Crops (ITSC), Private Bag X11208, Nelspruit 1200, South Africa.
2 Institute for Soil, Climate
and Water, P/Bag X79, Pretoria 0001, South Africa. Present address: HL & H
Forest Research Division, PO Box 1427, White River 1240, South Africa.
3South African Avocado Growers'
Association, PO Box 866, Tzaneen 0850, South Africa.
Additional index words: Persea americana, growth,
flowering.
Abstract
In South Africa the
'Pinkerton' avocado tree tends to flower over a long period, giving rise to
fruit of differing maturities at harvesting stage. This study was aimed at
investigating the effect of different fruit set periods on fruit maturity by
limiting fruit set to a specific period. Flowers and fruitlets were removed
during 1992 in two 'Pinkerton' orchards, one at Kiepersol and another at
Heidelberg (White River). During the first season the treatments consisted of
an August set, September set, October set and a control (no manipulations). The
second season included a pre-August set and the set period at Kiepersol was reduced
to 15 days instead of 30 days.
At Kiepersol the September
set had the effect of increasing fruit size whilst at Heidelberg both September
and October set showed increased fruit size with the control fruit set showing
smaller fruit sizes than the other sets. These size differences were also
reflected by the spatial shape of the fruit.
Moisture
percentages of the different treatments indicated that at Kiepersol, the early
fruit set was ready for marketing a month before the control fruit. Under the
cooler climatic conditions at Heidelberg the fruit from all the different
manipulations took at least a month longer than those at Kiepersol to reach the
same maturity stage. Data on post-harvest cold storage are also presented.
1. Introduction
In South Africa, flowering of
avocado trees normally starts around June/July with fruitset occurring during
September (Robertson, 1969). Local 'Pinkerton' trees flower profusely over a
very long period, ie. from June through to December. This long flowering time
causes fruit to be developed over a long period, resulting in fruit of
different maturities at harvest time. It was also shown (Sippel, et al., 1992)
that fruit setting late had a much faster growth rate and the potential to
become larger than early set fruit. These large late-set fruit, which at
harvesting time (May) can still be immature, can be wrongly picked if fruit
size is taken as a maturity index.
Fruit and flower manipulation
was investigated as a possible means to produce uniform- mature fruit. This
trial concentrated on limiting the fruitset period by physical removal of
unwanted fruit and flower panicles. The effect of these manipulations on fruit
size, yield, fruit maturity and fruit quality was studied, coupled to two
different climatic areas.
2. Materials and methods
The trial was conducted in
two different climatic areas namely Kiepersol and Heidelberg (Nelspruit/White
River). Uniform-sized trees were selected for manipulation purposes. Both
orchards received standard horticultural practices.
During the first season
(1992/93) four different treatments were laid out randomly in the orchards.
Single whole tree treatments were applied, replicated three times. The
treatments consisted of an August set, September set, October set and a control
(not manipulated). All unwanted flowers and fruit were physically removed to
allow set periods of about 30 days for each treatment. A fixed starting and end
date for each treatment was chosen beforehand, and these determined the fruit
set periods. Data collected at harvest consisted of moisture analysis as well
as quality assessments on fruit that were export simulated. These fruit were
subjected to four different cold storage regimes as well as a control at 18ēC.
During the second
season (1993/94) flower and fruit removal was done on 10 trees per treatment. A
pre-August manipulation was included together with the August, September,
October and control treatments. At Kiepersol the fruit set period was limited
to 15 days whilst it was limited to 30 days at Heidelberg. All fruit that set
before the required periods was removed and all flowers produced after the
fruit set period were also removed. Data were accumulated on yield, fruit
growth, moisture and oil analysis and fruit quality. Fruit were both exported
and export simulated and quality was analyzed.
3. Results
3.1 1992/93 season
3.1.1 Fruit maturity
August set fruit from the
Kiepersol site were ready for harvesting during mid-April whilst the other sets
and the control were still outside the picking norm of 75% moisture (Table 1).
One month later all the sets were at harvestable stage with the control fruit
and the October set fruit showing the highest decrease in percentage moisture,
and the August set fruit the lowest. At the Heidelberg site none of the sets
were ready to be harvested during mid-April (Table 1). One month later the
manipulated sets were ready for harvesting whilst the control fruit were still
outside the picking norm at 78,3%.
3.1.2 Cold storage
At the control
temperature (18ēC) early set fruit from both sites performed similar being of
better quality than fruit from the other sets. At Kiepersol, the warmer
production area, the August and September set fruit performed better at the
higher storage temperatures (7.5 and 6.5ēC) whilst the October and control sets
performed better at 5.5ēC. These last two were also judged to be the best
overall. Flowering at this site tended to be earlier than flowering at
Heidelberg.
