Integrated Control of Avocado Root
Rot
J. M. Kotzé
Dept.of Microbiology and Plant Pathology,
J. M. Darvas
Westfalia Estate,
This article describes how avocado root rot which
threatened the very existence of the avocado industry in
Root rot of avocado, caused by Phytophthora
cinnamomi
The severity of root rot in South Africa is probably
due to the prevailing high soil temperatures, summer rains which sometimes
cause water logging, badly drained soils, lack of rootstock resistance, and the
low calcium and magnesium status of the soils, (Zentmyer, 1979).
Since 1977, an intensive research program was
sponsored by the S A Avocado Growers' Association in
order to find means of controlling the disease. The objectives of this program
were threefold:
1. To evaluate fungicides and techniques of
application in order to save existing orchards.
2. To develop methods of establishing disease-free
nurseries, improve cultural practices which will suppress disease development.
3. To introduce resistant rootstocks from
It will be noted that these objectives were short,
medium, and long term. The research program yielded positive results within a
year when very promising results were obtained with soil applications of
fungicides (Darvas, Kotzé
& Toerien 1978).
The fungicide that gave the most significant control
of fully grown trees and seedlings was metalaxyl,
which was used as a sou application in a 5% granular
formulation. After registration of the product, thousands of trees were treated
with two applications per year. After the second year, the results with metalaxyl were less convincing and the trees deteriorated
despite treatment. There is evidence now that P. cinnamomi has become
resistant to metalaxyl in some localities in
Fosetyl-Al was introduced as a foliar spray, but up to six
applications were recommended per season. This control program was laborious
and expensive, and the recovery of infected trees was slow. Alternative methods
of application were explored, and Snyman & Kotzé(1983)
showed that fosetyl-Al applied as a stem paint or in
a plastic sponge-band around the trunk was as effective as were foliar sprays.
These techniques are easier and cheaper to the grower. Darvas,
Toerien, & Milne (1983) demonstrated that diseased trees recover remarkably
when injected with a 8 to 10% solution of fosetyl-Al. This technique is a significant break-through,
especially under severe disease conditions. The cost of material is
considerably reduced as 0,4g a.i. is applied per m2
canopy. Two injections per season were sufficient to effect
almost complete recovery. There is no doubt that this technique is by far the
most effective way of controlling Phytophthora root rot.
Chemical control of avocado root rot has become a
means to survive until such time that long term control measures, like
resistant rootstocks, can be fully adopted. The progress made with chemical
control is so remarkable that farmers consider root rot as a solved problem.
The truth is, however, that we have developed techniques to employ chemicals to
our best advantage. At the same tune, nursery techniques and plant hygiene play
major roles in the establishment of a sound avocado industry. The management of
avocado root rot depends on four principles.
1. DISEASE-FREE
NURSERY PLANTS
The objectives of an elite nursery are as follows:
Complete elimination of P. cinnamomi is
practiced. All the soil and potting media are heat or steam sterilized or
fumigated with methyl bromide before these materials are allowed to enter the
nursery. No worker is allowed to enter the nursery without washing of hands, sterilizing
of tools, and wiping of the feet in copper sulphate
powder. All the plant material used in the nursery is washed and treated with
fungicides. Water, which is an important distributor of Phytophthora inoculum, is closely scrutinized. Borehole water is
preferred because most of the rivers are heavily infected. River water is
sometimes filtered but always chlorinated and afterwards left to stand for 24
hours before use. Hygiene standards are high and are applied voluntarily by the
nurseryman. If there is relaxation of standards, Phytophthora claims a
high toll.
2. CLEAN
ORCHARD SOILS
The second line of defense against root rot is clean
orchard soils in which to plant clean plants from the nursery. New orchard
soils are seldom infested with P. cinnamomi, and it was proved that if a
grower fumigates the planting hole with methyl bromide hi which he plants a
clean tree, root rot takes much longer before tree decline sets in. It is
therefore recommended that planting holes be fumigated with methyl bromide at
least a month before planting. A cheaper method, namely solarizarion,
is making progress, however. A thin, transparent plastic cover over moist soil
6 weeks before planting is effective in reducing P. cinnamomi to a level
where it has no or little effect on the young plant. Solarization
before planting is a promising new method that might replace fumigation
completely. Apart from being cheaper, solarization
can be just as effective as methyl bromide, is cheaper and safer, provided that
the time of the year is chosen discreetly. A phenomenon was recorded hi
Queensland Australia where P. cinnamomi, although present in the soil,
causes no significant disease (Broadbent & Baker 1974). The soil which
behaves like this is called suppressive; and, although there is no lack of
theories, we do not understand a suppressive soil well enough to produce it or
to encourage its formation. The emphasis in our case is on the reduction of inoculum and on preventing any soil situation that will
predispose the tree to infection.
3.
RESISTANT ROOTSTOCKS
So-called resistant rootstocks, particularly Duke 6
and Duke 7 were introduced from
4.
JUDICIOUS USE OF FUNGICIDES
The fourth, but very important, principle of root rot
management involves the use of fungicides. The grower has only one fungicide to
rely on, viz: fosetyl-Al,
if he has metalaxyl-resistant Phytophthora.
However, there are several methods of application to choose from. When the
disease is mild, regular stem sprays of painting will halt the progress of the
disease. The sponge-band appears equally effective. If, however, the disease
gets out of hand, the trees can be saved by stem injections. The disease
management of root rot has become sophisticated. At present, fosetyl-Al offers the only practical short term solution.
However, where metalaxyl has not been used, or where
only limited use was made in the past, this chemical gives acceptable results
when it is applied to the orchard soil.
It is expected that the chemical control techniques
will yield even better results where the resistant rootstocks have been introduced.
Duke 7 performs well in greenhouse experiments and is undoubtedly more
resistant against the local strains of P. cinnamomi than the Guatemalan
seedlings.
Root rot control has become a science of an art. The
art lies in management, and not in recipes. The successful grower should
understand all the different facets of root rot and he must make it part of his
management program.
REFERENCES
BROADBENT,
P., BAKER, K.F., (1974). Behavior of Phytophthora
cinnamomi in soils suppressive and conducive to root rot. Australian
Journal Agric. Research 15: 121-137.
DARVAS,
J.M., (1983). Five years of continued chemical control of Phytophthora rootrot
of avocados. SA Avocado Growers' Association Yearbook 1983: 6: 72-73.
DARVAS, J.M., KOTZÉ, J.M. & TOERIEN, J.C. (1978). Preliminary results on chemical control of
Phytophthora root rot in avocados. Citrus
& Subtropical Fruit Journal,
537: 6-7.
DARVAS, J.M., TOERIEN, J.C. & MILNE, D.L. (1983). Injection of established avocado trees for the
effective control of Phytophthora root rot. SA Avocado
Growers' Association Yearbook 6: 76-81.
SNYMAN,
C.P., KOTZÉ, J.M. (1983). Efficacy of systemic fungicides applied as a trunk
paint and a spongeband for the control of root rot on
five year old avocado trees. SA Avocado Growers' Association Yearbook
1983: 6: 70-71.
ZENTMYER,
G.A., (1979). Report on Phytophthora root rot of Avocado in