Journal of Phytopathology and Pest Management 8(1): 79-91, 2021
pISSN: 2356-8577 eISSN: 2356-6507
Journal homepage: http://ppmj.net/
∗
Corresponding author:
Fakher Ayed,
E-mail: ayedfakher@yahoo.fr
79
Copyright © 2021
Selection of compost-derived actinomycetes with
plant-growth promoting and tomato stem rot
biocontrol potentialities
Fakher Ayed1,2,3*, Rania Aydi Ben Abdallah3, Hayfa Jabnoun-Khiareddine3, Mejda Daami-Remadi3
1National Agronomic Institute of Tunisia, 1082 Tunis, University of Carthage, Tunisia
2Technical Centre of Organic Agriculture, 4042 Chott-Meriam, Sousse, Tunisia
3UR13AGR09-Integrated Horticultural Production in the Tunisian Centre-East, Regional Research
Centre on Horticulture and Organic Agriculture, University of Sousse, 4042, Chott-Meriem, Tunisia
Abstract
Keywords:
actinobacteria, antifungal activity, growth enhancement, Sclerotium rolfsii, Solanum lycopersicum.
Seventeen actinomycetes isolates, recovered from 2 composts, were screened for
their ability to promote the growth of tomato seedlings and to suppress stem rot
disease caused by Sclerotium rolfsii. Tomato cv. Rio Grande seedlings inoculated
with S. rolfsii and treated with A2-3, A3-3, A4-3, A5-3, A8-3, A9-3, A1-4, A2-4, A3-
4, A4-4, A6-4, and A10-4 actinobacterial isolates showed 23.3-70% less disease
severity than the inoculated and untreated controls. A3-3, A2-4, and A4-4 based
treatments applied to S. rolfsii-infected tomato seedlings had significantly
enhanced all growth parameters as compared to control. The recorded increments
were estimated at 35.52-66.6% for height, 37.4-53.4% for the stem diameter, 38.5-
95.6% for the aerial part dry weight, and 81.8-151% for the root dry weight.
Treatments with A3-3 and A4-4 isolates had increased the majority of tomato
growth parameters by 15.8-56.5% over the pathogen-free control. Tomato
seedlings treated with A4-3 and A1-4 isolates showed between 35.2-22.8% and
42.3-43.3% higher aerial part dry weight and root dry weight, respectively, as
compared to pathogen-free and untreated control. This investigation
demonstrated that the tested composts can be explored as potential sources for the
isolation of actinomycetes acting as biocontrol and bio-fertilizing agents.
Ayed et al., 2021
80
1. Introduction
Composting is defined as a biochemical
process managed under principally
thermophilic and aerobic conditions by active
microorganisms to produce a renewable
organic resource (Bohacz, 2018). This process
is a sustainable option for recycling various
agro-industrial wastes, on small or large scale,
to obtain a mature and stable organic matter
(Scotti et al., 2016). The final product, i.e.,
compost, can be used to improve soil
physicochemical and microbiological
properties and suppress various soilborne
diseases (Pane et al., 2019; Stavi et al., 2016).
These effects are partly attributed to their
associated beneficial microorganisms and
bioactive metabolites (Hadar & Papadopoulou,
2012). Thus, composts are becoming an
alternative tool for the extensive use of
synthetic inputs in cropping systems and are
deemed safer for the environment and human
health (Coventry et al., 2006). In the past
decades, the added value of composts
attributed to their disease-suppressive effects
have been extensively demonstrated against
various soilborne pathogens such as
Rhizoctonia solani, Verticillium dahliae,
Fusarium spp., Sclerotinia spp., Phytophthora
spp., Pythium spp., and Thielaviopsis sp.
causing plant wilting, damping-off and
decaying in many important crops (Tubeileh &
Stephenson, 2020; Pane et al., 2013; Alfano et
al., 2011; Bonanomi et al., 2007). The compost
disease-suppressive potential is mainly related
to the biological activity of its associated
microbiota, which interacts with the soil
organic matter and the host plant by regulating
the microbial communities in the rhizosphere
(Hadar & Papadopoulou, 2012; Manici et al.,
2004), and to its capacity to improve plant
nutrition and growth (Martin, 2015). Among
the widely documented compost-associated
microbial agents, bacteria, actinomycetes, and
fungi were the focal driving forces for plant
diseases suppression (Coelho et al., 2020; Joshi
et al., 2009). These microbial agents acted
against target pathogens through antibiosis,
hyperparasitism, and competition for space
and nutrients (Larkin & Tavantzis, 2013).