At Heidelberg the
August set again performed better, together with the control set, at the higher
storage temperatures. The September and October sets performed better when
subjected to colder storage temperatures. Overall the early set fruit had the
better quality. Good quality were obtained with fruit between 75 and 76%
moisture content. At lower and higher figures fruit quality tend to be poor,
regardless of storage temperature.
3.2 1993/94 season
3.2.1. Moisture
determinations
Table 2 compares
freeze drying of moisture samples with conventional oven drying. A very good
correlation was found between the two methods. The table also shows differences
in moisture content between the two sites. Even with a ten day later sampling
date, maturity of the Heidelberg control fruit was retarded compared with that
of the Kiepersol control. In contrast, fruit maturity of all the manipulated
treatments at Heidelberg was more advanced than those at Kiepersol.
3.2.2. Yield & fruit size
Fruit size was affected by
fruit count per tree as well as the fruit set period. The best treatment at
Heidelberg was the mid-August set with an average of 244 fruit per tree and a
20% higher yield per tree than the control. At the Kiepersol site the control
treatment had about 10% fewer fruit than the best treatment (mid-August), but a
38% lower yield per tree due to smaller fruit.
Figure I shows
yield per hectare differences between set periods and sites. Note that the
mid-August set treatment was superior to the control treatment at both sites.
Heidelberg shows a poor fruit set at the beginning of the season while
Kiepersol shows a rapid decline from mid- September.
3.2.3. Fruit
quality
Table 3 presents
quality data from Heidelberg fruit that were exported to the UK. No clear
treatment differences were shown in fruit firmness. However, trends were
observed with black cold, lenticel damage and greypulp during both pre- and
post ripening of the fruit samples. Older fruit (early sets) had more problems
than fruit from later sets. Control fruit gave variable results. With dusky
cold only the post-ripening analysis showed a definite trend of older fruit
being more susceptible to damage.
4. Discussion
4.1 Moisture
determinations
Differences in fruit
set period resulted in definite differences in moisture content. If manipulated
into different sets, fruit from the early sets could be ready for harvesting up
to one month before that of later sets, which could also mean better prices due
to market demand for early fruit. Freeze drying and conventional oven drying
gave a very good correlation enabling the former to be reliably used for future
analysis. Freeze drying is a much easier method and a larger number of samples
can be dried more rapidly.
4.2 Yield& fruit size
Low average fruit counts from the pre-August
treatment at Heidelberg can be ascribed to unfavourable climatic conditions
during the early part of the flowering season. Kiepersol, on the other hand,
had good flowering from early in the season with a rapid decline after n-Ad-
September. This phenomenon explains the low fruit count during the October set
in this orchard. At both sites optimum fruit set was obtained during the
mid-August period.
Yield was influenced by fruit set period. If a producer
is prepared to manipulate this, his production could be 38% higher. An increase
in fruit size is an added benefit, which could possibly result in better market
prices. This benefit should be weighed against labour costs to manipulate the
trees.
4.3 Fruit quality
Results obtained
from the export fruit quality analysis indicate the importance of fruit age and
optimum fruit maturity towards improved fruit quality. This highlights the
disadvantage of a single harvest, when old and young fruit of different sets
are picked at the same time, a process followed by many producers. Post-harvest
problems can be avoided if the producer is prepared to manipulate his trees.
However, he should also be prepared to harvest the manipulated trees separately
to ensure optimum post-harvest quality.
Acknowledgements
The producers, namely Messrs
Vos & Son (Kiepersol) and Pienaar Packers (Heidelberg), willingness to
donate their orchards for the purpose of this study is greatly appreciated.
Thanks are also due to Mr & Mrs Hearne for accumulating climatic data at
Kiepersol and to Mrs Marie Hattingh for computerizing it. Also many thanks to
Miss Mariet Christie for constant field work and computerizing all the data and
Mrs Valerie Claassens and Mr Coenie Pretorius for valuable technical assistance
with the field work, as well as SAAGA and Jerome Hardy for assistance with the
analysis of exported fruit.
References
Robertson, B.L., 1969. The
morphogenesis of the flower and fruit of the 'Fuerte' avocado (Persea
americana Mill x P. drymifolia Chain. Schlect.). M.Sc (Agric)
thesis, Univ of Pretoria, Pretoria.
SIPPEL, A.D., CONRADIE, W.
& CLAASSENS, N J.F., 1992. Growth rates of 'Pinkerton' avocado fruit. S
A Avocado Growers' Ass Yrb 15, 72-74.