INTEGRATED MANAGEMENT OF AVOCADO: VALIDATION OF TECHNOLOGY IN PUEBLA, MEXICO.
Antonio Mora Aguilera, Daniel Teliz Ortiz, Jorge D.Etchevers Barra and Arturo Huerta De La Peña.
Colegio De Posgraduados, Montecillo, Texcoco, Mexico. 56230.
The
state of Puebla is a probable center of domestication of avocado. Root rot
caused by Phytophthora cinnamomi is the main parasitic limiting factor
in this area. An integrated pest management experiment was established in 1982.
The whole experimental plot was subjected to the following modifications in the
general management: irrigation, pruning, chemical fertilization and control of
aerial pests. Additionally the following contrasting treatments were applied:
1) Fresh bovine manure (E) 360 kg/ tree; 2) Alfalfa straw (A) 25 kg/ tree; 3)
Metalaxyl (M) 2.5 g.a.i./ m2; 4) EA; 5) EM; 6) AM; 7) EAM; 8)
Control (T)(without addition of E, A or M); 9) Double control (DT)(without
addition E, A or M and no modification of the cultural management).
Experimental data accumulated from 1982 to 1990 showed that E treatment
promoted the lowest incidence of P. cinnamomi in roots, the highest dry
weight of roots, the highest number of bacteria and antagonistic fungi, the
highest fruit production, foliage vigor and good macro and micro elements
content in leaves and soil. Control and E treatments gave the best marginal
return rate. This integrated technology (HVA) has been validated from 1992 to
1995 in Puebla, Mexico. HVA had several modifications in the general management
(irrigation, pruning and chemical fertilization). Additionally, 70 kg of fresh
bovine manure/ tree were added. HVA was compared with a traditional technology
(TT) (no modification in the general management and without bovine manure) and
also compared with groves with the best traditional technology (TAS) in the
region. HVA and TAS showed the highest frequency of isolations of Phytophthora
cinnamomi (HVA: 22 % = TAS: 19 % > TT= 12.7 %) and the highest
accumulated root production (TAS: 45 g = HVA: 35 g > TT: 10 g). The high
levels of infection were not detrimental to the growth and vigor of trees
managed with HVA and TAS technology. The effect of root rot on foliage symptoms
in TT trees was 91 %, which decreased to 9 % in TAS and down to 3 % in HVA.
Higher total populations of other soil fungi occurred in HVA (22.45 x 50
propagules/ g soil). The genera more frequently found were Phialophora,
Fusarium, Aspergillus, Trichoderma, Penicillium, Botrytis and Rhizopus. The
total population of bacteria was higher in TAS (700 x l0l3 cell/ ml)
and HVA (457 x 1013 cell/ ml). The longer extension of wood bored by
Copturus aguacatae was found in TT and TAS (TT: 19 cm/ m lineal = TAS: 6
cm/ m > HVA: 2.5 cm/ m). HVA showed the highest foliage index (HVA:
4.3 m2 foliage/ m2 soil > TAS: 3.0 > TT: 2.2) and
the least fruit abortion (HVA: 53 % < TAS: 80 % = TT: 78 %). Accumulated
fruit production in HVA (9.2 ton/ ha) was 31 % higher than TAS (7 ton/ ha), and
460 % higher than TT (2 ton/ ha). The best net benefit in 1993 was found in HVA
(44 %), higher than TAS (31 %) and TT (45 %). In 1994 net benefit in HVA was 55
%, and there were economical loses in TAS (-19 %) and TT (- 75 %)
Key
Words: Persea
americana, IPM, root rot, validation, Phytophthora cinnamomi, Copturus
aguacatae .
Mexico is the largest avocado producer in the world. Among the world production in 1991 (1.3 million of ton) 52 % was produced in Mexico (FIRA and Banco de Mexico, 1991) (Fig. 1)
Puebla,
a probable center of origin and domestication of avocado, is the third avocado
producing State in Mexico. Avocado root rot caused by Phytophthora cinnamomi
is the main parasitic limiting factor in this area. The fungus causes root
rot, wilting, defoliation, and reduces the quantity and quality of fruit;
finally attacked trees die (Broadbent and Baker, 1974; Téliz et al, 1989;
Zentmyer, 1980). Avocado branch borer (Copturus aguacatae ), usually
associated with P. cinnamomi , occurs in Puebla (Cabrera and Salazar,
1991; Valenzuela et al, 1985; Teliz et al, 1989; Muñiz, 1960). The fungus
defoliates branches, which more exposed to sun, are warmer and preferred by the
branch borer females for oviposition. The insect galleries destroy the internal
tissues of the current year branches. Trees simultaneously attacked by P.
cinnamomi and C. aguacatae decline faster in their vigor and
productivity until they die. Damage by branch borers are not significant in
trees with abundant foliage. Isolated control measures against P. cinnamonii
have not been consistently effective, including biological control
(Broadbent and Baker, 1974; Zentniyer, 1963), resistant rootstocks (Zentmyer,
1980) and chemical control, although more recently, the application of
fosfonate has been reported as giving good results (Kotz6 et a], 1987; Pegg and
Whiley, 1987; Young et al, 199 1; Whiley, 1991).
