Control of apple scab (Venturia inaequalis) with bicarbonate salts under controlled environment

Abstract

The effectiveness of potassium bicarbonate against Venturia inaequalis, the cause of apple scab, was studied. In vitro experiments with sodium, ammonium and potassium bicarbonate, as well as potassium phosphate used at 1% (w/v), reduced colony growth of V. inaequalis by 99, 98, 90 and 64%, respectively. Under controlled conditions in greenhouse experiments, a single spray of 0.5 or 1% (w/v) aqueous solution of sodium or potassium bicarbonate applied on young apple seedlings, 24 h before or 24 h after scab artificial inoculation, significantly controlled the disease. Greater effectiveness of potassium bicarbonate was recorded when the period of time before or after the inoculation was reduced. A significant increase of the fungicide activity of potassium bicarbonate was observed when salt was mixed with mineral oils. However, combining potassium bicarbonate with vegetable linseed oil and grapefruit seed extract did not increase its efficacy whereas these two vegetable products used alone reduced significantly scab infections. Formulated potassium bicarbonate, under the trade name Armicarb® 100 and containing surfactant compounds, was more effective than bicarbonate alone. A phyto toxicity effect of potassium bicarbonate was observed with a 0.75% dose. The potential and limitations of potassium bicarbonate used to control apple scab in the field are discussed.

Full text

Journal of Plant Diseases and Protection, 114 (5), 221–227, 2007, ISSN 1861-3829. © Eugen Ulmer KG, Stuttgart
J.Plant Dis.Protect. 5/2007
Control of apple scab (Venturia inaequalis) with bicarbonate salts under controlled environment
Bekämpfung des Apfelschorfs (Venturia inaequalis) mit Bicarbonatsalzen unter kontrollierten Bedingungen
L. Jamar*, B. Lefrancq & M. Lateur
Centre wallon de Recherches agronomiques, Département Lutte biologique et Ressources phytogénétiques, rue de Liroux 4, B-5030 Gembloux,
Belgium
*Corresponding author, e-mail jamar@cra.wallonie.be
Received 30 January 2007; accepted 8 March 2007
Summary
The effectiveness of potassium bicarbonate against Ven tu ria
inaequalis, the cause of apple scab, was studied.
In vitro
experiments with sodium, ammonium and potassium bicar-
bonate, as well as potassium phosphate used at 1% (w/v),
reduced colony growth of V. inaequalis by 99, 98, 90 and 64%,
respectively. Under controlled conditions in greenhouse
experiments, a single spray of 0.5 or 1% (w/v) aqueous solu-
tion of sodium or potassium bicarbonate applied on young
apple seedlings, 24 h before or 24 h after scab artificial inocu-
lation, significantly controlled the disease. Greater effective-
ness of potassium bicarbonate was recorded when the period
of time before or after the inoculation was reduced. A signifi-
cant increase of the fungicide activity of potassium bicarbon-
ate was observed when salt was mixed with mineral oils.
However, combining potassium bicarbonate with vegetable
linseed oil and grapefruit seed extract did not increase its
efficacy whereas these two vegetable products used alone
reduced significantly scab infections. Formulated potassium
bicarbonate, under the trade name Armicarb® 100 and con-
taining surfactant compounds, was more effective than bicar-
bonate alone. A phytotoxicity effect of potassium bicarbonate
was observed with a 0.75% dose. The potential and limita-
tions of potassium bicarbonate used to control apple scab in
the field are discussed.
Key words: alternative fungicides, Armicarb, copper, linseed
oil, natural substances, organic farming
Zusammenfassung
Die Wirkung von Kaliumbicarbonat gegenüber Venturi a
inaequalis, dem Erreger des Apfelschorfs, wurde untersucht.
Natrium-, Ammonium- und Kaliumbicarbonate sowie Kalium-
phosphat in einer Konzentration von jeweils 1% (w/v) ver-
minderten das Koloniewachstum von V. inaequalis
in vitro
um
99, 98, 90 bzw. 64%. Die Krankheit konnte unter kontrollier-
ten Bedingungen im Gewächshaus durch Einzelspritzungen
einer 0,5 oder 1%igen (w/v) wässrigen Lösung von Natrium-
oder Kaliumbicarbonat 24 h vor oder nach der Inokulation
junger Apfelsämlinge bekämpft werden. Eine Wirkungssteige-
rung von Kaliumbicarbonat wurde durch eine Verkürzung des
Anwendungszeitraums vor oder nach der Inokulation er-
reicht. Eine Wirkungssteigerung wurde auch durch Mischung
der Substanz mit mineralischen Ölen erzielt. Eine Zugabe von
Leinöl oder Grapefruit-Samenextrakten hingegen steigerte
die Wirkung von Kaliumbicarbonat nicht, während beide
Produkte eine deutliche Einzelwirkung gegenüber dem Apfel-
schorf aufwiesen. Mit Netzmitteln unter dem Handelsnamen
Armicarb® 100 formuliertes Kaliumbicarbonat wirkte stärker
als das reine Salz, das in einer Konzentration von 0,75% eine
phytotoxische Wirkung zeigte. Das Potential und die Grenzen
von Kaliumbicarbonat als Pflanzenschutzmittel gegen den
Apfelschorf werden diskutiert.
