Proceedings of The World Avocado Congress III, 1995 29 - 31
POSSIBLE MEANS TO INCREASE HASS AVOCADO FRUIT SIZE
S. Kremer-Köhne and
J.S.Köhne Merensky Technological Services P.O. Box 14, Duiwelskloof 0835 South
Africa,
Abstract
Various
approaches to solving the Hass small fruit problem are dealt with, viz. the
influence of Hass tree condition on the proportion of small fruit, the effect
of Ettinger as pollen donor for Hass, the effect of a paclobutrazol treatment
on the yield of export size fruit and an evaluation of new Hass selections. The
influence of Hass tree condition on the proportion of small 67uit was evaluated
over a period of three years. The percentage of small fruit increased when tree
condition deteriorated. Therefore maintaining good tree condition by the
correct application of cultural measures is essential in Hass. With regard to
testing Ettinger as pollen donor for Hass, no conclusive results have been
obtained on Hass yields and fruit size as yet. A foliar spray of paclobutrazol
(250 mg a.i. l-1) at full flowering increased total yield and the
yield of export size fruit in young Hass trees when compared to the untreated
control in two consecutive years. Four Hass-like selections and cultivars (T
142, Lamb Hass, Gil, Iriet) are being evaluated as possible replacements for Hass
and first results on their postharvest cold storage performance are presented.
1. Introduction
On the overseas
market, the cultivar Hass is very popular due to its excellent shelf life and eating
quality. Fruit with a mass exceeding 160g can usually be exported and best
prices are fetched for Hass fruit in the mass range 200g to 300g. However, the
major drawback of this cultivar is that it bears a large percentage of
undersized fruit causing high financial losses in the South African avocado
industry. The objective of this paper is to report on the current status of the
different approaches to solving the Hass small fruit problem at Westfalia
Estate, South Africa.
2. Tree Condition
Hass avocado trees on 'Duke
7 rootstock were used in this study. The trees were planted in 1982 at
Westfalia Estate. At harvest in July 1991, June 1992 and July 1993, individual
tree yields were recorded for 31 trees. From each tree, fruit were graded into
two size categories, i.e. export size ( > 160g) and small fruit ( <
160g). The percentage of small fruit was calculated on a mass basis. Tree
condition was rated in July, according to a disease index of zero (healthy) to
10 (dead) as described by Darvas et al. (1984). In healthy trees, the
quantity of export size fruit (kg tree-') increased when total yield increased
(figure 1). Apart from the export size yield, every Hass tree bore a proportion
of small fruit in the order of 5 - 20 %, independent of total yield, provided
tree condition rating was zero (figure 2). The percentage of small fruit
increased when tree condition deteriorated. At a tree rating of 5 or 6, almost
the entire crop consisted of small fruit. For a high yield of export size
fruit, good condition of Hass trees must therefore be ensured by the correct
application of cultural measures, such as irrigation, fertilisation, root rot
control and mulching.
3. Ettinger as Pollen Donor
for Hass
Three-year old
Hass trees planted at a spacing of 5 x 5m in square one hectare blocks
bordering an Ettinger row at two sides, were used. The yield of the Hass trees
was measured with increasing distance from Ettinger trees in 1995. The
proportion of export size Hass fruit was determined (as described above). Fruit
samples were taken at a distance of 5m and 50m from an Ettinger row and assayed
for the isozyme system triosephosphate isomerase (TPI) at the Institute for
Tropical and Subtropical Crops in Nelspruit to determine the pollen parent.
Cross
pollination between Ettinger and Hass occurred widely and small and large
seeded Hass fruit differed in their isozyme pattern. However, no conclusive
results with regard to Hass yield have been obtained yet in contrast to
findings by Guil and Gazit (1992).
4. Paclobutrazol foliar spray
Hass avocado trees on Duke 7
rootstock were used in this experiment. The trees were planted in 1991 at
Westfalia Estate. The plant growth regulator paclobutrazol (250 mg a.i. l-1)
was applied as a foliar spray at full flowering in September 1993 and 1994. The
control trees were left untreated. At harvest 1994 and 1995, individual tree
yields were taken. The fruit were then pooled per treatment and sent over a
mass sizer to determine the proportion of export size fruit.
In 1994
and 1995, paclobutrazol significantly increased total tree yield and in 1994
paclobutrazol also increased the proportion of export size Hass fruit when
compared to the control (table 1). In Australia, spraying of Hass trees with
higher rates of paclobutrazol than reported here resulted in similar
improvements regarding yield and fruit size (Whiley et al., 1991).
5. New Hass-like Selections
and Cultivars
Four Hass-like selections
and cultivars, (T142, Lamb Hass, Iriet, Gil) and Hass as standard, have been
topworked on 7 year-old Hass trees on Duke 7 rootstock. Twenty trees were used
per selection. Topworking started in 1993 and was completed in 1994.
In 1995, some of the topworked trees bore their first
crop. So far all four selections
have produced larger fruit
than Hass with T142 fruit tending to be too large (mean fruit mass 450g) and
Lamb Hass having a good size (mean fruit mass 274g). Hass ripened from June to
August, T142 ripened from August to September and Lamb Hass matured from July
to October. T142 and Lamb Hass were stored for four weeks at PC together with
Hass control fruit to simulate shipment to Europe. After cold storage the
temperature was increased to 18'C to induce ripening. Soft-ripe T142, Lamb Hass
and Hass fruit were cut open and were found to be free of physiological
disorders and diseases. However, with regard to taste T142 and Lamb Hass fruit
were slightly inferior to Hass fruit. Little information has been gained to
date about Gil and Iriet as they have only produced very few fruit yet. Both
cultivars seem to mature later than Hass under South African growing
conditions.
References
Darvas, J.M., Toerien, J.C. and Milne, D.L., 1984.
Control of avocado root rot by trunk injection with phosetyl-Al. Plant Disease
68: 691-693.
Whiley, A.W.,
Saranah, J.B., Wolstenholme, B.N. and Rasmussen, T.S., 1991. Use of
paclobutrazol sprays at mid-anthesis for increasing fruit size of avocado (Persea
americana Mill. cv. Hass). J. Hortic'. Sci. 66: 593-600.