In
Mexico, the work of Colegio de Posgraduados Interdisciplinary Research Group
during 1982- 1990 showed that the incorporation of bovine manure, alone or
mixed with alfalfa straw or with metalaxyl, combined with some modifications to
the traditional groves management, reduced P. cinnamomi and C.
aguacatae damage, promoted the best root growth and tree productivity,
increased the populations of antagonistic fungi and bacteria, improved the
foliage appearance and gave the best marginal return rate (Cabrera and Salazar,
1991; Jacobo et al., 1990; Téliz et al., 1991; Valenzuela et al., 1985). This
integrated crop management (ICM) was validated in two commercial avocado groves
during 1992-1995; this paper will show the benefit of ICM on the control of
avocado root rot and of avocado branch borer, measured by the incidence of the
two pests, and by the root weight, population dynamics of microorganisms
antagonistic to P. cinnamomi, foliage area index, yield and by
the marginal return rate.
Four
avocado groves (1 ha/ each) were selected in 1992 on the basis of age and
variety (13-15 year-old 'Fuerte' trees), and with high incidence of avocado
tristeza, branch borers, and very low productivity in Puebla. Two groves were
managed with the local traditional technology (TT) and the other two were
managed with the following ICM strategy: a) irrigation was modified from
general flooding to individual tree basins, b) general pruning of trees at 1.5
in high from the soil, c) periodic fertilization every 5 to 6 months, d)
rational control of other pests (scab, anthracnose, mites), e) incorporation of
70 kg of fresh bovine manure/ tree. Additionally, two other groves were
included due to their best local management and productivity, whose management
was not modified and which served as the local technology to surpass (TAS)
(pruning of bored branches at the end of harvesting, in September-October,
chemical fertilization every summer, 80-100 Kg of bovine and caprine manure;
irrigation by general flooding, 84% copper oxicloride 1 1/ha, sprayed every
year in February and April.
In
each orchard 13 trees were selected in completely randomized design. The three
types of groves were
compared by the percentage of roots infected by P. cinnamomi, root
weight, foliage area index, production of fruits and net income. Fungus incidence in the
rhizosphere was obtained from a mixture of 10 soil sub samples; after weighting
the roots, 20 to 30 small root pieces per tree were placed in PARPH medium
(Jeffers and Martin, 1986). Tree vigor was evaluated by the dry root weight,
fruit production (ton/ ha) and foliage index assessed with a foliage area
analyzer (LAI-2000: Ll-COR®).
The analyzer was placed at each of the four cardinal points, under the canopy,
at 1.5 m above the soil. Total fungi population was estimated in I ml of soil
suspension from the rhizoplane diluted 1:10 in 20 g agar/ 500 ml water+ 0.50 ml
tergitol, 0.050 g streptomycin and 0.0025 g tetracycline clorhidrate. The most
frequent fungal colonies were transferred to 25% corn meal agar, around a P.
cinnamomi colony to measure their antagonism. Total bacteria population in
the soil was estimated in 0.05 ml of a soil diluted 1: 1013 in nutritive agar.
Borer incidence and damage was estimated in 50 cm of a current year branch
terminal per cardinal point in the higher half of the canopy of 11 trees. The
extension of galleries was measured and number of larvae was counted in each
branch terminal. Total fruit yield and costs of management in 1993 in each
grove were registered. Data was analyzed in a complete randomized blocks and
regression between variables was performed.
RESULTS
AND DISCUSSION
The
highest accumulated root infection by P. cinnamomi was found in HVA
(147-161%) and TAS (125-144 %) (Tukey, 0.05 %), whereas TT trees had the lowest
root infection (56-62 %) (Fig. 2).
The
higher root infection in HVA and TAS may be due to a vigorous root growth (Fig.