Actinomycetes, gram-positive bacteria, and
members of the Actinobacteria group are well
known as secondary metabolites producers and
are widely explored for various agricultural
features (Qin et al., 2011). Actinomycetes have
been recovered from diverse natural
environments such as rhizospheric soil,
composts, and healthy plant tissues.
Commercial bio-molecules are mostly
produced by Streptomyces, Saccharo-
polyspora, Micromonospora, Amycolatopsis,
and Actinoplanes (Palla et al., 2018). These
microbial agents can improve plant growth and
support its establishment even under stress
conditions (Srivastava et al., 2015; Hamdali et
al., 2008). Moreover, their antagonistic
potential against phyto-pathogenic organisms
was demonstrated (Nurkanto & Julistiono,
2014; Anouar et al., 2012). They can protect
roots by inhibiting the fungal pathogen
development mostly through the production of
antifungal compounds or cell wall degrading
enzymes (Bhatti et al., 2017). Sclerotium
rolfsii Sacc. is a soilborne pathogen that affects
a wide host range of over 500 dicotyledonous
and monocotyledonous plant species (Sun et
al., 2020; Punja, 1985; Aycock, 1966).
Infection by this pathogen may occur at all
growing stages and lower stems at or near the
soil surface by forming water-soaked lesions.
These lesions spread quickly to girdle stems
where white mycelial mats may be observed on
the infected plant tissues. Severely infected
plants may wilt thus leading to partial or total
yield loss (Sun et al., 2020; Kator et al., 2015;
Fery & Dukes, 2002). Stem rot diseases are
usually managed using chemical fungicides
such as carboxin, carbendazim, benomyl,
propiconazole, methyl thiophanate, and
oxycarboxin (Sridharan et al., 2020).
Nevertheless, the hazardous use of these
chemicals represents a severe threat to the
environment, food safety, and human health
(Sridharan et al., 2020). Biological control of
soilborne phytopathogens is of increased
interest where various microorganisms
Ayed et al., 2021
81
recovered from diverse ecological niches are
largely explored for sustainable management
of stem rot disease (Singh et al., 2013). Among
the explored microorganisms, actinomycetes
are considered as promising potential
candidates for the suppression of S. rolfsii
(Anusree & Bhai, 2017). Therefore, the main
objectives of this investigation are: (1) to
isolate actinomycetes associated with two
selected composts; (2) to evaluate their ability
to promote the growth of tomato seedlings and
to control stem rot disease.
2. Materials and methods
2.1 Pathogen growth conditions
S. rolfsii isolate Sr1, used in the current study,
was originally recovered from potato plants
showing typical stem rot symptoms.
Identification and pathogenicity of this isolate
were previously investigated (Ayed et al.,
2018a; Daami-Remadi et al., 2010). The
pathogen was grown on Potato Dextrose Agar
(PDA) medium for 15 days at 30 °C, in the
dark, before used.
2.2 Composts preparation
Bioassays were carried out using 2 mature
composts. The first one (C3) are composed by
a 70% of cattle and 25% of chicken manures
mixture associated with 5% green waste, the
second one (C4) are issued from 70% of cattle
and 25% of sheep manures mixed with 5% of
olive-mill solid waste. The composting system
was carried out in parallel open-windrows in
the following dimensions: height 1.5 m × base
width 2.0 m × length 10 m. The compost mass
was mechanically homogenized and the
humidity of the mass during fermentation was
maintained using water at an optimal level (60-
70%) for the composting process for 8 months.
These two locally produced composts and their
teas were screened for their plant growth-
promoting potential and for their ability to
control stem rot disease on tomato seedlings
(Ayed et al., 2018b; 2018c).
