Proceedings of The World Avocado Congress III, 1995 132
- 139
LOCAL AND IMPORTED AVOCADO ROOTSTOCKS IN SOUTH AFRICA
D.J. Roe, S. Kremer-Köhne & J. S. Köhne
Merensky
Technological Services
P 0
Box 14
Duiwelskloof 0835
South
Africa
Abstract
South African avocado
production areas are in the subtropical summer rainfall regions of the eastern
escarpment. Soils are mostly deep, well drained and conducive to rapid growth.
Older avocado plantings were established mainly on Guatemalan seedling
rootstocks. The current avocado industry standard rootstock in South Africa is
Duke 7 due to its relatively good tolerance to Phytophthora cinnamomi and
good yielding potential. A large number of new orchards have also been planted
using Duke seedling rootstocks. More recent rootstock imports include, inter
alia, Barr Duke, Thomas, D9, G6 and G755. In this paper, a brief history of
the different rootstocks, their characteristics, and import sources are given.
Performance data of Hass and Fuerte on clonal rootstocks, and current status of
these rootstocks in the South African avocado industry, are presented. Recent
promising results of trials conducted at Westfalia Estate regarding dwarfing
rootstocks and interstocks are also presented.
Keyword: Avocado, Persea americana, rootstocks, South
Africa
I .
Introduction
The choice of avocado rootstocks
in South Africa has mainly been aimed at reducing root rot, caused by Phytophthora
cinnamomi, the major avocado disease in the country. The main commercial
avocado-growing areas of South Africa are located in parts of the eastern and
north eastern escarpment and the midlands of Kwazulu-Natal. In these areas,
avocado production is limited mainly to those areas which are frost-free, and
soils are deep, well drained, red or brown apedal oxisols and inceptisols.
These soils have a lower P. cinnamomi threat than in more
anaerobic soil types. High rainfall is usually experienced in these areas,
which falls mainly during the warm summer months. The warm, wet conditions with
good soils contribute to a most favourable environment for vegetative growth
(as well as P. cinnamomi) during summer, sometimes at the expense of
production, resulting in excessively large trees which are difficult to manage.
The climatic conditions are unlike those experienced in California and Israel,
the major avocado rootstock developers and rootstock choices may therefore
sometimes be different from those countries. The discovery of phosphorous acid
trunk injections at Westfalia Estate to control root rot (Darvas et al., 1978;
Darvas et al., 1983), has led to a wider choice of rootstocks, and other
factors such as level of management inputs, soil type, irrigation water
quality, the intended fruiting cultivar, tree size management, etc., have
become more relevant in rootstock choice. In this paper the seedling and clonal
rootstocks used in South Africa, both on experimental and commercial bases,
their origins, characteristics, status in the industry, and yield data where available, are
presented.
2. Seedling
rootstocks
Seedling rootstocks are still used in South Africa but to a much lesser extent than is the case with clonal rootstocks. Their use is mainly due to their relatively low prices (±60% of clonal rootstocks) and deep tap root systems which are more efficient than clonal rootstocks at taking up water during periods of drought (which is the current situation in most SA avocado growing areas). Some of the seedling rootstocks which have been in use throughout the years are listed below according to horticultural race.
2. 1. Mexican
Seedlings of Mexicola were
first used in South Africa during the late 1920's and it was considered the
first recognised rootstock to be used commercially in this country. Its
advantage was lower vigour than the existing West Indian seedling trees at that
time. However, the Mexicola seedlings had a high incidence of Avocado Sunblotch
Viroid (ASBV) and the seed also soon became hybridised with the Guatemalan and
West Indian races and high seedling variability resulted. This rootstock is not
currently used on a commercial scale in South Africa.
Seedlings of Duke 7 (also
called Duke seedlings) started gaining popularity by the 1970's. It is
presently the most popular seedling rootstock sold by Westfalia Estate nursery
because of mild tolerance to P. cinnamomi, although there is genetic
variability.
2.2. Guatemalan
In approximately 1948, the
Guatemalan cultivars Edranol and Nabal were imported as fruiting scions, but
they were also used as seedling rootstock due to the high incidence of ASBV in the
'Mexicola' population at that time. These seedlings had uniform germination and
vigorous, uniform growth, a distinct advantage to nurserymen. However, they are
more susceptible to root rot than those of the Mexican race (Gaillard, 1987).
Guatemalan seedling rootstocks remained popular well into the 1960's, but at
that stage orchards were in poor condition due to the high Phytophthora incidence.