Stichwörter: Alternativfungizide, Armicarb, Kupfer, Leinöl,
Naturstoffe, ökologischer Landbau
1 Introduction
Apple scab, caused by Venturia inaequalis (Cooke) G. Wint. is
the most important apple disease, causing significant eco-
nomic losses in many of the world’s apple production areas,
particularly in rainfed agricultural areas where intensive
fungicide control is necessary for commercial apple pro-
duction. With the cultivation of susceptible commercial apple
cultivars, apple scab control is becoming more difficult, such
that losses caused by apple scab would be about 70% if no
control measures were taken. Even in Integrated Pest
Management systems, scab is currently controlled by up to
15–20 applications of protective and curative fungicides
during the growing season, regardless of the presence of
ascospores in the orchards (DEMEYERE and DE TURCK 2002).
Prediction systems have been developed for apple scab and
used successfully to assist in timing fungicide applications
(MACHARDY 1996; TRAPMAN and POLFLIET 1997; JAMAR and
LATEUR 2005). There is, however, a growing concern globally
over the continuous use of synthetic chemicals on food crops
because of their potential effects on human health and on
the environment. Pathogen resistance is another factor
militating against the continuous use of synthetic fungicides
(BENAOUF and PARISI 1998; ANONYMOUS 2004; KOLLER et al.
2004).
Bicarbonate salts are one of several alternative control
options that have recently received attention. These ‘biocom-
patible’ chemicals are particularly interesting because they
have fungicidal properties combined with a very low mam-
malian and environmental toxicity profile. Bicarbonates are
generally regarded as safe by United States Environmental
Protection Agency (EPA) and therefore will be much easier to
register. They are common food additives allowed in many
applications by European and North American regulations.
The actual use of these products in the control of plant disease
is, however, still limited.
Bicarbonates have been shown to control a wide range of
fungi, including food spoilage organisms and plant patho-
gens. The fungicidal effects of the carbonate and bicarbonate
salts of ammonium, potassium and sodium have been report-
ed on soil-borne pathogens, including Sclerotium rolfsii Sacc.
(PUNJA and GROGAN 1982). HOMMA et al. (1981b) found sodi-
um bicarbonate to be inhibitive to powdery mildew on cucum-
ber and green mould on citrus, and the addition of surfactants
to improve the effectiveness of sodium bicarbonate against
green mould on citrus. HORST et al. (1992) showed that rose
powdery mildew (Sphaeroteca pannosa) and blackspot (Diplo-
carpon rosae) were significantly controlled by weekly sprays
of 0.5% (w/v) aqueous solution of either potassium or sodium
bicarbonate used alone or with 0.5% or 1.0% (v/v) Sunspray
oils. ZIV and ZITTER (1992) found pronounced detrimental
effects of bicarbonates on the
in vitro
growth and disease

222 Jamar et al.: Control of apple scab (Venturia inaequalis) with bicarbonate salts under controlled environment
J.Plant Dis.Protect. 5/2007
incidence of several cucurbit foliar pathogens: Alternaria
cucumerina, Colletotrichum arbiculare, Didymella bryoniae,
and Yulocladium cucurbitae. PALMER et al. (1997) reported the
inhibitory effects of bicarbonates on the
in vitro
colony growth
of Botr ytis cinerea and examined the contribution of pH and
buffering capacity of these compounds. The pre-harvest
application of 2% potassium bicarbonate on bell peppers
significantly reduced post-harvested gray mould, caused by
B. cinerea (FALLIK et al. 1997). Sodium bicarbonate controls
post-harvest green mould, caused by Penicillium digitatum on
citrus fruit, and is in common commercial use (SMILANICK et al.
1999). MLIKOTA GABLER and SMILANICK (2001) demonstrated
the potential of carbonate and bicarbonate salts for the control
of post-harvest gray mould on grapes. A novel fungicide
(Armicarb SP) containing potassium bicarbonate reduced
defoliation in citrus caused by Mycosphaerella citri (MCGOVERN
et al. 2003). Potassium bicarbonate salt reduced powdery
mildew (Microsphaera pulchra) in flowering dogwood in the
field (MMBAGA and SAUVE 2004).
