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Nagy G. et al. / Not Bot Horti Agrobo, 2014, 42(1):109-114
In Vitro and in Planta Activity of Some Essential Oils
against Venturia inaequalis (Cooke) G. Winter
Géza NAGY
1
*, Tamás HOCHBAUM
1
, Szilvia SÁROSI
2
, Márta LADÁNYI
3
1
Corvinus University of Budapest, Faculty of Horticultural Sciences, Department of Plant Pathology, 44. Ménesi Str., H-1118 Budapest,
Hungary; geza.nagy@uni-corvinus.hu (*corresponding author); HochbaumT@nebih.gov.hu
2
Corvinus University of Budapest, Faculty of Horticultural Sciences, Department of Medicinal and Aromatic Plants, 29-43 Villányi Str., H-1118
Budapest, Hungary; szilvia.sarosi@uni-corvinus.hu
3
Corvinus University of Budapest, Faculty of Horticultural Sciences, Department of Biometrics and Agricultural Informatics, 29-43 Villányi Str.,
H-1118 Budapest, Hungary; marta.ladanyi@uni-corvinus.hu
Abstract
The effect of the essential oils of thyme (Thymus vulgaris), cinnamon (Cinnamomum verum) and sweet orange (Citrus sinensis)
on conidial germination of Venturia inaequalis was investigated in vitro. Moreover, the effect of cinnamon oil was further studied by
analyzing its preventive and curative activity as well as its rainfastness in an apple seedling-Venturia pathosystem under growth
chamber and greenhouse conditions. The essential oils suppressed the germination of conidia at concentrations of 0.01% and 0.1%
effectively. The highest inhibition level was achieved with thyme oil. Cinnamon oil (0.2%) with Silwet Star adjuvant (0.02%) was
protective and curative against Venturia inaequalis when applied 72 h, 24 h, 1 h before or 24 h, 72 h after inoculation of plants.
Effective control was achieved when essential oil was applied by 24 h, 1 h or 24 h pre- or post-inoculation, respectively. The 72 h
application was less effective, however, it still showed decent control level. Although the simulated rainfall of light (1 mm),
moderate (3 mm) and heavy (5 mm) rain decreased the control efficacy of cinnamon oil, the water amounts did not influence
control level. The oil application resulted in a moderate and long lasting control (92% efficacy without rainfall, 65% efficacy with
rainfall). The results of this study show that the tested essential oils are appropriate candidates for the development of biofungicides
against apple scab.
Keywords: apple scab, conidia, essential oils, preventive and curative activity, rainfastness
Introduction
Apple scab (Venturia inaequalis (Cooke) G. Winter) is a
widespread and common disease around the world
wherever apples are grown (Vaillancourt and Hartman,
2000). The pathogen causes significant loss in the
Hungarian apple production. In epidemic years, the
number of fungicide applications against the disease often
exceeds twenty sprayings in a vegetation period. In recent
years, the use of fungicides against apple scab is strictly
regulated by EU legislation (eg. Directive 2009/128/EC,
Commission regulations No 834/2013, No 1004/2013, No
1138/2013, No 61/2014). In organic production the use of
copper fungicides is limited due to soil accumulation as well
(Commission Regulation No. 473/2002, Council
Regulation (EEC) No. 2092/91). Therefore, the research of
alternative solutions to synthetic or copper based fungicides
is particularly needed due to the potential risks of the
environment pollution and of fungicide resistance. Essential
oils of medicinal and aromatic plants are suitable candidates
for this purpose. Their antimicrobial and antifungal activity
has been known for centuries and it is still an intensively
studied field. Since essential oils contain several chemical
compounds, the inhibition is based on different biological
modes of action. The primary targe
t of essential oils is the
fungal plasma membrane. Active compounds cause lesions
and reduce ergosterol biosynthesis resulting in severe
morphological changes of the hyphal wall (Pinto et al.,
2006; Soylu et al., 2010; Tian et al., 2012).
Although there are numerous data available on the
research of the effect of plant extracts and essential oils on
plant pathogens, fewer results can be found about the effect
on Venturia species. The plant extracts from Citrus sp.,
Galega officinalis, Inula viscosa, Quillaja saponaria,
Polypodium vulgare and Saponaria officinalis inhibited
conidial germination of Venturia inaequalis (Cooke) G.
