Proceedings
of The World Avocado Congress III, 1995 pp. 408 - 411
EFFECT
OF DIFFERENT FUNGICIDES AND METHODS OF APPLICATION TO CONTROL AVOCADO ROOT ROT
IN SOUTHERN SPAIN
C.J. López-Herrera R.M. Pérez-Jiménez
Estación
Experimental La Mayora J.
García-Faraco
C.S.I.C. C.I.D.A.
29750,
Algarrobo-Costa
Cortijo La Cruz, 29140, Churriana
Málaga, Spain Málaga, Spain
Phosphorous acid
(20 %) and Phosetyl-Al as foliar applications, Phosphorous acid (20%) and
Phosetyl-Ca as trunk injections, and Metalaxyl as soil drench have been assayed
in an avocado orchard with trees severely affected by Phytophthora cinnamomi
in southern Spain. The trees received four applications of fungicides
during two consecutive years (1992 and 1993). Preliminary results revealed an
incipient improvement of avocado trees treated with Phosphorous acid or
Phosetyl-Ca as trunk injections, although in the first case, the improvement
was more noticeable.
Additional index words: chemical control, Phytophthora
cinnamomi, avocado root rot.
1. Introduction
One of the two most important avocado diseases on the
Mediterranean coastal plain of southern Spain is Phytophthora root rot,
caused by P. cinnamomi Rands (López- Herrera and
García-Rodriguez, 1987), with a continuous increase of the affected area since
1989 (data unpublished).
This paper report preliminary
results from an experiment in which different formulations of fungicides and
methods of application were used in an established avocado orchard for control
of Phytophthora root rot.
2. Material and Method
Ten-year-old Hass
avocado trees grafted on Topa-Topa seedlings rootstocks were used for this
experiment in an orchard, severely infested by Phytophthora cinnamomi, located
in Málaga (on the coastal plain of southern Spain). At the beginning of the
experiment the trees presented a height index of severity, around 7, in a scale
disease severity, 0 to 10 (0 = healthy, 10 = dead) (Darvas et al., 1984).
The treatments used were as
follows: Phosphorous acid (PA) (20% solution potassium phosphonate with pH
adjusted to 5.8 with potassium hydroxide) and Phosetyl-Ca (AC)
(Aliette-Ca-10%/EF 2008-13) as trunk injections (Pegg et al., 1985;
Darvas et al., 1984) at rates of 15 ml of commercial formulation per
meter of canopy diameter. PA and Phosetyl-Al (AA) (Aliette 80% WP) at rate at
60 ml and 7.5 g of commercial formulation respectively per meter of canopy
diameter, as foliar applications (5 L of spray per tree) and Metalaxyl (R)
(Ridomil 5G) at rate 40g/m2 of commercial formulation as soil
drenches. These applications were made on July and November, 1992 and 1993. Twenty trees were
used per treatment and there were five treatments and one control replicated
four times in randomized blocks. Data were processed using a general analysis
of variance with LSD test.
The assessment of results was
based on: a) Rating of trees, according to the previous cited scale disease, on
July 1992, December 1993, and November 1994. b) Root samples collected on March
1993 and 1994, from eight trees in each treatment (two per block and
treatment). Each time, freshly killed roots were removed from each tree, rinsed
with tap water to remove soil particles and washed with sterile water.
Afterwards, the roots were surface sterilized with 1% NaClO, followed by
rinsing in sterile water and plated (ten roots per plate) on acidified (pH 4.8)
PDA and CMA with antibiotics and incubated at 24 C in darkness. After two days
the colonies of P. cinnamomi were identified and the mean frequency, for
each treatment and date of isolation, was calculated. c) Fruit yield (Kg/tree)
was recorded from each tree at maturity in 1993 and 1994.
3. Results
At 16 months (in
1993), after 3 fungicide applications, the injected PA treatment, was the most
effective and decreased the average severity index of trees by 16.41 %. The
beneficial effects of PA and AA as foliar applications, and injected AC, were
similar among them and smaller than injected PA. The R was no different from
the control. At 28 months (in 1994), with one more fungicide application, PA
(foliar and injected) and AC decreased the severity of the disease, although
the injected PA was 7% more effective. The R showed no differences with the
control while the appearance of AA f6liar treated trees was slightly better
(table 1).
The frequency of isolation of
P. cinnamomi from rootlets between the two dates of sampling (March,
1993 and 1994) decreased for all treatments except for the control and was nil
in the second year for the PA treatments.
The fruit yield (table 2) was
increased only in the case of the fungicides injected, although the PA
increased the yield nearly 5 times more than the AC. The rest of treatments
decreased the yield.
4. Discussion
We have obtained
an important recovering in the health of trees for some of the treatments
assayed, considering the high level of mean disease initial rating (around 7)
at the beginning of this experiment in contrast with experiments of other
authors in which the mean disease initial ratings of trees were around 5 (Wood et
al., 1987).
After 3 fungicide
applications, the PA injected treatment improved tree health significantly with
respect to the rest of treatments assayed. Besides, the nil isolation frequency
of P. cinnamomi from feeder roots obtained when using PA (injected or
foliar) in contrast to the 4 % obtained with AC injected, revealed a better and
more rapid translocation of the fungicide into the plant in the first case
allowing a better and major protection of the tree roots to invasion by P.
cinnamomi from the soil.
However, after 4 fungicide
applications the AC injected treatment had improved in health to the same
extent as the PA injected and similar to PA foliar. Although the improvement
was 1.39 units and 1.13 units for PA injected and AC injected respectively, in
tree rating over two years, as opposite to the improvement of 0.7 units for PA
foliar during the same time, for trees with a similar initial rating (around
7). This revealed the main absorption power of these fungicides when injected.
Although the PA injected was taken up more readily and was more effective than
AC injected as noticed by Pegg et al (1985). Besides the AC injected
effect was smaller than PA injected in the first year but it was dramatically
improved in the second year, in agreement with the results of Darvas et
al.(1984), and Onsando and Gathungu (1988). The increase of severity index for
the AA foliar treatment can be explained by the slow absorption of this
fungicide through the foliage, since in this experiment the trees presented a
slight foliar mass. The nil effect of R can be explained by a rapid degradation
(biological or chemical) of fungicide in the soil, and it should not be used in
orchards with a rapid decline of trees as a result of P. cinnamomi infection
(McKenzie (1984).
In reference to fruit yield,
it only seems profitable the applications with fungicides by trunk injection;
the low absorption the commercial product by foliar application could be due to
the slight foliar mass in trees with a high severity index.
These preliminary results on
the control of P. cinnamomi in an established avocado orchard on
southern Spain, reveals that it is possible to get an incipient recovering of
avocado trees severely affected by P. cinnamomi only through four
applications, during two consecutive years, by using Phosphorous acid or
Phosetyl-Ca as trunk injections. However, the former seems to be more effective
in basis to the decrease of severity index, the nil isolation frequency of
pathogen from feeder roots, and the increase of fruit yield, of the treated trees.
References
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