There are very few studies on the bicarbonate effects on
apple scab: SCHULZE and SCHÖNHERR (2003) reported that
calcium hydroxide prevents spore germination and kills the
germ tubes of apple scab (V. inaequalis) at a concentration of
4.3 g l–1. A successful field trial with potassium carbonate to
reduce fruit and leaf scab has been conducted by GRIMM-WET-
ZEL and SCHÖNHERR (2005). More recently, ILHAN et al. (2006)
reported the effectiveness of sodium bicarbonate alone or in
combination with a reduced dose of tebuconazol for control-
ling apple scab under field conditions.
The objective of this study was to evaluate the effectiveness
of bicarbonates used alone or combined with horticultural oils
for the control of apple scab in order to develop a successful
strategy using environmentally friendly substances compati-
ble with the organic production system. This is the first study
we know of that demonstrates the effectiveness of potassium
bicarbonate for controlling apple scab (V. inaequalis) on
in
vivo
plantlets raised from seeds.
2 Materials and methods
2.1 Plant material
Experiments were carried out with highly susceptible apple
seedlings from open-pollinated trees of cv. ‘Golden Delicious’
that had probably been pollinated by cv ‘Gala’, two highly scab
susceptible cultivars. After dry storage, the seeds were strati-
fied in moist peat at 2°C for 80–90 days. The apple seeds were
raised in commercial potting soil mixture under greenhouse
conditions at 18°C ± 2°C and 80% relative humidity in a 12-h
light regime as described by OLIVIER and LESPINASSE (1980).
Four-week-old plants at the four-leaf stage were used for the
experiment.
2.2 Controlled inoculation
A mixture of strains of Venturia inaequalis isolated from
diseased leaves from unsprayed orchards of various cultivars
in central Belgium was used for the experiments. Dry leaves
were conserved in the deep freeze at –18°C. The inoculum
was prepared as described by SZKOLNIK (1978). For infection
experiments, conidia were collected in distilled water and the
suspension was adjusted to 1.5 x 105 living conidia ml–1, using
a heamocytometer. Quantitative seedling inoculations were
carried out with an automatic bench sprayer machine in
laboratory. The conidial suspension was sprayed at the
‘just-before-run-off’ stage. Immediately after inoculation, the
plants were incubated in a dew chamber at 100% relative
humidity for 48 h at 18°C to provide optimal infection
conditions. The treatments were randomised within the mist
chamber in a complete block design.
2.3 Fungicide preparations
The chemicals tested included potassium bicarbonate (99.5%
KHCO3, Sigma-Aldrich, Bornem, Belgium), sodium bicarbon-
ate (99.5% NaHCO3, Sigma-Aldrich), Armicarb®100 (85%
KHCO3, Helena Chemical Company, Collierville, TN, USA)
and both ‘Candit’ (a synthetic fungicide containing 50%
kresoxim-methyl, BASF, Antwerp, Belgium) and Thiovit jet
(80% micronised sulphur, Syngenta Agro, Saint Cyr l'Ecole
Cedex, France) as the positive control. The treatments includ-
ed bicarbonate salts alone or combined with emulsified
linseed oil (Vandeputte Oleochemicals, Mouscron, Belgium),
Citripur grapefruit seed extract (containing 33% grapefruit
seed extract without benzethonium from Pro-vera, Chaumont-
Gistoux, Belgium) and Oviphyt mineral oil (a refined petro-
leum distillate marketed by Belchim Benelux, Londerzeel,
Belgium). Armicarb is registered in US by the EPA and labelled
as a biocompatible fungicide.
2.4 Performance of bicarbonate salts at varying concentrations
for foliar disease control
In the first experiment, freshly prepared aqueous solutions
containing 0.25, 0.5 and 1% of KHCO3, 0.25, 0.5 and 1% of
NaHCO3, and 0.25, 0.5 and 1% of Armicarb were sprayed
from a bench sprayer machine in the laboratory onto the
upper surface of seedling leaves until just before run-off.
A solution containing 0.02% Candit (50% kresoxim-methyl)
was used as a reference treatment. Water-treated samples
were used as the control. There were 160 seedlings per treat-
ment (four replicates of 40 seedlings for each treatment).
Pre-inoculation protective treatments were applied once 24 h
before inoculation and post-inoculation curative treatments
were applied 24 h after inoculation when the germination
period had ended and the infection period had begun
(MACHARDY 1996). The plants were then placed on the green-
house bench at 18°C and 80% relative humidity for 3 weeks to
promote plant and disease development. Disease incidence
was assessed 21 days after inoculation by estimating the scab
severity on the most infected leaf of the plant (LATEUR and
POPULER 1996; LATEUR and BLAZEK 2002).