Winter effectively and gave good control of apple scab on
seedlings in greenhouse studies, however, simulated rainfall
decreased the efficacy significantly (Pfeiffer, 2002; Pfeiffer et
al., 2004). Plant extracts containing high contents of active
ingredients of artemisinin (Artemisia annua), chelidonine
(Chelidonium majus), menthol (Mentha piperita), populin
(Populus nigra), linalool + linalyl acetate (Salvia sclarea) and
thymol (Thymus vulgaris) were tested against the ascospore
and conidia germinaton of Venturia inaequalis (Cooke) G.
Winter. Almost all extracts showed high inhibition of
ascospore germination. Artemisinin, thymol and po
pulin
Available online: www.notulaebotanicae.ro
Print ISSN 0255-965X; Electronic 1842-4309
Not Bot Horti Agrobo, 2014, 42(1):109-114
Nagy G. et al. / Not Bot Horti Agrobo, 2014, 42(1):109-114
extracts effectively inhibited conidial germination as well.
The 1% Populus extract gave similar control efficacy against
apple scab on apple trees as synthetic fungicides in a 2 years
field study (Bálint et al., 2014; Theisz et al., 2007). The
preventive application of six plant oils (canola, corn,
grapeseed, olive, soybean and sunflower) reduced scab
infection on apple leaves significantly under controlled
condition, however, orchard application of canola and
soybean oil with emulsifier did not give better control level
than that of the application of emulsifier alone (Northover
and Schneider, 1993). The essential oils of Phellodendron
chinense, Russowia sogdiana and Zanthoxylum bungeanum
inhibited in vitro the mycelial growth of Venturia pirina and
other fungi effectively. The wide antifungal activity could be
attributed to the high contents of monoterpene compounds
(Gong et al., 2009, 2011; Tan et al., 2007). Berngtsson et al.
(2006, 2009) compared the effect of plant extracts and the
resistance inducer acibenzolar-S-methyl on conidial
germination with their effect on scab infection on apple
seedlings. Several extracts inhibited effectively conidial
germination, however, showed no or moderate in vivo
efficacy. They pointed out that besides the direct fungistatic
or fungicide effect of the plant extracts, induction of plant
resistance might also play an important role in their mode of
action.
Although the effect of plant extracts on Venturia
inaequalis (Cooke) G. Winter has been actively
investigated, the effect of essential oils is little-known. This
is particularly true to their in vivo mode of action, duration
of efficacy and rainfastness, which latter highly influence the
control efficacy in outdoor conditions.
The effect of the essential oils of thyme (Thymus
vulgaris) and cinnamon (Cinnamomum verum) in different
plant – pathogen relationships has been investigated in the
Department of Plant Pathology, Corvinus University of
Budapest since 2005 (eg. Hochbaum and Nagy, 2013;
Horváth et al., 2013; Petróczy et al., 2006).
The objectives of this study were to (i) investigate the
effect of thyme, cinnamon and sweet orange oil on the
germination of the conidia of Venturia inaequalis (Cooke)
G. Winter, (ii) reveal in vivo mode of action of cinnamon oil
against scab on apple seedlings, (iii) assess the length of
efficacy of cinnamon oil and (iv) evaluate the effect of rain
amount on the efficacy of essential oil treatment.
Materials and methods
Essential oils and adjuvant
The essential oils of cinnamon (Cinnamomum verum)
(Aromax Inc.), thyme (Thymus vulgaris) (Aromax Inc.) and
sweet orange (Citrus sinensis) (Naturol Ltd.) were selected
for in vitro and in vivo assays. Chemical composition was
determined by GC 6890N Gas Chromatograph equipped
with 5975 Inert mass selective detector (Tab. 1). According
to previous studies (Hochbaum and Nagy, 2013), Silwet
Star adjuvant was added in 0.02% concentration to aqueous
essential oil dilutions in order to facilitate the dispersion of
the oils. The applied concentration did not influence
conidium germination. In the wash-off test the adjuvant
was not applied.