3) in soil improved in its physical and biological condition. Increment in
succulent, more metabolically active feeder roots of trees treated with bovine
manure give a higher and more susceptible substrate for fungal invasion (Teliz
et al 1989; Zentmyer, 1980). The higher fungal root invasion in HVA and TAS did
not affect tree vigor. Tree recuperation in HVA was very satisfactory; only 24
out of 440 trees did show tristeza symptoms (Fig. 7) and the accumulate fruit
yield in 1993-94 was 460% superior to TT. In previous works, bovine manure gave
the lowest pathogen population after the third year of its periodic
incorporation, and from the fourth year on the response was consistent and
stable throughout the years (Teliz et al 1989). The validation groves will be
observed in the following years to confirm the previous results. Other factors
related with the host and environment combined with the fungus are probably
involved in the susceptibility, since TT trees showed the highest disease
severity despite having the lowest root infection. Other root factors like
suberization, presence of tannins, phenols and the nature and low concentration
of root exudates, might be influencing the chemiotaxy and low efficiency in
fungal infection.
The
highest accumulated root weight was observed in HVA plots (45 g/ tree) and TAS
(35 g) (Fig. 3) probably due to the vegetative condition of the trees. Amount
of roots is related with the trees canopy (Westwood, 1982). TAS and HVA trees
had more foliage area index (Fig. 10) and did not have water stress. TT trees
had the lowest amount of roots, probably due to the low tree vigor from
compacted soils, water stress, scarce and chlorotic foliage (Fig. 10), and
severe damage from branch borers (Fig. 8). Probably the low tree vigor
incremented the severity of root rot, despite the pathogen incidence was low
(Fig. 2).
Total
fungi were significantly incremented after the incorporation of bovine manure
in HVA groves, whereas in TAS and TT fungi populations were almost constant
throughout the time (Fig. 4). Fungi genera most frequently isolated were Phialophora,
Fusarium, Aspergillus, Trichodenna, Penicillium, Botrytis and Rhizopus. The
effect of organic matter in the increment of populations and activity of
microbial populations in the soil has been registered before (Patrick and
Toussoun, 1965). This effect might be due to the continuous liberation of
nitrates, nitrites, sulfates, phosphates, water vapor, oxygen and other organic
constituents that nourish the soil micro flora (Chapman, 1965).
Population
dynamics of total fungi in TAS and TT groves remained almost unchanged, despite
the content of organic matter in some of these groves (1.6 - 7.6 %) was higher
than RVA groves (3.0 - 3.9%). This might be due to the low water holding
capacity and deficiency of air interchange that reduced the activity of
saprophytic microorganisms in TT groves (Raney, 1965). The constancy of
microbial population in TAS groves was probably due to the stabilization of
organic matter. Soil microorganisms logarithmically increased during the first
months after the incorporation of bovine manure, then the populations
stabilized for a while and finally they declined. This behavior is due to the
gradual decay of the organic matter that liberates organic metabolites, which
are rapidly consumed by the soil microorganisms (Juarez et al, 1986).
Fungal
antagonism against P. cinnamomi
P.
cinnamomi is
attacked by many antagonistic agents (Malajezuk and Theodorus, 1979). The
antagonistic fungi isolated in Puebla were Aspergillus, Trichoderma and
Penicillium, whose detrimental effect on P. cinnamomi has been
previously reported (Sneh et al, 1977). The regression analysis between the
isolated antagonistic fungi and P. cinnamomi was not significant;
however, there was a relation (R2= 0.743) between the total fungi in
soil and the survival of P. cinnamomi (Fig. 5). These results do not coincide
with those of Juarez et al (1986) found a correlation of P. cinnamomi only
with antagonistic fungi. In vitro tests of antagonism do not detect the
complex relations and interactions between soil microorganisms (comensalism,
parasitism, predation, competence, antagonism, etc.) and the physic-chemical
factors in soils (Malajezuk and Theodorus, 1979). It is interesting that total
fungi populations were less abundant in TAS groves, however, they had the
highest populations of P. cinnamomi and bacteria (Figs. 2 and 6).
Total
bacteria.
The
highest accumulated bacteria] populations occurred in TAS groves (621-790xl013
propagules/ml),
followed by TAS (363-562) and TT (100-250 x1013 propagules/ ml) (Fig. 6). It
seems that
bacterial populations were related with the soil and crop management which was
more intensive in HVA and TAS than in TT groves. Jensen (1934) and Cook and
Papendick (1972) found higher bacterial populations in soils enriched with
organic matter than in virgin soils. Water stress in TT groves, evident by the
severe necrosis and defoliation of branches, probably also reduced bacterial
populations.
HVA
groves showed the lowest tristeza incidence (2-4%), followed by TAS groves (7-12%),
significantly lower than TT groves (88-95%) (Tukey, 0.05 %) (Fig. 7).