2.5 Use of potassium bicarbonate and oils for foliar disease
control
The set-up of this experiment was as described above except
for the following modifications. KHCO3 was used as an active
ingredient at a concentration of 0.5% (w/v). It was applied
alone or mixed with soluble vegetable linseed oil, grapefruit
seed extract or mineral oil at 0.5% (v/v). The choice of linseed
oil and grapefruit seed extract was based on unpublished
results that had been obtained in our laboratory. Armicarb
was used as a comparative commercial formulated potassium
bicarbonate. A sulphur solution (0.2% w/v) was used as a
reference treatment. In this experiment, all treatments were
applied with a hand sprayer until runoff 24 h before or 12 h
after inoculation with V. inaequalis.
2.6 Effect of treatment timing on potassium bicarbonate
effectiveness
In a third set of experiments, the plants were sprayed once up
to run-off with a freshly prepared salt solution of KHCO3
0.85% (w/v) plus Tween-20 0.05% (v/v) as a surfactant com-
pound, Armicarb 1% (w/v) and Thiovit 0.25% (w/v). Accord-
ing to previous assessment, Tween-20 at 0.05% did not
express any inhibitory effects on apple scab. The treatments
were applied 48, 24 and 3 h before the inoculation or 3, 24
and 48 h after inoculation. The seedling treatments, inocula-

Jamar et al.: Control of apple scab (Venturia inaequalis) with bicarbonate salts under controlled environment 223
J.Plant Dis.Protect. 5/2007
tion, incubation and experimental design were carried out as
described above.
2.7 Phytotoxicity studies
In this experiment, solutions of sodium and potassium bicar-
bonates alone or combined with 0.5% mineral oil were
prepared in distilled water at 0.5, 0.75, 1.0 and 2.0% (w/v).
For comparative purposes, solutions of Armicarb were pre-
pared with the same active ingredient dose. Each solution was
applied with the bench sprayer machine in the laboratory onto
the upper surface of 40 healthy seedlings until just before
run-off. The plants were 4 weeks old at the time of treatment.
Visible leaf phytotoxicity (necrotic area) was recorded on the
10th day after treatment. A qualitative assessment was con-
ducted on the third and fourth leaf of each seedling. Leaf
phytotoxicity was scored thus: – = no damage; + = 0 to 2%;
++ = 2 to 5%; +++ = 5 to 20%; and ++++ = >20% of the leaf
surface damaged.
2.8 In vitro effect of bicarbonate salts on mycelial growth of
V. inaequalis
Two monoconidial strains of V. inaequalis isolated from
Belgium apple cultivars were used to evaluate the bicarbonate
inhibition of
in vitro
colony growth. The first strain was isolat-
ed from Malus floribunda and the second provided by ‘Golden
Delicious’ (strain EU-B-04 INRA Anger). Malt extract at 25.0 g
l–1 and agar at 20.0 g l–1 (ROBERTS and CRUTE 1994) were
mixed with Na, K, NH4+ bicarbonate or K2HPO4 at three con-
centrations (100, 1000 and 10.000 ppm) and were autoclaved
for 20 min, then incubated at 60°C and then poured onto
sterile plastic Petri plates. Non-autoclaved captan at 10 and
100 ppm was used as a control. The V. inaequalis conidia were
transferred to solidified plates with a heat-sterilized glass rod;
the plates were then sealed with Parafilm. The media pH was
not adjusted after amending with bicarbonate. Six plates
differentiated by treatments and by strain were inoculated
and then incubated at 18°C under dark conditions. The colony
diameter was determined by measuring the average radial
growth at 7, 14, 21 and 28 days. The control consisted of
pathogen grown on standard malt agar.
2.9 Experimental design and data analysis
All the greenhouse experiments were arranged in a com-
pletely randomized split-plot design with four replicates of 40
seedlings for each treatment and repeated at least twice. The
percentage data were transformed into arcsine angles before
performing an analysis of variance. The data were analysed
using statistical SAS software and the Student-Newman-Keuls
test was applied as a mean variance analysis. All statistical
analysis was conducted at a significance level of
P
<0.05. For
in vitro
experiments, six replicates per strain and treatment
were conducted and the experiment was repeated twice. The
Student-Newman-Keuls multiple range test at
P
<0.05 was
also used to establish the differences among treatments.