110
Component RT LRI Cinnamon
(%)
Thyme
(%)
Orange
(%)
α-pinene 5.56 938 - 0.67 0.31
camphene 5.95 952 - 1.44 -
sabinene 6.52 976 - - 0.19
β-myrcene 6.99 995 - 1.47 1.25
α-terpinene 7.79 1018 - 2.15 -
ρ-cymene 8.09 1026 1.77 24.58 -
limonene 8.19 1029 0.86 - 97.64
1,8-cineol 8.38 1034 3.58 6.54 -
γ-terpinene 9.2 1056 - 7.42 -
linalool 10.76 1097 3.65 5.5 0.22
borneol 13.43 1162 - 0.63 -
terpinen-4-ol 13.96 1175 - 0.93 -
α-terpineol 14.55 1189 1.12 - -
phenylethyl
alcohol 14.83 1201 1.64 - -
thymol 18.81 1290 - 40.64 -
carvacrol 19.2 1300 - 3.6 -
cinnamyl
aldehyde 20.8 1351 67.9 - -
eugenol 21.44 1361 6.83 - -
geranyl acetate 22.43 1388 - 0.52 -
β-caryophylene 23.68 1420 2.5 3.08 -
methyl trans-
cinnamate 24.14 1436 2.09 - -
ethyl
cinnamate 24.2 1440 0.75 - -
α-humulene 25.07 1454 - 0.32 -
cinnamyl-
acetate 26.13 1488 6.62 - -
caryophylene
oxide 30.2 1590 - 0.21 -
benzyl
benzoate 35.67 1738 0.33 - -
Total 99.64 99.7 99.61
Note: RT - Retention time, LRI - Linear retention index relative to C8-C23 n-
alkanes on a HP-5 column
Tab. 1. Compos
ition (%) of the selected essential oils
Test plants and fungal inoculum
Apple seedlings with 4-6 fully expanded leaves were used
for in vivo assays. The seedlings were produced from seeds of
apple cultivars ‘Gala’ and ‘Jonagold’. Plants were grown in a
light-room at 18-24 °C with 12 h photoperiod in vegetable
trays and were later transferred into individual pots.
Diseased apple leaves were collected from non-treated
orchards and home gardens from different cultivars. Leaves
were stored in paper bags in refrigerator (4 °C) until
inoculation. Conidia were harvested from the leaves by a
small brush into sterile distilled water to obtain conidial
suspension of a mixed population. Inoculum concentration
was measured by haemocytometer.
Nagy G. et al. / Not Bot Horti Agrobo, 2014, 42(1):109-114
rainfall on the rain
fastness of the essential oil different
quantity of distilled water was sprayed onto the seedlings for
simulating light (1 mm), moderate (3 mm) and heavy (5
mm) rainfall. Treated plants were left to dry for 4 hours (15
°C, 80% RH) before artificial inoculation. Untreated, no-
water-exposed (0 mm) and water-exposed seedlings were
inoculated with conidium suspension (4.9 × 10
5
conidia/ml) in the same manner as described earlier.
Evaluation of rainfastness was carried out on the 14
th
and
21
st
days by the assessment of 30 leaves per treatment in six
blocks according to their infected leaf area.
Statistical analysis
In order to normalize the data and homogenize the
variances, in the case of the rate of germinated conidia,
arcsin transformation; in the cases of infected leaf area data,
ln(x+0.0001) transformation were used. Normality was
proved by skewness and kurtosis (Tabachnick and Fidell,
2006). The effect of essential oil treatment on conidial
germination was evaluated by univariate ANOVA model
having fixed factors as treatments (essential oil types
cinnamon, thyme and orange in concentration of 0.001%,
0.01% and 0.1%, Silwet Star and Untreated control) as well
as germination media (water, apple leaf brew). The
preventive and curative effect of cinnamon oil was evaluated
by univariate ANOVA with random block design having
fixed factors as treatments (Control, 1 h, 24 h, 72 h or
Control, 24 h, 72 h, respectively) as well as the length of
time period after inoculation (days 14 and 21) and blocks as
different plants. The effect of cinnamon oil under simulated
rainfall was evaluated by univariate ANOVA with random
block design having fixed factors as treatments (Control, 0
mm rain, 1 mm rain, 3 mm rain and 5 mm rain) as well as
time period after inoculation (days 14 and 21) and blocks as
different plants. Homogeneity of variances and the
normality of the residuals of the ANOVA models run with
the transformed data was yet slightly violated, however,
considering the homogeneity of sample sizes and the
robustness of ANOVA in case the ratio of maximal and
minimal variances is under 2, we accepted the results as
reliable. In cases of significant factor effects Games-Howell’s
post hoc test was run.