2.3 Isolation of compost-associated
actinomycetes
A sample of 10 g of air-dried compost was
suspended into a 90 ml volume of sterile
distilled water (SDW) in a 250 ml flask, and
shaken for 30 min at 200 rpm. Serial dilution
of compost was carried out, and then a sample
of 100 µl was spread onto Petri-dish
containing Actinomycetes Isolation Agar
medium amended with 500 µg/ml of
streptomycin sulfate, 100 µg/ml of
chloramphenicol, 50 µg/ml of ampicillin, and
100 µg/ml of cycloheximide (Joshi et al.,
2009). Plates were maintained at 28 ± 2 °C for
14-21 days, in darkness. Developed colonies
showing distinct macro-morphological traits
were isolated separately on Yeast Malt Agar
(ISP-2), and stored in glycerol (20%) at -20 °C
for further use (Himaman et al., 2016).
2.4 Morphological characterization of the
actinomycete isolates
Obtained isolates were grown in ISP-2
medium at 28 ± 2° C for 10 days.
Actinobacterial colonies were characterized
based on the colony appearance, the type of
areal hyphae, and the growth of vegetative
hyphae. The color of diffusible pigment
production was visually estimated with the
help of RHS-color code (RHS color chart,
Fifth Edition-Royal Horticultural Society)
(Anusree & Bhai, 2017).
2.5 Screening for actinomycetes growth-
promoting ability
Actinomycete isolates were assessed for their
ability to promote the tomato seedlings
growth. Tomato cv. Rio Grande seeds were
Ayed et al., 2021
82
surface-sterilized by immersing in 5% of
sodium hypochlorite (NaOCl) for 3 min, then
rinsed 6 times with SDW and air-dried (Aydi
Ben Abdallah et al., 2018). Disinfected seeds
were soaked for 30 min into actinobacterial
cell suspensions (~108 CFU/ml), prepared in
ISP-2 liquid cultures incubated at 28 ± 2 °C for
48 h. They were sown in alveolated trays (4 cm
× 4 cm × 4 cm) filled with commercialized peat
(Klasmann-Deilmann, Bio-Substrate,
Germany), and watered regularly with tap
water. A volume of 10 ml of each
actinobacterial suspension was applied per
alveolus by substrate drench immediately, 15
days, and 30 days post-sowing. Controls were
soaked in SDW and watered later regularly
with tap water. Ten seedlings were used for
each treatment. The bioassay was carried out
under plastic tunnels at 26-32 °C with 12 h
(light) / 12 h (dark) photoperiod and 60% air
relative humidity. At 35 days post-sowing,
tomato seedlings were uprooted and washed
for removing adhering peat. Different growth
parameters were recorded (plant height, stem
diameter, aerial part, and root dry weights).
Data were analyzed according to a completely
randomized design. The whole experiment was
repeated twice.
2.6 Screening for actinomycetes stem rot
suppression ability
Tomato cv. Rio Grande seeds, disinfected as
detailed above, were soaked for 30 min into a
water cell suspension (108 CFU/ml) of
collected actinomycetes isolates and sown in
alveolated trays filled with commercialized
peat (Aydi Ben Abdallah et al., 2018). All
tested treatments were applied directly, 15
days and 30 days post-sowing (10 ml per
alveolus per individual treatment). Challenge
inoculation with S. rolfsii was performed as
substrate drench with 20 ml of a mixture of
mycelium and sclerotia (≈25-30 sclerotia per
100 ml) 21 days post-sowing (Daami-Remadi
et al., 2012). Uninoculated and inoculated
controls were watered with the same volume
of SDW and watered later regularly with tap
water. The bioassay was carried out using ten
seedlings per individual treatment and
maintained under the same greenhouse
conditions as indicated above. At 35 days post-
sowing, disease severity was recorded based
on a 1-5 scale (De Curtis et al., 2010), where 1
= no stem lesion, 2 = lesions girdled ≤ 25% of
the stem circumference, 3 = lesions girdled 26-
50% of the stem circumference, 4 = lesions
girdled > 51% of the stem circumference and
5 = stem completely girdled. Plant height, stem
diameter, root, and aerial part dry weights
were also noted. Data were analyzed according
to a completely randomized design. The whole
experiment was repeated twice.