3Results
3.1 Effect of bicarbonate salts at varying concentrations under
greenhouse conditions
All sprays of aqueous solutions of NaHCO3 and KHCO3
applied on apple plantlets 24 h before or 24 h after artificial
inoculation with a conidial suspension of V. inaequalis signifi-
cantly reduced the scab severity on the leaves (Table 1). Sim-
ilar effects were recorded with both Na and KHCO3, although
KHCO3 performed slightly better than NaHCO3. When applied
1 day before inoculation, sodium and potassium bicarbonates
at 1% (w/v) reduced scab severity to rates of 6.9 and 4.9%,
respectively, compared with the infection rate of the controls
that ranged from 41 to 43.8%, respectively. Similar values
were recorded when treatments were applied 1 day after
inoculation. The results indicated that Armicarb was more
effective than KHCO3 alone at the same active ingredient (a.i.)
rate. Slight phytotoxicity was observed only when bicarbon-
ates and Armicarb were used at a 1% a.i. dose.
3.2 Effect of potassium bicarbonate when mixed with oils
KHCO3 at 0.5% applied 24 h before or 12 h after inoculation
significantly reduced scab severity with an effectiveness of
69% compared with the water control. When KHCO3 treat-
ments were combined with vegetable oils, no significant
Table 1: Effect of KHCO3, NaHCO3 and Armicarb applied at increasing concentrations on apple scab development on artificially
inoculated apple seedlings. The chemicals were applied 24 h before or 24 h after the conidial suspension was added
Treatment a.i. dose % Leaf area covered with scab (%) Phytotoxicitya
Pre-inoculation treatments Post-inoculation treatments
Water control … 41.0 ab43.8 a –
NaHCO30.25 30.6 b 33.9 b –
KHCO30.25 31.6 b 33.0 c –
Armicarb 0.25 19.5 cd 22.7 c –
NaHCO30.5 19.3 cd 16.6 d –
KHCO30.5 17.9 d 15.6 d –
Armicarb 0.5 06.5 e 08.4 e –
NaHCO31.0 06.9 e 06.5 e +
KHCO31.0 04.9 ef 04.1 ef +
Armicarb 1.0 02.2 f 01.6 f +
Kresoxym-methyl 0.01 01.0 f 01.0 f –
a – = no phytotoxicity., + = slight phytotoxicity.
b Means of four replicates, each replicate including 40 seedlings. Two leaves per seedling were assessed. Means followed by the same letter
are not significantly different according to the Student-Newman-Keuls multiple range test at
P
< 0.05. The experiment was repeated three
times and similar results were recorded.

224 Jamar et al.: Control of apple scab (Venturia inaequalis) with bicarbonate salts under controlled environment
J.Plant Dis.Protect. 5/2007
increase of the fungicide activity of KHCO3 was observed.
However, significant improvement in the effectiveness of
KHCO3 was observed when it was mixed with 0.5% (v/v)
mineral oil (Table 2). Apple scab was reduced by 88 and 86%
compared with the controls on plants sprayed with Armicarb
24 h before or 12 h after inoculation, respectively. The effec-
tiveness of KHCO3 mixed with mineral oil at 0.5% (v/v) and
Armicarb, were fairly similar. Linseed oil and grapefruit seed
extract at 0.5% used alone significantly reduced infection,
whereas mineral oil used alone did not reduce infection signif-
icantly. In another set of experiments, our observations
indicated that the effectiveness of linseed oil increased as the
concentration increased (data not shown).
3.3 Effect of treatment timing on potassium bicarbonate
effectiveness
The effects of single spray of aqueous solutions of KHCO3,
Armicarb or sulphur applied from 48 h before inoculation to
48 h after inoculation are given in Figure 1. In this set of
experiments, KHCO3 at 0.85% reduced apple scab by 95 and
94.5% when applied 3 h before or 3 h after inoculation,
respectively. Control of apple scab was significantly better
with Armicarb than with KHCO3 when the treatments were
applied 48 h before inoculation. From 24 h before inoculation
to 24 h after inoculation the effectiveness of KHCO3 and Armi-
carb were as affective as sulphur. When the treatments were
applied 48 h after inoculation, the control of the disease was
significantly reduced. Hence, KHCO3 was more effective when
treatments were applied just before or just after artificial
inoculation. Any phytotoxicity on leaves from either KHCO3 or
Armicarb was observed at this concentration (0.85% w/v).