At that stage, expected avocado orchard life on Guatemalan rootstock was about
12 years before replanting was necessary (Maddison1, pers. comm.,
1995). Trees with Edranol seedling rootstocks are still sold on fairly large
scale by nurseries in South Africa. The highest yielding orchard at Westfalia
Estate over a number of years has been a combination of Ryan on Nabal rootstock
(Westfalia farm records).
The Australian selected rootstock, Velvick, was
imported in 1989. It apparently produces uniform seedlings which are said to be
fairly true-to-type and have moderate tolerance to salinity and root rot. This rootstock
has not been tested in South Africa yet.
2.3. West Indian
Rootstocks of the West
Indian race have received little attention in this country. Their main
advantage is their relative tolerance to salinity (Gaillard, 1987). During the
first quarter of this century, seedlings of the race were used as rootstocks
but they were found to be vigorous with low uniformity and high root rot
susceptibility (Maddison, pers comm., 1995). There has been renewed interest
during the present drought, since irrigation water in some areas became more
saline and sodic; and leaf bum and abscission led to crop losses and severe
sunburn of fruit and bark.
2.4. Hybrids (Guatemalan x Mexican)
A number of older Fuerte
orchards were established on Fuerte seedling rootstock after vigorous
selection, and this combination has produced acceptable crops. However,
Maddison (pers. comm., 1995) reported that Fuerte seedling was only successful
as a rootstock when planted in optimal avocado soils, and that it generally
failed elsewhere. No recent commercial plantings on Fuerte seedlings have been
made.
3. Clonal
rootstock
With the discovery of
reliable clonal propagation techniques for avocado (Frolich & Platt, 1972)
the rootstock situation was revolutionised worldwide. The predictability of
genetically uniform, root rot tolerant, productive trees was a major advantage
for growers and researchers in South Africa as they could investigate other
factors without the added variability of seedling rootstocks.
3. 1. Mexican
The majority of clonal
rootstocks tested and used in South Africa belong to the Mexican race. Duke 6
was imported from California in 1962 (Bijzet, unpubl.), and it was only in the
late 1970's that this clonal rootstock was planted on a commercial scale. It
was said to have moderate tolerance to Phytophthora and was
non-vigorous. During the early 1980's Duke 6 was planted on commercial scale at
Westfalia Estate, but after 2 to 3 years the trees started dying, with stem
pitting and gumming in the branch forks being the symptoms (Maddison, pers.
comm., 1995). To date, this disease remains to be identified. Possibly,
infected budwood was imported, but the rootstock has never been tried since.
Hectares of orchards and all mother trees were destroyed in an attempt to
eradicate all sources of budwood, and therefore this rootstock no longer exists
in South African orchards.
Duke 7, which was imported together with Duke 6, turned out to be the
most successful and widely planted rootstock in South Africa, especially for
the Hass cultivar. The major advantages of this rootstock are its moderate
tolerance to root rot and its uniform, productive Hass trees. It presently
dominates Hass (about 50% of all trees planted during the last 9 years are Hass
(SAAGA, 1995) plantings and has proven itself under South African conditions
(Kremer-Köhne & Köhne, 1992) and in California (Brokaw, 1987). It is
included in all the rootstock trials at Westfalia Estate as the comparative
control.
Budwood of Thomas, an escape
tree in a diseased orchard (Coffey, 1987) developed by UC, was imported in 1987
and has undergone testing at Westfalia Estate for the last four years. From
California, it is reported to have good Phytophthora tolerance (as high
as Martin Grande (Coffey et al., 1988), but is a bit weak regarding salt
tolerance (Brokaw, 1987). It has performed satisfactorily in rootstock trials
in California (Menge et al., 1991; Arpaia el al., 1993) but in a
Westfalia semi-commercial rootstock trial (Fig. 1) it has not performed up to
expectations. Its production of Hass fruit has been lower than Duke 7, and it
imparted greater vigour to Hass than did Duke 7, an undesirable characteristic
these days when dwarfism is the buzzword in the industry. As a consequence, its
commercial use in South Africa has not yet been recommended.