2.7 Statistical analysis
The data were statistically analyzed by
analysis of variance (ANOVA) with the
statistical package SPSS software (Version 20)
and subjected to mean separation by Duncan
Multiple Range test (at P ≤ 0.05). Correlations
between disease severity and seedling growth
parameters were performed using bivariate
Pearson’s test at P ≤ 0.05.
3. Results
3.1 Isolation of compost-associated
actinomycetes and their morphological
characterization
A collection of 17 actinomycetes isolates,
exhibiting diversity in their macro-
morphological traits, was recovered from the 2
composts C3 and C4. The color of aerial
mycelium of actinomycetes isolates varied
from white to brownish-black, whereas the
Ayed et al., 2021
83
color of submerged mycelium varied from
white to black. Colonies were circular or irregular, raised or flat or punctiform, and
entire or undulate (Table 1).
Table 1: Cultural characteristics of actinomycetes isolates on ISP-2 medium after incubation at 28 ± 2°C for
10 days.
Isolates
Composts
Color of aerial mycelium
Color of submerged mycelium
A1-3
C3
White
White
A2-3
C3
Brownish-black
Brown
A3-3
C3
Brownish-black
Brown
A4-3
C3
Brownish-black
Brown
A5-3
C3
Greyed-brown
Brown
A 8-3
C3
Greyed-white
White
A9-3
C3
White
White
A10-3
C3
Greyed-black
Black
A1-4
C4
White
White
A2-4
C4
White
Greyed-yellow
A3-4
C4
Greyed-brown
Greyed-brown
A4-4
C4
Greyed-brown
Greyed-brown
A5-4
C4
White
Greyed -white
A6-4
C4
Greyed-green
Green
A7-4
C4
White
White
A8-4
C4
Greyed-white
White
A10-4
C4
Grey
Greyed-white
3.2 Disease suppression ability of compost-
associated actinomycetes
ANOVA analysis revealed that stem rot
severity, noted after 35 days post-sowing and
14 days post-inoculation, varied significantly
depending on the tested actinobacterial
treatments. As shown in Figure (1), a
significant decrease in disease severity by
23.33 to 70% over pathogen-inoculated and
untreated control was noted on tomato
seedlings infected with S. rolfsii and treated
with A2-3, A3-3, A4-3, A5-3, A8-3, A9-3, A1-
4, A2-4, A3-4, A4-4, A6-4, and A10-4
isolates. Tomato seedlings inoculated by the
pathogen and treated with these isolates
showed a significantly disease severity similar
to the pathogen-free and untreated ones. The
highest decreases in stem rot severity, of about
56.67 and 70%, were noted on tomato
seedlings treated with A9-3 and A4-4 isolates,
respectively. However, no disease-suppressive
effect, as compared to the inoculated and
untreated control, was noted on those treated
with A1-3, A10-3, A5-4, A7-4, and A8-4
isolates.
Figure 1: Effect of compost-associated actinomycetes isolates on stem rot severity noted after 35
days post-sowing on Sclerotium rolfsii-inoculated tomato cv. Rio Grande seedlings as compared to
the untreated controls. UC: Uninoculated and untreated control; IC: S. rolfsii-inoculated and
untreated control. Results are presented as means ± SE (n = 10, P ≤ 0.05). Bars sharing the same
letter are not significantly different according to Duncan's Multiple Range test (at P ≤ 0.05).
Ayed et al., 2021
84
3.3 Growth-promoting ability of compost-
associated actinomycetes on Sclerotium
rolfsii-inoculated tomato seedlings
Growth parameters of tomato seedlings, noted
after 35 days post-sowing and 14 days post-
inoculation with S. rolfsii, varied significantly
(at P ≤ 0.05) depending on tested biological
treatments. As illustrated in Table (2),
treatments with A2-3, A3-3, A4-3, A5-3, A8-
3, A9-3, A10-3, A1-4, A2-4, A3-4, A4-4, A5-
4-, and A6-4 isolates significantly improved
the seedling height by 21.1 to 66.6% over
pathogen-inoculated and untreated control,
where the highest increment was induced by
A4-4 treatment. Furthermore, a significant
enhancement in the stem diameter, by 37.4 to
62.2% over pathogen-inoculated control, was
recorded on tomato seedlings treated with A2-
3, A3-3, A2-4, A3-4, and A4-4 isolates. A3-3
and A4-4 based treatments exhibited the
highest growth-promoting effect based on the
aerial part dry weight which was significantly
improved by 95.6% and 66.2%, respectively,
over the pathogen-inoculated and untreated
control.