3.4 Phytotoxicity studies
Phytotoxicity in the form of beige to light brownish necrotic
areas was noted on healthy apple leaves treated with 0.5,
0.75, 1 and 2% of NaHCO3 and KHCO3 used alone or com-
bined with oil and Armicarb (Table 3). Tests with 4-week-old
seedlings showed that phytotoxicity symptoms were related to
the concentration of the bicarbonate salts used. The impor-
tance of injury level is directly correlated to the level of salt
concentration. With all the treatments, at 0.5% no injury at all
was observed on young leaves. A few beige necrotic spots
appeared on leaves treated with NaHCO3 and KHCO3 when
used at concentrations of 0.75%. When bicarbonates salts
were mixed with mineral oil leaves were far less susceptible to
phytotoxic reactions. At a 0.75% concentration, neither bicar-
bonate plus oil (0.5% v/v) nor Armicarb showed any injury
symptoms. Armicarb was less phytotoxic than potassium
bicarbonate at the same active ingredient dose. In all cases, it
was noted that the application of bicarbonate solutions that
resulted of distinct droplets on the leave surface produced
more phytotoxicity compared with a better standard leaf
coverage like a continuous film.
3.5 In vitro screening
The
in vitro
results show that potassium and sodium bicarbon-
ates reduced the growth development of V. inaequalis colony
as the bicarbonate concentration increased (Table 4). At
1000 ppm, both NaHCO3 and KHCO3 significantly reduced
colony growth in comparison with malt agar controls, but
NH4HCO3 did not express any fungicide effect. No colony ex-
pansion was measurable with NaHCO3, KHCO3 and NH4HCO3
at 10.000 ppm. At low concentration, potassium phosphate
dibasic stimulated colony growth and at 1000 ppm some
inhibition was recorded. At 10.000 ppm, potassium phosphate
Table 2: Effect of pre-inoculation and post-inoculation appli-
cation of potassium bicarbonate alone or combined with a
vegetable oil, a mineral oil and a vegetable extract on the
severity of apple scab in the greenhouse
TreatmentsapH Leaf area covered with scab (%)c
Pre-inoculationbPost-inoculation
Water control 7.6 52 a 52 a
0.5% MPO 7.8 48 a 46 a
0.5% VGE 7.6 26 b 23 bc
0.5% VLO 7.7 17 cd 20 bcd
0.5% KHCO38.5 15 d 16 d
0.5% KHCO3 + 0.5% MPO 8.6 08 e 07 e
0.5% KHCO3 + 0.5% VLO 8.5 17 cd 19 cd
0.5% KHCO3 + 0.5% VGE 8.5 17 cd 18 cd
0.5% Armicarb 8.5 05 e 06 e
0.25% Sulphur control 7.7 02 e 02 e
a MPO = mineral paraffinic, VLO = soluble vegetable linseed oil,
VGE = soluble vegetable grapefruit seed extract.
b The pre-inoculation and post-inoculation treatments were made,
24 h before and 12 h after inoculation, respectively.
c Means followed by the same letter are not significantly different
according to the Student-Newman-Keuls multiple range test at
P
<0.05
Fig. 1: Effect of a single foliar
spray of 0.85% potassium bicar-
bonate, 1% Armicarb and 0.3%
wettable sulphur solutions on
apple scab severity (leaf area
infected). Freshly prepared salt
solutions of KHCO3, Armicarb and
wettable sulphur were sprayed
until run-off on the upper surface
of each leaf of the plants 48, 24 or
3 h before inoculation and 3, 24 or
48 h after inoculation. The num-
bers are means of 160 plants per
treatment, including four repli-
cates. Different letters denote a
significant difference (
P
< 0.05)
among treatment means accord-
ing to the Student-Newman-Keuls
multiple range test.
a
a
a
a
a
a
b
c
c
c
c
bb
c
cc
c
c
ccccc
b
0
10
20
30
40
50
–48 –24 –3 3 24 48
Time before or after inoculation (hours)
Affected leaf area (%)
Control KHCO3 Armicarb Sulphur

Jamar et al.: Control of apple scab (Venturia inaequalis) with bicarbonate salts under controlled environment 225
J.Plant Dis.Protect. 5/2007
reduced colony growth but was less effective than bicarbonate
salts. Similar pH for all bicarbonate agar solutions were mea-
sured: 5.9, 6.4, 7.2 and 7.9 at 0, 100, 1000 and 10.000 ppm,
respectively. A higher pH was observed for the phosphate agar
solutions: 6.4, 7.4 and 8.2 at 100, 1000 and 10.000 ppm,
respectively.
4Discussion
The results presented in this paper describe the ability of
NaHCO3 and KHCO3 to reduce the growth (
in vitro
and
in vivo
) of V. inaequalis. The ability of bicarbonate salts to
reduce
in vitro
fungal development and disease incidence in
several disease systems had already been reported (PUNJA et
al. 1982; HORST et al. 1992; ZIV and ZITTER 1992; REH and
SCHLÖSSER 1995; OSNAYA-GONZALEZ et al. 1998; MLIKOTA GABLER
and SMILANICK 2001; MMBAGA and SAUVE 2004). Controlling ap-
ple scab with bicarbonate salts has previously been demon-
strated in an
in vitro
study using isolated cuticles (SCHULZE and
SCHÖNHERR 2003) and in three field studies conducted by
BERESFORD et al. (1996), GRIMM-WETZEL and SCHÖNHERR (2005)
and ILHAN et al. (2006).
