Proceedings
of The World Avocado Congress III, 1995 119 - 123
AN
OVERVIEW OF AVOCADO BREEDING AND EVALUATION OF GENOTYPES IN THE REPUBLIC OF
SOUTH AFRICA
Zelda Bijzet, A.D. Sippel, B. Snijder, HJ. Breedt and
B. Cilliers
Institute for Tropical and Subtropical Crops,
Private Bag X1 1208, Nelspruit 1200,
South Africa.
Persea americana, Phytophthora cinnamomi, rootstocks, scions
Abstract
An avocado breeding programme
was initiated in 1991 by the Institute for Tropical and Subtropical Crops
(ITSC) in order to develop superior cultivars for Southern Africa. The
programme consists of two main parts, namely rootstock and scion breeding,
followed by extensive Phase II evaluation. The breeding programmes were founded
on the establishment of a gene source, continued introduction of improved
overseas material, controlled self- and cross pollinations and evaluations in
Phase I orchards. Since the inception of the scion breeding programme, 5,240
seedlings have been established. Eleven local seedlings were selected for Phase
II evaluation.
The rootstock breeding
programme is devoted exclusively to the selection of Phytophthora tolerant
types. Over the past three seasons 25,000 seedlings were grown under conditions
of severe artificial infection with Phytophthora. Thirty three seedlings
were selected for further screening.
The breeding and selection
programmes are followed by Phase II evaluations. Various rootstock/scion combinations
in different production areas are being evaluated for characteristics such as
yield, fruit quality, shelf life, etc.. Thus far rootstocks Duke 7, Thomas and
Barr Duke have been used in the trials. ne rootstock selections are evaluated
in combination with established scion cultivars, Fuerte, Hass, Pinkerton and
Ryan. Ninety two scion/rootstock combination are currently being tested in two
localities.
1. Introduction
Although the Avocado has been
known to westerners for nearly five centuries, and has experienced a great deal
of selection pressure over the centuries, there is still considerable potential
for the improvement of the current cultivar range (Berg, 1987). Knight &
Winters (1971), Slor & Spodheim (1972) and Terblanche (1988) offer various
reasons for the need to produce improved cultivars:
* low production efficiency of the available scion/rootstock
combinations, alternate bearing
* the need to extend the
marketing season with earlier and later cultivars
* the shortage of resistance
to scions and rootstock diseases
* the need to improve keeping
quality and to reduce postharvest physiological disorders
Cultivars developed in California currently form the
backbone of the avocado industries of Israel, Chile, Mexico, New Zealand and
South Africa (Du Plooy, 1991). Wolstenholme (1987) emphasized that a country
has to run its own breeding programme in order to remain competitive. The
complexity of climatic, soil and other factors make it unwise for South Africa
to rely on California and Florida for improved germplasm.
The ITSC consequently
embarked on a local breeding programme in 1991 under the guidance of Dr. Du
Plooy.
2. Breeding strategy
2.1 Scion
breeding strategy
The programme launched in
1991 provided for a comprehensive effort including genebank maintenance, a
planned pollination programme followed by three evaluation phases (Du Plooy et
at, 1992).
Open pollinated seeds are
collected and planted at an approximate rate of 1250 a year. Selections from
these orchards are used for Phase 11 and Phase HI evaluations or as breeding
parents.
Records are carefully kept
since offspring performance is used to judge breeding values of parent genotypes
as the relative breeding value of a cultivar or tree cannot be judged from its
phenotype (Bergh & Whitsel, 1975 & Berg, 1987). It is envisaged that a
hundred progeny per breeding parent will be produced.
Pollen parent control is
expected to accelerate selection progress. Bergh (1987) recommends self
pollination as a means of identifying superior breeding parents. Excessive
vegetative vigour is reduced by self pollination which may result in greater
fruitfulness. Parents of known merits can be used more wisely in corrective
mating, repeated back crossing and exploiting favourable combining abilities.
(Bijzet & Cilliers, 1995)
Controlled cross pollination
is promoted by using trees top worked with various cultivars and caged during
pollination. Isozymes are used to distinguish between cross and self pollinated
progeny (Bijzet, et al, 1994)
Phase I evaluation of
seedlings is followed by extensive Phase II evaluation after grafting on the
best commercial rootstock or rootstocks at the time, for evaluation of
horticultural traits in Phase II trials. (Sippel et al. 1994) Currently the
rootstock in use is the Phytophthora tolerant Duke 7. Phase U trials are
replicated in different environments. The best selections per location are
considered for cultivar status in conjunction with a third phase consisting of
a semi- commercial planting. (Sippel et al. 1994)
The breeding and evaluation
criteria were discussed by du Plooy et al (1992) and Sippel, et al (1994) and
these will be adjusted as consumer preferences change. The main selection
criteria are quality, yield and disease resistance.