Results and discussion
Effect on conidial germination
Germination was more intensive in water than in apple
leaf brew, however, similar tendency was observed in the
inhibition of the germination by the essential oils in both
types of applied germination media. In the lowest applied
concentration (0.001%), none of the essential oils
suppressed germination significantly in comparison with
Silwet Star control. Inhibition level was more remarkable in
0.01% concentration. Thyme and sweet orange suppressed
germination in apple brew significantly at 94.3% and 82.9%
efficacy, respectively. In water, only the inhibition of thyme
(96.0% of efficacy) proved to be significant. In 0.1%
concentrations, all essential oils suppressed germination in
apple brew effectively (p<0.05) at 98.9-100.0% efficacy,
111
In vitro
assay of the inhibition of conidial germination
Two-fold dilutions of essential oils dissolved either in
water or in 0.1% apple leaf (‘Jonagold’) brew were prepared.
Oil dispersion was facilitated by Silwet Star (0.02%). 60-60 µl
of conidium suspensions (7.2 × 10
5
conidia/ml) was
measured into the wells of single concave microscope slides.
60-60 µl essential oil suspensions were added to the conidium
suspensions. Final concentrations of essential oils were
0.001%, 0.01% and 0.1%. The wells were covered by cover
slips to prevent evaporation of the oils. Conidia suspensions
were incubated on 20°C in the dark. Germination was
assessed microscopically after 96 h by counting 3 × 30 spores
per treatment and compared to the germination in water, in
apple leaf brew and in 0.02% Silwet Star solution. Conidia
were considered germinated when the germ tube was at least
twice as long as the one of the conidia. The trial was
performed in triplicates. Efficacy of essential oils was
calculated by Abbott’s formula (Abbott, 1925).
In vivo plant assays
The essential oil of cinnamon was selected for plant
assays. According to our earlier in vitro studies (Cseh et al.,
2014), the essential oil of thyme could be harmful to plants
under forced conditions. Orange oil does not show
significant in vivo inhibition against Venturia inaequalis
(Cooke) G. Winter in ongoing orchard trials.
Assessment of preventive and curative activity of cinnamon oil
The experiment was carried out in a greenhouse during
the winter of 2013-2014. Potted ‘Gala’ seedlings were placed
into plastic boxes. Plants were kept between 10 °C and 27 °
C
under natural photoperiod supplemented with 12 h artificial
light. Relative humidity ranged between 70% and 97%.
Artificial inoculation was performed by spraying conidium
suspension (4.9-5.4 × 10
5
conidia/ml) onto the upper and
lower side of the leaves. Seedlings were sprayed with 0.2%
cinnamon oil diluted in 0.02% Silwet Star 72 h, 24 h and 1 h
prior to inoculation as protective treatments and 24 h or 72 h
after inoculation as curative treatments. Conidium and
essential oil suspensions were sprayed onto the plants by hand
sprayer until “run-off”. After inoculation, relative humidity
was maintained above 90% for three days by covering boxes
by a plastic lid in order to facilitate conidial germination.
Evaluation was carried out on the 14
th
and 21
st
days after
inoculation by the assessment of 30 leaves per treatment in six
blocks according to their infected leaf area. Inhibition of
essential oil treatments was compared to inoculated control
plants.
Wash-off study of cinnamon oil
The experiment was carried out in a growth room (14-
16 °C, 12 h photoperiod, 80-95% relative humidity). Potted
‘Jonagold’ seedlings were sprayed until “run-off” by a single-
nozzled hand sprayer with 0.2% cinnamon oil diluted in
distilled water without adjuvant. In order to ensure dilution
homogeneity sprayer was being shaken continuously.
Treated plants were left to dry for 4 hours (15 °C, 80% RH)
before rainfall simulation. To assess the effect of natural
Nagy G. et al. / Not Bot Horti Agrobo, 2014, 42(1):109-114
however, cinnamon oil did not show significant inhibition
in water. Both the effects of essential oil treatment and
germination media were significant on conidial germination
(F
tr
(10;44)=46.35 with p<0.001; F
med
(1;44)=12.84 with
p<0.001, resp.). The interaction was also significant
(F
tr*med
(10;44)=2.71 with p<0.05). The treatment effect size
was high, meanwhile the media effect size was low (partial
η
2
=0.91 and 0.27, resp.). The observed power was above 0.9
in all cases (Fig. 1).