The mechanisms of fungistatic or fungicidal activity of
bicarbonate salts have not been conclusively established.
Because K2HPO4 was much less effective than NaHCO3 and
KHCO3 in our
in vitro
experiments, it seems clear that bicar-
bonate anions are involved directly in the reduction of
V. inaequalis colony growth. As reported by PALMER et al.
(1997), bicarbonate anion appears to be the active portion of
the bicarbonate salts even if cations might have some minor
effects, as demonstrated by differing sensitivities to various
bicarbonate salts. The same author reported that other bicar-
bonates salts, such as ammonium bicarbonate, were more
effective than sodium or potassium bicarbonate salts in con-
trolling the colony growth of Botrytis cinerea.
Bicarbonates might have several modes of action against
fungi, including buffering and action to raise the pH level and
the osmotic pressure of cells at the leaf surface, both factors
leading to detrimental conditions for fungal spore (PALMER et
al. 1997). MLIKOTA GABLER and SMILANICK (2001) demonstrated
that bicarbonate solutions were less toxic when tested at pH
7.2 than at a higher pH in the case of controlling post-harvest
gray mold (Botrytis cinerea) in grapes. Bicarbonate concentra-
tion in solution is directly related to the pH of that solution.
Bicarbonates are ineffective under acidic conditions because
carbonic acid predominates in solutions below ph 6.5. H2CO3
is unstable and decomposes into carbon dioxide and water. As
the pH increases to pH 8.5, the concentration of bicarbonate
increases. Above pH 8.5, bicarbonate concentration decreases
and the level of carbonate rises. However, high pH on leaf and
fruit surfaces was shown to be effective as a control strategy
for scab (WASHINGTON et al. 1998).
Our results and those described by HORST et al. (1992) show
that the effectiveness of disease control by bicarbonates can be
improved when bicarbonates are used in combination with
horticultural oils. This improved effectiveness was attribut-
able to the following factors: bicarbonate ions, the fungicidal
characteristics of oils, the improved leaf coverage ability and
the spreader-sticker characteristics of oils that keep the bicar-
bonate ions on foliar surfaces. Therefore, in order to enhance
the stability on leaves and hence scab control effectiveness,
bicarbonates could be mixed with mineral oils and with
surfactant supplies in a well-buffered alkaline solution. This
raises the question of whether the greater activity observed
with Armicarb was due to the active ingredient itself (KHCO3)
or to other components of the unspecified formulation.
The remarkable reduction of apple scab with a single foliar
spray of linseed oil and grapefruit seed extract was shown in
our work. Other authors have reported detrimental effects of
various vegetable oils on the disease incidence of several plant
foliar pathogens (COHEN et al. 1991; NORTHOVER and SCHNEIDER
1996; STEINHAUER and BESSER 1997; OSNAYA-GONZALEZ and
SCHLÖSSER 1998). Grapefruit seed antimicrobial effects as well
as grapefruit seed effectiveness against apple scab had been
reported (TRAPMAN 2004) althought its relation to the preser-
vative substances contained has been established earlier
(WOEDTKE et al. 1999). CLAYTON et al. (1943) showed that oils
from cottonseed, corn, linseed, peanut and soybean were fun-
gicidal against Phytophthora tabacina and that the oils from
castor bean, coconut, olive and palm were non-fungicidal.
They concluded that linoleic acid “occurs in large amounts in
most of the fungicidal oils, but not to any extent in the no
fungicidal oils”. They also concluded that there were “strong
indications that linolenic acid (in linseed oil) is associated
with positive fungicidal activity”. These conclusions contrast
with NORTHOVER and SCHNEIDER (1993) who showed that
against three foliar pathogens there was no difference in fun-
gicidal activity between two groups of oils which had either a
high or a low linoleic acid concentration. Corroborating
evidence was obtained by C
OHEN et al. (1990) using water
sonicates of free unsaturated fatty acid instead of oils. Against
Phytophthora infestans, they found that linoleic and linolenic
acids were fungicidal, whereas oleic acid was not fungicidal.
Linseed oil was more effective than paraffinic oil when used
alone in our experiment, whereas linseed oil mixed to KHCO3
did not have positive additive effect, unlike parraffinic oil.
This might be related to the negative effect on the pH of the
linseed mixed solution.
