CONTROL
OF DEAMTOPHORA NECATRIX AND PHYTOPHTHORA CINNAMOMI IN ESTABLISHED AVOCADO ORCHARDS BY SOIL
SOLARIZATION
C.J.
López Herrera Estación Experimental La Mayora, C.S.I.C. 29750-Algarrobo Costa,
Málaga, Spain
M.J. Basallote Ureba C.I.D.A.
Apdo.
4240 14080-Córdoba, Spain
R.M.
Pérez Jiménez B. Verdú Valiente C.I.D.A. 29140-Churriana, Málaga, Spain
J.M.
Melero Vara Instituto Agricultura Sostenible, C.S.I.C. Apdo. 4084
14080-Córdoba, Spain
Abstract
Soil solarization in avocado
orchards in the Coastal areas of Andalucia increased average maximum soil
temperatures 5-7 C in the unshaded areas specially at shallower depths, whereas
shaded areas reached temperatures lower than those in unshaded areas of control
plots. Solarization periods of 5-8 wk determined a drastic or complete loss of
viability of mycelium, of D. necatrix infecting avocado roots as well as
a significant reduction of viability in roots sampled from shallow soil layers
in the unshaded control. Although those solarization periods achieved a good
control of P. cinnamomi infecting rootlets and in the 10-20-cm-upper
soil layer, but this pathogen remained viable in deeper layers from which a
recolonization process would probably initiate. This would account for the
reinfestation of the upper layer of soil noticed after ca. 1 yr since the
beginning of solarization.
Additional index words: inoculum density, soilborne
fungi, Rosellinia necatrix, avocado root rot, avocado white rot.
1. Introduction
Avocado White Rot (AWR)
caused by Deniatophora necatrix Harting (anamorph Rosellinia necatrix
Prill.) and Avocado Root Rot (ARR), which causal agent is Phytophthora
cinnamomi Rands, are the most important diseases of avocado in the Coastal
areas of Andalucia (Southern Spain) since the early eighties (López-Herrera and
Garcia-Rodríguez, 1987; López-Herrera and
Melero-Vara, 1991).
This work is aimed at
determining the effectiveness of soil solarization in avocado fields naturally
infested with either D. necatrix (1991, 1993 and 1994) or P. cinnamomi
(1993 and 1994) in different orchards of Andalucia.
2. Material and methods
Treatments of soil solarization
were performed in four orchards affected by AWR and two other affected by ARR.
These orchards consisted of trees 10-20 years old. Transparent polyethylene
(TPE) films 75 um thick were laid down at both sides of trees, covering
completely the road, by mid July of every year and TPE films were removed after
5 or 8 wk.
Mean
hourly temperatures were recorded during the solarization period at the different depths and
locations considered in each plot.
Nylon nets containing 10
segments of roots naturally infected by D. necatrix, were buried at
different soil depths (15, 20, 30, 45 and 60 cm) in two locations (unshaded and
shaded) of each plot. These roots, buried in both solarized and control plots,
were recovered immediately after the 5-wk solarization period in 1991, at 4 and
8 wk in 1993, and at 3, 6 and 8 wk after initiation of the treatment in 1994.
Mycelial viability of the pathogen and the effectiveness of soil solarization
in eradicating D. necatrix from infected trees were assessed by incubating
the root segments on PDA at 24 C in the dark and by incubating 10 root segments
taken from each avocado tree both in a chamber of saturation humidity and in
PDA plates, respectively. Assessments of viability were conducted both
immediately before and immediately after the total period of solarization.
In the case of P.
cinnamomi, the effect of the treatment was evaluated by: a) determining
isolation frequency from avocado roots before and after solarization, by
plating onto CMA, b) determining inoculum density (propagules/g) in soil
sampled around avocado roots (Gees and Coffey, 1989), c) isolating the pathogen
from avocado roots of plants grown in soil sampled before and after
solarization, and d) determining the survival of inoculum grown on a nutrient substrate
(Juárez-Palacios et al. 1991) buried at the different depths and
locations in the orchards of 1994, 3, 6 and 8 wk after initiating solarization.
3. Results
Average maximum hourly soil
temperatures in the unshaded areas of solarized plot were 40.3-32.0 C at the
20-45 cm depth in 1991 and 1993. In contrast, temperatures in shaded areas of
the solarized plots were 27.0-29.2 C, only slightly higher than their controls
and lower than in the unshaded areas of the control plots, particularly for the
shallower (20 cm) depth (table 1). In 1994, soil solarization determined a
temperature increase over the control which was more pronounced at depths of 15
and 30 cm and in the unshaded location. Whereas for unshaded solarized
locations maximum hourly temperatures reached 40 and 38 C at 30 and 45 cm
depth, respectively, for shaded areas those were only ca. 29 C, i.e. ca. 5 C
higher than their control plots.
A drastic or complete
reduction of the viability of D. necatrix was observed after 5-8 wk of
solarization at any of the depths and locations in the solarized plots. In
unshaded areas, the pathogen lost the viability even after solarization periods
as short as 3 wk and at depths up to 60 cm, the maximum considered in our
studies. There was also a significant reduction in pathogen viability in the
samples buried at the shallower depth in the unshaded control plots.
Inoculum density was nil
after solarization whereas a reduction of ca. 50% in the frequency of isolation
of P. cinnamomi from avocado rootlets occurred in 1993 (table 2). The
frequency of isolation of P. cinnamomi from naturally infected avocado
rootlets was reduced to negligible levels for 6 months after the end of
solarization although it greatly increased after 14-18 months (figure 1).
4.
Discussion
The effect of soil
solarization on temperature increase is considerable when unshaded areas are
compared, whereas soil solarization of shaded areas has little effect on
temperature increase. As a consequence, the use of soil solarization is
suggested for young orchards with a reduced canopy or else pruning of older
trees is recommended in order to maximize the area of effective soil
solarization.
In agreement with previous
studies (Sztejnberg et al., 1987), D. necatrix shows a very high
thermal sensitivity. This explains the loss of effectivity at deepest soil
layers and under short periods of soil solarization. Even in shaded areas of
solarized plots, the control of the pathogen was quite good, as well as in the
upper 15 cm of soil in the unshaded control plots.
The control of P.
cinnamomi achieved by soil solarization was similarly observed by the
different determination procedures using root and soil samples taken at 10-20
cm depth. However, this effect is not persistent, since P. cinnamond was
isolated with a similar frequency to that of the control plots when sampling
was conducted 14 mo after the end of the solarization (figure 1). This could be
related to a rapid recolonization process from lower soil layers.
Acknowledgements
This research was supported
by grant INIA SC93-085 from Instituto Nacional de Investigaciones Agrarias,
M.A.P.A. Madrid, Spain.
5. References
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1989. Evaluation of a strain of Myrothecium rolidum as a potential
biocontrol agent against Phytophthora cinnamomi. Phytopathology
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Juóaez-Palacios, C., Fé1ix-Gastelum,
R., Wakeman, R.J., Paplomatas, E.J. and DeVay, I.E. 1991. Thermal sensitivity
of three species of Phytophthora and the effect of soil solarization on
their survival. Plant Disease 75:1160-1164.
López-Herrera, C.J., and
Garcia-Rodriguez, J.C. 1987. Survey of soil fungi associated with avocado crops
in southern mediterranean coast of Spain (Málaga- Granada). Proc. of 7th
Congress of M.P.U.:189-190.
Sztejnberg, A., Freeman, S.,
Chet, I., and Katan, J. 1987. Control of Rosellinia necatrix in soil in
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