2.2 Rootstock breeding
strategy
The rootstock breeding
programme is based on rapid screening of large numbers of seedlings for
potential resistance to Phytophthora cinnamomi (Du Plooy et al. 1992,
Koekemoer, et al. 1994). However, when choosing breeding parents and in
subsequent trials, attention is also given to other rootstock characteristics,
for example, dwarfing and salinity resistance. Open and controlled pollinated
seed, collected from parents known to be tolerant of Phytophthora cinnamomi,
are screened at 10 000 a year. The first screening is done by germinating
these seeds in Phytophthora infected soil followed by drenching the
seedlings with a mycelium suspension. Surviving seedlings are clonally
multiplied for a second and statistical screening (Bijzet, 1993). In both
screenings clonal Duke 7 and Edranol plants are used as controls (Koekemoer et
al. 1994 & Breedt, et al. 1995). Selections significantly more tolerant than
Duke 7 are grafted with Hass for subsequent field evaluation.
3. Current status of the South African breeding
programmes
3.1 Scion breeding
3.1.1 Genebank, pollination
and Phase I evaluation
After extensive evaluation of
cultivars a new genebank was established in 1994 in a single orchard. It
currently includes 41 cultivars, 17 rootstocks and 12 selections planted in two
tree replicates. The genebank will constantly be updated in order to include
all the possible breeding material in the industry (Bijzet et al., 1994).
By the end of the current season 5090 seedlings,
derived from 57 different open pollinated parent cultivars will be established
at three localities for Phase I evaluation. Eight selections imported from California
were used to produce an additional 150 self pollinated progeny.
Self pollination of caged trees produced a
disappointing average of only 21 seeds per tree per season. The self
pollination programme will be continued, and the possible problems with this
mode of pollination will be investigated and rectified.
Controlled cross pollination will in future be facilitated with 116
pollen parents, planted in transportable pots which will be caged with selected
seed parents. Trees were cut back during 1995 to be top worked with multiple
cultivars for cross pollination.
Two hundred seedlings of open pollinated sources,
planted in 1987 and girdled in 1991 have yielded six selections for further
testing. The seed parents were Ettinger, Wurtz, Edranot and Hass. Some Phase I
seedlings in other orchards have started flowering and evaluation will commence
next season.
An orchard at Levubu,
containing imported material and regarded as Phase I, produced fruit that were
evaluated. Another season's data will however be obtained before any decisions
with regard to the imported selections can be made. At this early stage none of
the imported selections except BL 122 and 3-29-5 show any potential. Farmers
and homeowners across the country were requested to advise the ITSC of their
material with potential. To date more than 100 seedling trees were submitted to
the ITSC for evaluation. Very promising material was recovered and five
selections are about to be entered in phase II trials. These include a black
skin selection (1-14-2) similar to Hass but with significantly larger fruit.
3.1.2 Phase II scion
evaluation
The first phase H trials were
established in March 1993 at two sites (Burgershall and Levubu). These include
four Fuerte selections (vegetative), Hass, Pinkerton, Ryan, Gwen and two local
selections. In the following summer another 13 imported and local selections
were added to these trials. Each planting is replicated on Duke 7, Thomas and
Barr Duke rootstocks. (Sippel, et al., 1994)
Early results indicate that
scions on Thomas are growing more vigorously. First yields are better on Duke
7, and Pinkerton and Ryan gave the highest yields. Another 12 imported and
local selections will be added in 1996. (Sippel, et al. 1995)
3.2 Rootstock breeding
3.2.1 Pollination programme
and Phase I screening
Controlled pollination
commenced in 1993 with Duke 7, D9 and Teague being encaged with bees for self
pollination. The cross pollination programme was initiated by top working 14
four year old trees with two selected rootstocks each (table 1). A large
polycross facility with room for 90 trees and covered with shade cloth will be
finalized in November 1995. In the interim, 24 897 seedlings from open
pollinated sources were screened for resistance to Phytophthora during
1992-1995 (Koekemoer, et al., 1994). Fifty three seedlings survived drenching
with the mycelium suspension and after transplanting another 20 of the weaker
selections succumbed to Phytophthora. The remaining 33 seedlings are
currently multiplied for further screening (Breedt, et al., 1995).
