BIOCONTROL OF PHYTOPHTHORA
CINNAMOMI ON AVOCADO: IDENTIFICATION AND FIELD TESTING OF LOCAL NATURAL
ANTAGONISTS, AND EVALUATION OF ROOTSTOCKS FOR RESISTANCE
Merensky
Technological Services
P.O. Box 14, Duiwelskloof 0835
South
Africa
Abstract
Soils suppressive to root
rot of avocado (caused by Phytophthora cinnamomi), were first identified
in South Africa in 1990. Mcro-organisms from these soils were tested for in
vitro antagonism to Phytophthora cinnamomi and subsequently
evaluated for suppression of root rot of avocado seedling plants in a mistbed.
Three fungal antagonists, Paecilomyces filacinus, Aspergillus candidus and
Trichoderma hamatum were effective in suppressing root rot. These
antagonists have been evaluated since 1992 for control of root rot in avocado
trees in the field. Populations of the antagonists have been found to increase
in the root zone of newly planted trees (after antagonist treatment in the
nursery and in the orchard) and in the root zone of established Hass and Fuerte
trees planted in 1981 and 1980 (after antagonist treatment in the orchard).
In an isolated planting site
consisting of various ungrafted avocado rootstocks (eg. G1033, Dusa, Latas, D9,
Duke 7, Barr Duke, Thomas, Velvic) trees undergo open pollination. Seeds from
this orchard, situated at Westfalia Estate, are germinated and the seedlings
evaluated for resistance to root rot in a mistbed. A number of successful
selections have been made and these are being clonally propagated and grafted
with Hass. These trees are to be planted in a root rot infested field site
where they will be evaluated for yield and resistance to root rot.
1. Introduction
Root rot of avocado (Persea
americana Mill.), caused by Phytophthora cinnamomi Rands (Pc), is
the most important avocado disease in Australia (Broadbent and Baker, 1974;
Pegg et al. 1982), California (Zentmyer 1984) and South Africa (Kotzé et
al. 1987).
The South African avocado
industry relies on chemical control of root rot with phosphite compounds and
Duke 7 rootstock (tolerant to root rot), which is widely used. Thus, resistance
of Pc to phosphite compounds would constitute a serious threat to the avocado
industry. However, it has been reported that resistance of Pc to fosetyl-Al
occurs on Chamaecyparis lawsoniam (Vegh et al. 1985), and a
tendency has also been reported of Pc. from soil of avocado trees treated with phosphites
over a prolonged period to be less sensitive to phosphites in vitro than
Pc from soil of untreated trees (Duvenhage 1994). This stresses the importance
of the search for biocontrol methods root rot control such as the use of
antagonistic micro-organisms or another rootstock genetically different from
Duke 7 with tolerance or resistance to root rot, but which is high yielding.
This would facilitate less reliance on Duke 7 as a rootstock, and possibly less
reliance on chemical control of root rot.
2. Materials methods and result
2. 1. Identification and
field testing of local natural antagonists
In 1990 soils suppressive to
avocado root rot were identified in South Africa
(Duvenhage and Maas, 1990;
Duvenhage et al. 1991). Several antagonists from these soils were
consequently found to effectively control avocado root rot under glasshouse
conditions (Duvenhage and Kotzé, 1993). Field trials are currently under way to
evaluate the ability of three fungal antagonists to effectively control avocado
root rot in the orchard.
During November to December
1992 one trial block each of Fuerte and Hass on Duke 7 rootstocks was planted
with nursery trees of which the growth medium had been amended with three
fungal antagonists; Paecilomyces filacinus, Aspergillus candidus and
Trichoderma hamatum, and also a mixture of all three (Duvenhage and Köhne,
1995). Antagonist treatments are repeated annually in the orchard (Duvenhage
and Köhne, 1995), and the tree condition and yield, antagonist and Phytophthora
cinnamomi populations, and suppressiveness of the soils are monitored
annually. Also, two avocado blocks, a Hass block planted in 1981 on Duke 7
rootstocks, and a Fuerte block planted in 1980 on Guatemalan seedling
rootstocks, have been treated since 1992 with antagonist spore suspensions
(Duvenhage and Köhne, 1995) and evaluated on the same criteria mentioned above.
Although there was a tendency for the antagonist populations to increase in
soil of all antagonist treatments, over the period from 1992 (before the start
of the treatments) to 1995, the increases were not always significant (data not
shown). Figure 1 illustrates the increased populations for the Fuerte block
treated with different antagonists, during the period of 1992 (before planting)
to 1995. Antagonist populations in the untreated soil (control) did not change
significantly during this period (data not shown). To date, tree condition,
yield, populations of Phytophthora cinnamomi, and suppressiveness of
soils have not been influenced by antagonist applications (data not shown).
2.2. Evaluation of
rootstocks for resistance
In order to obtain
rootstocks with better resistance or tolerance to root rot and better
horticultural characteristics than currently used rootstocks, a breeding
programme was started a number of years ago. In an isolated planting site,
situated on Westfalia Estate, consisting of various ungrafted avocado
rootstocks (eg. G1033, Dusa, Latas, D9, Duke 7, Barr Duke, Thomas and Velvic)
trees undergo open pollination. Seeds from this orchard are germinated and the
seedlings evaluated for resistance to root rot in a inistbed by planting in Pc
inoculated vermiculite, and subsequent visual evaluation of the root system of
each seedling for root rot symptoms. The breeding programme has produced approximately
13 00 seedlings to date, and 10 promising new rootstock selections have been
made. Material of each selection is now being propagated and the first trees
(36 of each rootstock selection) will be planted in a root-rot-prone field site
during 1996. All rootstocks will be grafted with Hass and will be compared with
Hass on Duke 7 rootstock (as the standard) for root rot symptoms and yield.
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