Table 2: Growth-promoting potential of compost-associated actinomycete isolates noted 35 days post-sowing
on tomato cv. Rio Grande seedlings inoculated with Sclerotium rolfsii as compared to the untreated controls.
Biological treatments
Plant height (mm)
Stem diameter (mm)
Aerial part dry weight (mg)
Root dry weight (mg)
UC
81.5 ± 1.35 b
2.28 ± 0.1 a
189.87 ± 7.10 bcdef
26 ± 1.13 cdefg
IC
59.12 ± 1.17 de
1.38 ± 0.11 ef
142.25 ± 13.91 fg
17.87 ± 1.25 fgh
A1-3
39.75 ± 5.28 f
0.44 ± 0.24 g
58.37 ± 23.03 h
11 ± 1.88 h
A2-3
73 ± 6.16 bc
2.24 ± 0.23 a
145.75 ± 8.81 efg
18.37 ± 0.71 fgh
A3-3
80.12 ± 6.96 b
2.03 ± 0.21 abc
278.25 ± 17.78 a
39.37 ± 5.08 ab
A4-3
82.25 ± 5.43 b
1.77 ± 0.16 bcde
214.62 ± 17.86 bc
36.37 ± 4.97 abc
A5-3
81.75 ± 2.39 b
1.57 ± 0.11 cde
176.75 ± 10.71 cdef
24.37 ± 0.98 defg
A8-3
73.12 ± 2.53 bc
1.78 ± 0.13 bcde
184.75 ± 17.60 cdef
30.62 ± 6.61 bcde
A9-3
72.37 ± 2.8 bc
1.65 ± 0.66 cde
146.5 ± 11.25 efg
19.75 ± 0.90 efgh
A10-3
73 ± 2.1 bc
1.76 ± 0.14 bcde
169.37 ± 15.82 cdefg
25 ± 1.80 defg
A1-4
76.37 ± 5.53 bc
1.65 ± 0.20 cde
202.5 ± 16.86 bcd
27.87 ± 3.60 cdef
A2-4
80.5 ± 3.9 b
2.12 ± 0.08 ab
197 ± 11.27 bcd
32.5 ± 3.02 bcd
A3-4
80.37 ± 4.23 b
2.02 ± 0.09 abc
208.12 ± 10.34 bc
24 ± 2.15 defg
A4-4
98.5 ± 3.57 a
1.9 ± 0.07 abcd
236.37 ± 18.32 ab
44.87 ± 6.16 a
A5-4
73.62 ± 3.8 bc
1.63 ± 0.08 cde
195 ± 20.54 bcde
36.25 ± 6.39 abc
A6-4
71.62 ± 1.77 bc
1.69 ± 0.09 bcde
184.37 ± 3.73 cdef
27.37 ± 2.07 cdef
A7-4
49.12 ± 0.55 ef
1.14 ± 0.08 f
140.37 ± 4.66 fg
22 ± 2.36 defgh
A8-4
64.62 ± 4.5 cd
1.53 ± 0.19 def
127.75 ± 25.73 g
16.37 ± 3.13 fgh
A10-4
50.87 ± 2.71 ef
0.34 ± 0.03 g
153.37 ± 5.55 defg
14.62 ± 1.89 gh
UC: Uninoculated and untreated control; IC: S. rolfsii-inoculated and untreated control. Results are presented as means ± SE (n =
10, P ≤ 0.05). For each column, values followed by the same letter are not significantly different according to Duncan's Multiple
Range test (at P ≤ 0.05).