The effectiveness of bicarbonates salts in controlling scab
on apple, as reported here, together with the improvement of
several disease controls using bicarbonate salts (HOMMA et al.
Table 3: Phytotoxicity of bicarbonates on apple seedlings
under greenhouse conditions
Treatment Active ingredient concentration (% w/v)
0.5 0.75 1 2
KHCO3– + ++ +++
NaHCO3– + ++ +++
KHCO3 + MO 0.5% – – + ++
NaHCO3 + MO 0.5% – – + ++
Armicarb ––+++
Phytotoxicity was recorded on the 10th day after treatment on
healthy leaves of 4-week-old seedlings. A qualitative assessment
was conducted on the third and fourth leaves of each seedling.
– = no damage, + = 0 to 2%, ++ = 2 to 5%, +++ = 5 to 20% of the leaf
damaged.
Tab le 4: Mean diameters (mm) of Venturia inaequalis colonies
28 days after placement onto malt extract agar amended with
KHCO3, NaHCO3 or NH4HCO3 applied at increasing concentra-
tions
Treatment 100 ppm 1000 ppm 10.000 ppm
Control 16.6 b 16.7 b 16.6 b
K2HPO422.8 a 16.4 b 06.0 d
KHCO315.0 b 10.0 c 01.8 e
NaHCO317.8 b 10.2 c 00.2 e
NH4HCO315.6 b 16.4 b 00.4 e
Fungicide 00.1 e – –
Mean of 12 replicates. Values followed by the same letters are not
significantly different according to the Student-Newman-Keuls
multiple range test at
P
<0.05.

226 Jamar et al.: Control of apple scab (Venturia inaequalis) with bicarbonate salts under controlled environment
J.Plant Dis.Protect. 5/2007
1981a; HORST et al. 1992; ZIV and ZITTER 1992; SCHULZE and
SCHÖNHERR 2003; SMILANICK et al. 2005), suggest that these
simple compounds are good ‘biocompatible’ fungicides. The
compounds are ubiquitous in nature, naturally present in
human food, available to the general public for non-pesticide
uses, and available for normal functions in human, animal,
plant and environmental systems. The US EPA and the Euro-
pean Commission DG Health & Consumer Protection ruled
that NaHCO3 and KHCO3 are exempt from residue tolerances.
Bicarbonate salts have minimal environmental or worker
safety issues associated with their use; they pose a minimal
ingestion hazard because of their very low toxicity to animals.
Some issues need further examination, however, before the
technology can be recommended for commercial adoption
against apple scab. The fact that effectiveness of potassium
bicarbonates fell when the time before or after inoculation
increased indicates the short longevity activity of these salts
when applied alone on the upper surface of the leaves.
Potassium bicarbonate acts as a contact fungicide and is not
likely to be systemic or curative. Therefore, it is very important
to apply such compounds with a very high foliar coverage
quality. Armicarb is formulated with a surfactant system that
increases its coverage ability. Consequently, the performance
of Armicarb is much better than that of the pure potassium
bicarbonate salt. The timing of their application is crucial
since V. inaequalis causes deep-seated fungal infections after
germination in contrast with Podosphaera leucotricha, the
causal agent of powdery mildew. As shown in this study, a
long-lasting action of potassium bicarbonate cannot be
expected. Bicarbonate salts are quickly converted into an inef-
fective compound and are highly water soluble, and they will
be washed off the leaves by a small amount of precipitation.
They will therefore require frequent spray applications consid-
ering the presence of ascospores in the orchards and infection
risk periods determined by modern local warning systems.
The practical relevance of this work on the use of bicarbon-
ate salts for controlling apple scab includes the following
points: (i) NaHCO3 and KHCO3 used at up to 0.5% were effec-
tive in controlling apple scab in greenhouse seedlings inocu-
lated with a V. inaequalis suspension; (ii) the stability and per-
formance of Armicarb is much better than that with straight
KHCO3; (iii) the addition of mineral oil to KHCO3 improved its
effectiveness in controlling apple scab; (iv) NaHCO3, KHCO3
and Armicarb could not be used at up to 0.75% without a
phototoxic risk on seedling leaves under greenhouse condi-
tions. Additional research is necessary to determine the effec-
tiveness of KHCO3 under field conditions, with an appropriate
treatment formulation, timing and frequency. The results of
field trials will be the subject of a future paper.
Acknowledgements
This research is funded by the Ministry of the Walloon Regional
Government, General Department of Agriculture, Research
Direction, project RW D31 – 1105. The authors would like to
thank Dr Robert Oger (CRA-W Gembloux) for his important
help in the statistical analysis and Ir Piet Creemers (PCF- KOG,
St Truiden) for stimulating discussions during this study.
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