All tested essential oils suppressed conidial germination
effectively. The highest inhibition level was achieved with
thyme oil. Bálint et al. (2014) and Theisz et al. (2007) found
thyme extracts effective as well. The high contents of
phenolic monoterpens, especially thymol (40.6%) and its
precursore p-cymene (24.6%), might be responsible for the
antifungal effect of thyme oil (Gong et al., 2009), however,
the synergistic effect of minor oil components might also
contribute to the antifungal activity (Zambonelli et al.,
2004). According to Tan et al. (2007), the high content of
limonene (97.6%) might have caused the good inhibitory
effect of orange oil.
In apple leaf brew the germination rates of conidia were
lower than in water in most treatment, which could be
explained by the accumulation of antifungal substances in
old apple leaves (Agrios, 2005).
Preventive and curative activity of cinnamon oil
The preventive and curative applications of 0.02%
cinnamon oil decreased the disease severity on inoculated
plants significantly compared to inoculated control plants.
Oil application 1 h or 24 h before inoculation showed
almost complete inhibition (100 to 99% efficacy) by the
first while an outstanding inhibition (91 to 92% efficacy) by
the second evaluation. Cinnamon oil applied 72 h before
inoculation decreased infection level at a rate of 65% (first
evaluation) and 51% (second evaluation) efficacy.
Curative application of cinnamon showed good disease
control even when applied 72 h after inoculation. At the
first evaluation, efficacy ranged from 97% (24 h curative
application) to 83% (72 h curative application). By the
second evaluation, efficacy of cinnamon almost remained
the same (96.8%) when applied 24 h after inoculation and
decreased by 20% when applied 72 h after inoculation.
Both the effects of preventive and curative application
were significant (preventive: F
tr
(3;165)=37.03 with
p<0.001; curative: F
tr
(2;123)=65.13 with p<0.001) while
both the effect of time length after inoculation and the
interaction were insignificant in both cases (preventive:
F
time
(1;165)=3.73 with p=0.06; F
tr*time
(3;165)=0.04 with
p=0.99, resp.; curative: F
time
(1;123)=0.093 with p=0.76;
F
tr*time
(2;123)=0.28 with p=0.76, resp.). The treatment
effect size was moderate (partial η
2
=0.51 and 0.40 for
preventive and curative treatments, respectively) with
observed power above 0.99 (Fig. 2).
Foliar application of cinnamon oil inhibited scab
infection on apple seedlings effectively under controlled
conditions. Interestingly, curative applications gave similar
control efficacy as that of the preventive applications. Since
curative activity of active substances is based on the
inhibition of mycelial growth rather than spore
germination, it can be concluded, that the main compounds
of the essential oil (cinnamyl aldehyde (67.9%), cinnamyl-
acetate (6.6%) and eugenol (6.8%)) effect both spore
germination and mycelial growth (Soylu et al., 2010).
112
0%
20%
40%
60%
80%
Average conidial germination
Apple brew
Water
*
** *
ab
a
ab
a
ab
abc
b
bc
bc cc
AB
A
A
A
AB
ABC
ABC
B
ABC C C
Fig. 1. Mean and standard deviation of conidium germination
(%) of Venturia inaequalis (Cooke) G. Winter in 0.05% apple
brew and in water under different treatments . Different letters
are for significantly different groups (apple leaf brew: lower
case; water: upper case; p<0.05; Games Howell’s test). Asterisk
is for significantly different types of media per treatment level.
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
Control 1 h 24 h 72 h
Infected leaf area
Time prior to inoculation
1st evaluation
2nd evaluation
B
c
ab a
b
A
A
A
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
Control 24 h 72 h
Infected leaf area
Time after inoculation
1st evaluation
2nd evaluation
a
A
b
B
C
c
Fig. 2. Mean and standard deviation of infected leaf area (%) of
apple leaves affected by preventive (above) and curative
(below) treatments of 0.2% cinnamon oil, compared to
inoculated untreated control. Different letters are for
significantly different groups (1
st
evaluation: lower case; 2
nd
evaluation: upper case; p<0.05; Games Howell’s test).