Treatments with A4-3, A1-4, A2-4, A3-4, and
A5-4 isolates led to a significant increment in
this parameter by 37.1-50.9%. As for their
effects on the root dry weight, A3-3, A4-3, A4-
4, and A5-4 isolates had significantly
improved this parameter by 102.8-151.1%
over pathogen-inoculated and untreated
control. It should be also highlighted that A8-3
and A2-4 based treatments led to a 71.3% and
81.8% increase in the root dry weight over
control, respectively. However, A1-3, A7-4,
A8-4, and A10-4 isolates had no positive
effects based on all studied growth parameters.
3.4 Correlation between stem rot severity
and seedling growth parameters
Pearson’s correlation analysis revealed
that the lowest stem rot severity,
estimated based on a necrosis index, led
to significant increases in all seedling
growth parameters. Indeed, the seedling
height was negatively correlated to
necrosis index (r = -0.471; n=190;
P
=
Ayed et al., 2021
85
0.000). Moreover, the stem diameter was
negatively linked to stem rot severity (r =
-0.431; n=190;
P
= 0.000). Furthermore,
the aerial part dry weight (r = -0.392;
n=190;
P
= 0.000) and the root dry weight
(r = -0.268; n=190;
P
= 0.001) were
negatively correlated to the necrosis
index.
3.5 Growth-promoting ability of compost-
associated actinomycetes on pathogen-free
tomato seedlings
ANOVA analysis revealed that all growth
parameters (height, stem diameter, aerial
part dry weight, and root dry weight),
varied significantly (at
P
≤ 0.05)
depending on tested actinobacterial
treatments. As presented in Table 3, the
A4-4 isolate exhibited the highest growth-
promoting ability on tomato pathogen-
free seedlings where the height was
enhanced by 27% over the untreated
control. This parameter was also
significantly increased by 15.8 and 12.1%
over control following seedling treatment
with A3-3- and A3-4 isolates,
respectively. However, no positive effect
was noted based on the stem diameter.
Treatment of tomato seedlings with A3-3,
A4-3, A1-4, and A4-4 isolates resulted in
a significant increment of the aerial part
and root dry weights by 20.1-56.5% and
42.3-51%, respectively, over control. A3-
3 and A5-4 isolates exhibited the highest
growth-promoting potential based on the
aerial part dry and the root dry weight,
respectively (Table 3).
Table 3: Growth-promoting potential of compost-associated actinomycete isolates noted 35 days post-sowing
on pathogen-free tomato cv. Rio Grande seedlings as compared to the untreated control.
Biological treatments
Plant height (mm)
Stem diameter (mm)
Aerial part dry weight (mg)
Root dry weight (mg)
UC
81.5 ± 1.35 def
2.28 ± 0.10 ab
189.87 ± 7.10 de
26 ± 1.13 bcdef
A1-3
69.37 ± 2.54 g
1.86 ± 0.06 bc
143.5 ± 11.10 f
19.37 ± 0.82 def
A2-3
81.37 ± 3.74 def
1.81 ± 0.01 bc
142.75 ± 1.38 f
18 ± 0.27 ef
A3-3
94.37 ± 2.56 b
2.43 ± 0.16 a
297.12 ± 23.67 a
39 ± 5.63 a
A4-3
90.25 ± 4.66 bcd
2.29 ± 0.12 ab
256.75 ± 16.49 b
37 ± 5.73 a
A5-3
83.37 ± 2.09 cde
2.02 ± 0.16 abc
225.25 ± 28.20 bcd
36 ± 4.46 ab
A8-3
83.12 ± 3.33 cde
1.99 ± 0.14 abc
188.75 ± 3.25 de
29.5 ± 1.22 abcd
A9-3
81.75 ± 4.07 def
1.67 ± 0.08 c
134.62 ± 7.03 f
18.375 ± 1.03 def
A10-3
80.62 ± 1.64 ef
2.04 ± 0.06 abc
210.12 ± 7.72 cde
31.25 ± 2.62 abc
A1-4
87.12 ± 1.76 bcde
1.79 ± 0.1 bc
233.12 ± 10.99 bc
37.25 ± 4.25 a
A2-4
83.75 ± 2.25 cde
1.71 ± 0.06 c
184.5 ± 4.32 e
25.75 ± 1.37 bcdef
A3-4
91.37 ± 1.37 bc
2.30 ± 0.08 ab
186.12 ± 7.75 e
28.75 ± 3.62 abcde
A4-4
103.5 ± 9.67 a
1.99 ± 0.11 abc
228.12 ± 16.32 bc
39.25 ± 5.99 a
A5-4
83.5 ± 2.46 cde
1.87 ± 0.16 bc
219.5 ± 8.87 cde
39.25 ± 6.12 a
A6-4
84.87 ± 2.56 cde
1.98 ± 0.04 abc
199.5 ± 2.84 cde
33.5 ± 2.08 abc
A7-4
55.12 ± 1.53 h
1.75 ± 0.19 c
145.25 ± 4.63 f
24.12 ± 1.8 cdef
A8-4
69 ± 2.07 g
1.8 ± 0.03 bc
148.87 ± 3.39 f
24 ± 1.99 cdef
A10-4
73.5 ± 2.16 fg
1.93 ± 0.11 bc
141.37 ± 5.25 f
16.87 ± 2.02 f
UC: Uninoculated and untreated control. Results are presented as means ± SE (n = 10, P ≤ 0.05). For each column, values followed
by the same letter are not significantly different according to Duncan's Multiple Range test (at P ≤ 0.05).