3.2.2 Phase II rootstock
evaluation
The first phase II rootstock
evaluation trials were established during March 1993 at Levubu and Burgershall.
These include Duke 7, G6, Thomas, D9 and Martin Grande (G755) each grafted with
Fuerte, Hass, Pinkerton, Hass and Ryan (Sippel, et al. 1994).
Initial
results indicate that Thomas, G6 and D9 produced more vigorous trees. First
yields of Pinkerton on Thomas and Duke 7 are particularly good. Hass performed
better on D9 and Duke 7, whilst Ryan did the best on Martin Grande and Duke 7.
Fuerte trees hardly set any fruit at this early stage of the trial. (Sippel, et
al. 1995)
Current plantings will be expanded in 1996 with five
imported and two local selections grafted with Hass.
4. Summary
Since 1991 a total of 5240
seedlings have been established for a Phase I scion evaluation of which 150 are
self pollinated. Thus far, six phase I seedlings have been selected for further
testing. Almost 25 000 seedlings have been screened for Phytophthora resistance
resulting in 33 selections. Controlled pollination has been successfully
initiated in the programme. PhaseII trials have produced their first crop. Firm
conclusions can not however be contemplated yet. An effective South African
avocado breeding programme is now a reality. Future development will be
followed with great interest and expectations.
Bibliography
Berg,
B.O., 1987. Avocado breeding in California. S. Afr. Growers Assoc. Yrb.
10:22-23. Berg, B.O., and Whitsel, R.H., 1975. Self pollinated Fuerte
seedlings. Calif. Avo. Soc. Yrb.
Bijzet, Zelda, Sippel, A.D.,
and Koekemoer, P.JJ., 1993. Avocado breeding: a progress report. S. Afr.
Growers Assoc. Yrb. 16:86-88
Bijzet, Zelda, Sippel, A.D.,
and Snijder, B., 1994. Current status of the avocado breeding programme. S.
Afr. Growers Assoc. Yrb. 17:64-66.
Bijzet, Zelda, and Cilliers.
B., Breeding strategy and the selection of seed parents in the ITSC avocado
programme. 1995. S. Afr. Growers Assoc. Yrb. 18:1-3
Breedt, HJ., Koekemoer, PJJ.,
and Bijzet, Zelda., 1995. Vorderingsverslag: Seleksie van Phytophthora-bestande
avokado onderstarnme. S. Afr. Growers Assoc. Yrb. 18:4-6
Du Plooy, C.P., 1991. Avocado
breeding and selection. Report on a visit to the University of California.
ITSC, Nelspruit, RSA
Du Plooy, C.P., Marais,
Zelda, and Sippel, A.D., 1992. Breeding and evaluation strategy on avocado. S.
Afr. Growers Assoc. Yrb. 15:75-77.
Koekemoer, PJJ., Breedt, HJ.,
Manicom, B.Q, and Bijzet, Zelda, 1994. Selection of Phytophthora tolerant
avocado rootstocks. S. Afr. Growers Assoc. Yrb. 17:72-74.
Knight, RJ., and Winters,
H.F., 1971. Mango and avocado selection in southeastern Florida. Proc. Fla. st.
Hort. Soc. 84:314-317
Sippel, A.D., Bijzet, Zelda,
Snijder, B., and Du Plooy, C.P., 1994. The current status of the avocado phase
H evaluations at the ITSC. S. Afr. Growers Assoc. Yrb. 17:67-69.
Sippel, A.D., Snijder, B.,
and Bijzet, Zelda, 1995. Progress with the phase 11 avocado evaluation
programme and its value to the industry. S. Afr. Growers Assoc. Yrb. 18:7-9
Slor, E., and Spodheim, R.,
1972. Selection of avocado varieties in Israel. Calif. Avocado Soc. Yrb.
55:156-157
Terblanche, J.H., 1988. Noodsaaklikheid
van 'n doeltreffende avokadoteelprogram. S. Afr. Growers Assoc. Yrb. 11:3-4
Wolstenholme, B.N., 1987.
Some aspects of avocado research worldwide. S. Afr. Growers Assoc. Yrb.
10:8-11.