Nagy G. et al. / Not Bot Horti Agrobo, 2014, 42(1):109-114
According to Velluti
et al
. (2004) cinnamyl aldehyde and
eugenol are responsible for the antifungal activity. A strong
curative effect of cinnamon oil was observed against
Fusarium head blight of winter wheat in previous field
experiments as well (Horváth et al., 2013; Nagy et al., 2013).
However, Berngtsson et al. (2006) found curative
application of plant extracts less effective.
The oil components eugenol, cinnamyl aldehyde,
cinnamyl-acetate belong to phenylpropanoid molecules,
which play an important role in plant defense mechanisms
(Bhattacharya et al., 2010; Dixon et al., 2002). Therefore,
according to Berngtsson et al. (2006, 2009), beside direct
antifungal activity cinnamon oil may influence plant
defense as well.
The application of cinnamon oil within 24 h pre- or
post-inoculation resulted in very effective and long lasting
disease control. Decent control level was achieved with its
application within 72 h, especially by the curative timing
(Fig. 3). This observation emphasizes the importance of
disease forecasting for outdoor application of essential oils.
Further research is needed for the determination of the
translocation of cinnamon oil within the plant.
Rainfastness of cinnamon oil
Without simulated rainfall cinnamon oil inhibited scab
infection on apple seedlings effectively. Control efficacy
decreased as a result of rain. However, significant difference
in disease severity was not observed among the different rain
amounts. The effect of treatment was significant
(F
treatm
(4;149)=5.96 with p<0.001) that is to say, untreated
control revealed significantly different results that those of
treated ones. Both the effect of time length after inoculation
and the interaction were insignificant (F
time
(1;149)=1.32
with p=0.25; F
treatm*time
(4;149)=0.48 with p=0.75, resp.).
The treatment effect size was low (partial η
2
=0.14) with
o
bserved power above 0.98 (Fig. 4
).
Although control efficacy decreased by simulated
rainfall, the water amounts did not influence control level. It
can be concluded that part of the active compounds of
cinnamon oil remains on the leaf surface due to their
lipophilic properties. However, Pfeiffer et al. (2004) found
that larger amount of rain decreased the efficacy of plant
extracts on apple seedlings more considerably.
In spite of simulated rainfall, oil application resulted in
moderate (approximately 65% efficacy) and long-lasting
disease control in laboratory conditions.
Conclusions
The tested essential oils, especially Thymus vulgaris
suppress the conidial germination of Venturia inaequalis
effectively. Cinnamon oil applied with adjuvant has
preventive and strong curative effect against apple scab on
plants which could be observed within 3 days prior- or post-
inoculation. Although the efficacy of cinnamon oil
application decreased by simulated rainfall, a moderate
control effect was still exhibited even after the plants were
exposed to relatively high amount of rain. The results of this
study show that the tested essential oils may represent bases
for the development of biofungicides against apple scab.
However, further field experiments are needed for the
investigation of control efficacy in outdoor growing
conditions. Moreover, the translocation of essential oils in
plant should also be tested in laboratory.
Acknowledgments
This research was supported by the European Union
and the State of Hungary, co-financed by the European
Social Fund in the framework of TÁMOP 4.2.4. A/1-11-1-
113
Fig. 3. Scab symptoms on the leaves of inoculated control plant
(above) and that of curatively (72 h) treated plant by
cinnamon oil (below) on the 21
th
day
0%
10%
20%
30%
40%
50%
60%
70%
Control 0 mm 1 mm 3 mm 5 mm
Infected leaf area
applied rain
1st evaluation
2nd evaluation
a
ab ab ab
b
A
AAB AB
B
Fig. 4. Mean and standard deviation of infected leaf area (%) of
apple leaves affected by cinnamon oil treatment followed by
different amount of simulated rainfall, compared to inoculated
untreated control. Different letters are for significantly
different groups (1
st
evaluation: lower case; 2
nd
evaluation:
upper case; p<0.05; Games Howell’s test).
Nagy G. et al. / Not Bot Horti Agrobo, 2014, 42(1):109-114
2012
-
0001 ‘National Excellence Program’.
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