The G-6 series consists of
Guatemalan selections of Mexican type, introduced into California by Dr
Zentmyer. It has tolerance to Phytophthora by a mechanism apparently different
from Duke 7 (Brokaw, 1982). This rootstock has been tested in South Africa for
the cultivars Fuerte (Kremer-Köhne & Köhne, 1994) (Table 1) and Hass (Köhne
1991) (Table 2), and is in the process of being tested for Ryan. Some doubt was
cast over this rootstock at Westfalia Estate when a large number of trees on G6
died rapidly about 2 to 3 years after planting. At the time, isolations were
done to determine the causal organism, Verticillium being isolated in
only one case. Today G6 is still used on an experimental scale in South Africa,
but there have been no recent plantings on this rootstock
A seedling of
Duke 6 which survived in a UCR infested site (Brokaw, 1982), Barr Duke was
imported into South Africa in 1987. It is reported to be very vigorous by
Brokaw (1982), but Menge2
(pers comm, 1994), as well as our own results (Roe et al, in press),
have found Barr Duke to have lower vigour than Duke 7. Its production has so
far been slightly higher than Thomas (Fig. 1) and its smaller stature would be
beneficial if first thinning of Hass; were delayed by at least a year. Status
of Barr Duke in the South African avocado industry is experimental.
D9 is a product of Dr
Bergh's irradiated Duke budwood (Menge et al., 1991). It apparently
establishes slowly and young trees are often smaller than other rootstocks (Menge et al., 1991), a characteristic which is evident in rootstock
trials at Westfalia Estate (Roe et al, in press). Production at
Westfalia has been satisfactory compared to Duke 7 (Fig. 1), particularly when
taking into account its slower growth rate. Status in South Africa is
experimental.
Colin V-33 a
dwarf scion cultivar from CICTAMEX, has been used successfully as an interstock
to impart lower vigour to Fuerte avocado trees (Barrientos-Priego et al., 1987).
It was imported into South Africa in 1988 with the purpose of studying its
interstocking effect on Hass trees. As mentioned previously, ideal growing
conditions in the South African avocado growing areas result in excessively
large trees which are difficult to pick. At Westfalia, three-year old Hass
trees interstocked with Colin V-33 on Duke 7 have displayed little growth
retardance compared to Hass on Duke 7 (Roe et al., in press) although
this may only become evident at a later stage. It is also being tested as a
rootstock and has so far exhibited only slight reduction in vigour (Fig. 2).
Colin V-33 is an experimental interstock.
3.2. Hybrids
G755 (Martin Grande), a
hybrid between Persea americana and P. schiedeana from Guatemala
(Coffey et al., 1988) was imported in 1983 from California. It possesses
moderate resistance to Phytophthora and was originally collected in the
market at Coban in the Guatemalan highlands (Coffey el al., 1988).
However, it has faired disappointingly in terms of production (Table 1) and
also caused excessive tree vigour of Hass in South Africa (Köhne, 1991;
Kremer-Köhne & Köhne, 1992; Conradie et al., 1994) and in California
(Bender et al., 1991). Small-scale plantings and topworks of Pinkerton
on G755 have taken place recently.
3.3 local
Selections
Dusa is a
seedling of Duke 7 which survived heavy Phytophthora pressure and was
selected at Westfalia Estate (Darvas3, pers. comm., 1995). In trials
carried out by Botha (1991), young Hass on Dusa out-produced Hass on Duke 7,
but was also much more vigorous. Dusa (as well as Thomas, G755 and Barr Duke)
showed no signs of root rot, while -Duke 7 showed slight symptoms. This
rootstock has been included in Dr Menge's rootstock evaluations at UC,
Riverside, and has made it to the field evaluation stage. It is not used on
commercial scale in South Africa.
The two rootstocks Latas and Jovo were selected at Westfalia Estate where they were growing in waterlogged conditions. Latas was named by Dr Joe Darvas and is the Hungarian word meaning 'vision'. Botha's (1991) evaluations showed that Latas is a more vigorous rootstock than Duke 7, and was similar to Dusa, G755, Thomas in its root rot tolerance. However, it tended to be vegetative at the expense of production. This rootstock is still under evaluation. Jovo is Hungarian for 'future'. It has not faired as well as Dusa and Latas in rootstock evaluations at Westfalia (Botha, 1991). It imparted lower vigour to Hass, but also had low production. It is currently being evaluated on larger scale at Westfalia but is still not a commercial rootstock.
'Wilg' was
growing in a root rot infested orchard. It was a healthy tree with a growth
habit similar to a weeping willow. This growth habit gave Wilg its name
(meaning "willow"
in Afrikaans). All tests for viroids have been negative. Hass has been
significantly (P < 0.05) stunted on Wilg rootstock (Fig. 2) and this
combination was also very precocious, with fruit borne during the first season
from planting. However, there are problems with its propagation because of its
willowy growth habit, and research is under way to rectify this.
4. Conclusion
The South African avocado
commercial rootstock scene is dominated by a small number of seedling and clonal
rootstocks. Seedlings of Duke 7 are the most popular seedling rootstocks sold
by South African nurseries. Duke 7 is by far the most planted clonal rootstock
and performs well under good soil and management conditions. Future rootstock
selection apart from the obvious root rot tolerance, will lay emphasis on lower
vigour rootstocks and interstocks, so that orchard efficiency is improved.