Tristeza
symptoms in HVA trees disappeared by the effect of pruning which contributed to
a better foliage-root balance (Figs. 3,7,10). Tristeza incidence in TT groves
was increased by deficiencies in water and fertility management. Trees in TT
groves had the lowest incidence of P. cinnamomi in roots (Fig. 2) but
the highest visual incidence (Fig. 7). The low level of root infection in TT
trees might be a limiting factor only in groves with poor management. High
levels of inoculum. (Fig. 2) are not so limiting if the trees are well managed
(HVA and TAS groves); these trees show abundant roots, foliage and acceptable
yields (Figs. 3, 10 and 11).
Damage
by Copturus aguacatae
Trees
in TT groves had a significantly longer bored wood (16.6 - 20.8 cm) by meter of
branch than
trees in HVA groves (Tukey, 0.05 %) (Fig. 8). There were also significant
differences in the number of borer larvae in 24 m of the current year wood: TT (103-116
larvae) > TAS (55-83) > HVA (3-30) (Tukey, 0.05 %) (Fig. 9).
The
lack of significant correlation between branch borer damage and incidence of
tristeza in this work might be due to the pruning of bored branches in TT-2 and
TAS-2. Previous works have reported and indirect correlation (- 0.61 Spearman
correlation coefficient) between tristeza symptoms and wood borer damage
(Cabrera, 1989). Branches of trees affected by tristeza become defoliated and
consequently warmer from sunlight; borer females prefer these warmer branches
to oviposit on (Cabrera and Salazar, 1991). In this work trees with vigorous
foliage did not show significant borer damage. Trees in TT groves had a very
scarce foliage during the year (Fig. 10) and were severely attacked by the branch
borer (Figs. 8 and 9). Trees in RVA groves had a very vigorous vegetative
growth after the severe pruning and showed the lowest borer damage. Cabrera and
Salazar (1991) also found the trees recovered from tristeza, with abundant
foliage area were less damaged by C. aguacatae. These results
verify the recommendation of a severe and uniform pruning of groves with high
incidence of tristeza to recuperate their health, to obtain a faster
equilibrium between foliage and root growth and to promote a more efficient
cultural control of branch borers.
No
relation was found between borer damage and the orientation of branches.
Probably the scarce foliage and the exposition of the branches to sun heat in
YF trees resulted in equal damage in the four cardinal points. The overgrowth
of canopies in TAS trees and the compact and vigorous canopies in HVA trees
influenced the lack of a pattern of damage.
Good
vigor of HVA trees was related with the best index of foliage area index (IAF)
(Fig. 10) and the highest yield (Fig. 11). HVA trees had the highest IAF (4.0 -
4.5'm2 foliage/ m2 soil), followed by TAS trees (2.3 - 3.2 m2/m2) and TT (1.8 -
2.2 m2/m2) (Tukey, 0.05 %) (Fig. 10). IAF is the most used and reliable
estimate of the parameter of foliage structure and volume (Welles and Norman,
199 1). Phytopathologically, IAF represents an estimate of the foliage density
and therefore, an indicator of the recuperation of trees defoliated by P.
cinnamomi and by a deficient management in the past. Diseased trees
recovered satisfactorily in HVA groves based in the non significant disease
incidence (Fig. 7), despite of having a substantial inoculum. in the roots
(Fig. 2). These trees have a vigorous vegetative growth and the best yield
(Fig. 11). IAF has a quadratic relation with yield. The optimum IAF for the
best yields has not been determined for avocado; it will be defined for 'Fuerte'
avocado trees. This index will be fundamental for the criteria ol pruning to
standardize the volume and architecture of canopies to make more efficient the
lighi energy reception and the physiological events like flower
differentiation, fruit growth, etc.
The
effect of a better management in yield is shown in Fig. 11. The accumulated
yield in HVA groves (9.2 ton/ha) during 1993-94 was 31% superior than TAS (7
ton/ha) and 460% superior than TT (2 ton/ha). These results show the
effectiveness of HVA management in the short-term recovery (24-30 months) of
non-productive trees affected by P. cinnamomi and by a deficient
management.
The
global economical analysis shows that HVA trees had the largest difference
between costs and income with a 55% net income; TAS trees had a net income of
31% in 1993 and -19% in 1994. Net income in TT trees was 20% in 1993 and -75% in
1994 (Fig. 12). TT trees had the lowest yields due to the scarce vigor of the
trees, lowest index of foliage area, least shoot growth and root weight, the
highest flower abortion and wood borer damage.
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