Proceedings
of The World Avocado Congress III, 1995 pp. 400 - 403
INFLUENCE
OF SOIL SOLARIZATION ON PHYTOPHTHORA CINNAMOMI RANDS IN AVOCADO (PERSEA
AMERICANA MILL.)
L. Gallo-Llobet and F. Siverio
Apartado 60 - 38200 La Laguna
Tenerife (Canary Is.), Spain
Abstract
Two field trials were
conducted to evaluate soil solarization effectiveness in controlling avocado (Persea
americana Mill.) root rot caused by Phytophthora cinnamomi Rands.
The soils at the experimental site have shown a high degree of infection for
the last 20 years.
In the first
trial solarization was applied to 80 diseased adult trees and 60 were used as
control. Disease severity index (DSI) was measured visually on a scale from 0
(healthy plant) to 5 (dead plant). All trees were indexed as DSI 3-4 at trial
commencement, improving to DSI 2-3 during the first year of solarization; a
second treatment further improved trees to DSI 1-2. Maximum temperatures
measured at 5 cm depths were 40ºC under tree canopy and 45ºC in the sun, higher
than those described as effective for pathogen elimination.
In the second
trial, two plots were used to test 450 West Indian race avocados and 78 Persea
indica (L.) K. Spreng, the latter as control due to reputed susceptibility
to P. cinnamomi. DSI was recorded over a three-year period. Results
showed 88% of avocado and 92 % of P. indica survived in the solarized
plot; control showed 21 % for avocado and 8 % for P. indica survival.
The DSI for plants from the solarized plot were consistently better than those
of the control.
Both trials show
the high efficacy of this technique for control of P. cinnamomi.
1. Introduction
Root rot caused by Phytophthora
cinnamomi Rands is the main disease of avocados (Persea americana Mill.)
throughout all the producing areas of the world. In Spain, root rot has caused
serious losses in the Canary Islands as well as in the plantings along the
Andalusian coast.
P. cinnamomi is a moderate temperature
species with an in vitro maximum growth in the range 30-36ºC with few isolates
growing slightly at 36ºC (Zentmyer, 1980). South African trials (Barbercheck
and Broembsen, 1986), using soil solarization in nursery greenhouses to control
both parasitic nematodes and P. cinnamomi, showed that three weeks of
solarization were sufficient to eliminate 91 % of the P. cinnamomi population,
achieving total elimination after six weeks. Previous trials in Israel (Pinkas et
al., 1984) established that after soil solarization capable of sustaining
36ºC during 4 h only 10% of P. cinnamomi inoculum. survived.
In this work we studied the
effectiveness of solarization for the control of P. cinnamomi in
naturally-infected avocado plots in the subtropical conditions of the Canary
Islands.
2.
Materials and Methods
Field trials were performed
in a seriously infected plot of adult trees in Valle Guerra (Tenerife), where P.
cinnamomi was first diagnosed over 20 years ago. The existence of P.
cinnamomi in the soil was confirmed by avocado fruit trap. The plots
were lightly tilled, irrigated until soil saturation and finally covered with
transparent polyethylene film of 30-37.5 pm of thickness. The soils were
solarized for 4-6 weeks between the months of August and September.
Temperatures of the solarized areas were recorded during five years. In 1994
temperature sensors were installed at 5 and 20 cm. depths, placed in sun and
shade tree-canopy areas in the solarized treatment and in the control treatment
only in the sun.
Given that
inoculum density was difficult to quantify, two trials were set up to compare
the effect of solarization on the symptoms of the disease. In the first trial
between three and six rows of trees (5 x 5 in planting distance) were solarized
per year, covering all ground between the trees and up to each trunk. Plants in
solarized soil were not irrigated during the treatment. Disease severity index
(DSI) was evaluated on a scale of 0 to 5, where 0 = healthy and 5 = dead. The
diseased trees, all about 15 years old, had an initial DSI of 3-4. Eighty trees
were solarized and 60 were used as control for the whole trial. Solarizations
were begun in 1988 and continued up to 1994. In the second trial a plot from
which diseased trees had been removed was solarized, after which I-year-old seedlings
of 135 West Indian avocado rootstocks and 28 viñátigos (Persea indica L.)
were planted. A further 315 avocados and 50 viñátigos were planted as control.
Viñátigo was used due to its reputed susceptibility. Results were expressed as
percentages of living trees and DSI.
3. Results
In the first trial, the adult
avocados from solarized plots with an initial DSI of 3-4 improved to 2-3 and
their foliage was comparatively greener. After the second solarization DSI
decreased to 1-2. In the second trial, percentage of dead plants and average
DSI were higher for non-solarized soil compared to solarized soil (Table 1 and
Figure 1). Three years later, 88 % of avocados and 90 % of viñátigos survived
in solarized soils, whereas only 21 % of avocados and 8% of viñátigos survived
in the non-solarized soil.
Temperature
evolution during solarization is given in Figure 2. At 5 cm depth, compared to
control, solarization temperatures were 10ºC higher in the sun and 6ºC higher
in the shade. The maximum temperatures reached at 5 cm were 46ºC in sun and 41ºC
in shade. At 20 cm,
maximum temperatures of 34ºC in the sun and 30ºC in the shade were registered.
4. Discussion
Several authors have cited reduction
in inoculum density of natural population or artificially introduced P.
cinnamomi after solarization (Pinkas et al., 1984; Barbercheck and
Broembsen, 1986; Juárez-Palacios et al., 1991). Our results for both
trials seem to confirm the effectiveness of this practice for the control of
the avocado root rot. Increased health and vigour of woody perennials following
transparent mulching have been previously described (Ashworth and Gaona, 1982;
Stapleton and De Vay, 1985). This coincides with the results of our
solarization trials, as disease expression in naturally-infected plots was
clearly reduced and established adult avocado trees did not experience further
damage. On the other hand, the percentage of dead avocado and viñátigo
seedlings planted on solarized soils is very low compared to non-solarized
plots. In addition, the DSI for plants from the solarized plot were
consistently better than those of the control.
According to the
results of Zentmyer (1980), Pinkas et al. (1984), Barbercheck and Broembsen
(1986) and Juárez-Palacios et al. (1991), the intervals of maximum
temperatures registered in our trials (41-46ºC) will either eliminate the
pathogen or reduce its viability in those soil depths where most of the avocado
root system is located. These results show the effectiveness of solarization in
controlling P. cinnamomi in naturally-infected avocado plantings in the
subtropical climate and in the soil conditions of the Canary Islands.
Acknowledgements
We thank R. Muñoz-Carpena for
his advice on the system for recording of soil temperature and T.P. Redard for
her help in preparing the manuscript. This work has been funded by the
Instituto Nacional de Investigación y Tecnologia Agraria y Alimentaria.
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