Recent results from rootstock trials in South Africa should be seen in the
light of an ongoing drought during the last six years, and Phytophthora pressure
has not been as great as in years of normal rainfall.
1Mr Robbie Maddison, Retired Section Manager,
Westfalia Estate, P 0 Box 14 Duivelskloof 0835, South Africa.
2Dr J A Menge, Department of
Plant Pathology, University of California, Riverside CA 92521-0122, USA
3Dr J M Darvas, Consultant,
Letsitele South Africa (Former Plant Pathologist, Westfalia Estate).
5.
References
Barrientos-Priego, A., Lopez
Jiminez, A. & Sanchez Colin, S., 1987. Effect of Colin V- 33 as interstock
on avocado (Persea americana Mill.) growth, cv Fuerte. S. A. Avocado
Growers' Assoc. Yrb. 10: 62-64.
Bender, G.S., Arpaia, M.L.
& Witney, G., 1991. Increasing production of avocados... the potential is
there. California Grower, December: 28-29
Bijzet, Zelda, unpubl.,
1994. Lys van materiaal reeds vrygestel. Unpublished report for the Cultivar
Evaluation Sub-committee of the South African Avocado Growers' Association.
Botha, T., 1991. Evaluasie
van avokado-onderstamme - voorlopige verslag. S. A. Avocado. Growers'Assoc. Yrb. 14:87 88.
Brokaw, W.H., 1982. Clonal
rootstocks: personal observations and a peek into the future. California
Avocado Society Yearbook 66: 81-92.
Brokaw, W.H., 1987. Field experiences
with clonal rootstocks. S. A. Avocado Growers' Assoc. Yrb. 10: 34-36.
Coffey, M.D., 1987. A took
at current avocado rootstocks. California grower 11(4):15-17.
Coffey, M.D., Guillemet,
F.B., Schieber, E.& Zentmeyer, G.1988. Persea schiedeana and Martin
Grande: the period from 1920 to 1975. California Avocado Society Yearbook 72:107-120.
Coffey, M.D., Guillemet,
F.B., Schieber, E., Campbell, S.D., Greer, G., Gabour, B. & Forester, H.,
1988. Avocado rootstocks with resistance or tolerance to P. cinnamomi.
Summary of Avocado Research. Prepared for the Avocado Research Advisory
Committee Meeting, April 13, 1988, pp 36-46.
Conradie, W., Smith,
D.G., Köhne, J.S. & Kremer-Köhne,
S., 1994. Evaluation of new scion and rootstock cultivars. S. A. Avocado Growers'
Assoc. Yrb. 17: 70-71.
Darvas, JM., Kotzé, J.M.
& Toerien, J.C., 1978. Preliminary results on chemical control of
Phytophthora root rot of avocados. Citrus & Subtropical Fruit
Journal 537: 6-7.
Darvas, J.M., Toerien, J.C.
& Milne, D.L., 1983. Injection of established avocado trees for effective
control of Phytophthora root rot. S. A. Avocado Growers' Assoc. Yrb. 6:76-81.
Frolich, E.F.& Platt,
R.G., 1972. Use of the etiolation technique in rooting avocado cuttings. Ca1ifornia
Avocado Society Yearbook 55: 97-109.
Gaillard, J-P., 1987. Races
et variétés d'avocatiers. In: L'Avocatier sa Culture, ses Produits, Ch. IV.
Techniques Agricoles et Productions Tropicales, G-P. Maisoneuve & Larose,
Paris. Pp 51 - 66.
Köhne, J.S., 1991.
Performance of Hass on three clonal rootstocks. S. A. Avocado Growers'
Assoc. Yrb. 14: 39.
Kremer-Köhne, S. &
Köhne, J.S., 1992. Yield and fruit quality of Fuerte and Hass on clonal
rootstocks. S. A. Avocado Growers' Assoc. Yrb. 15: 69.
Kremer-Köhne, S. &
Köhne, J.S., 1994. Horticultural performance of Fuerte avocado on Duke 7 and G6
Rootstocks. S. A. Avocado Growers' Assoc. Yrb. 17: 78-79.
Roe, D.J., Conradie, W.
& Köhne, J.S., in press. Progress with rootstock research at Merensky
Technological Services. S. A. Avocado Growers' Assoc. Yrb. 18:
SAAGA, 1995. SAAGA 1995
Avocado Tree Census, South African Avocado Growers' Association, P 0 Box 866,
Tzaneen 0850, South Africa.