4. Discussion
Several investigations have reported that
microorganisms isolated from composts could
enhance the growth of various crops and
controlling several phytopathogens (Coelho et
al., 2020; Joshi et al., 2009). In previous work,
we demonstrated the capacity of composts and
compost teas to suppress stem rot disease
caused by S. rolfsii and to promote tomato
growth (Ayed et al., 2018b; 2018c). Therefore,
this study was more focused on the isolation
and screening of actinomycetes naturally
associated with 2 composts as biocontrol and
Ayed et al., 2021
86
growth-promoting agents. A total of 17
actinomycetes isolates were recovered from 2
selected composts C3 and C4. When assessed
for their ability to suppress stem rot disease, 12
actinobacterial isolates (namely A2-3, A3-3,
A4-3, A5-3, A8-3, A9-3, A1-4, A2-4, A3-4,
A4-4, A6-4, and A10-4) had successfully
decreased disease severity on S. rolfsii-
inoculated tomato cv. Rio Grande seedlings.
The noted antifungal activity of actinomycetes
isolates confirmed the results obtained by
Anusree and Bhai (2017) who demonstrated
their ability to protect pepper by 98.10% from
Sclerotium foot rot disease. Also, Errakhi et al.
(2007) reported the antagonistic potential of
actinobacterial isolates against S. rolfsii
infecting sugar beet. Several other
investigations indicated that actinomycetes are
promising microbial agents for the biological
control of numerous fungal and bacterial plant
pathogens (Dandan et al., 2018; Mingma et al.,
2014; Golinska & Dahm, 2013; Kobayashi et
al., 2012; Patil et al., 2010). Previous studies
have also shown that actinomycetes can
produce plant growth promoters (Srivastava et
al., 2015; Gopalakrishnan et al., 2014). In the
current study, some compost-associated
actinomycetes were screened for their ability
to promote the growth of tomato seedlings.
Interestingly, our results demonstrated that
A3-3, A4-3, A2-4, A3-4, and A4-4 isolates
exhibited the highest growth-promoting
potentials on S. rolfsii-inoculated tomato
seedlings and that A3-3 and A4-4 were also the
most effective on pathogen-free seedlings.
Nevertheless, A1-3, A7-4, A8-4, and A10-4
isolates did not exhibit any positive effect on
seedling growth. Thus, actinomycetes
associated with composts can be explored as
potential agents for seedling growth-
stimulation and stem rot disease biocontrol.
These effects may be explained by the ability
of plant root exudates to stimulate the growth
of actinomycetes and the last ones use these
exudates for the synthesis of antimicrobial
substances active against plant pathogens
(Yuan & Crawford, 1995). Most
actinomycetes belong to the genus
Streptomyces and are well known as the main
sources of bioactive secondary metabolites
(Khanna et al., 2011; Goodfellow & Simpson
1987). Therefore, different aspects of these
microorganisms have been studied, such as the
production of metabolites that control plant
disease and/or improve plant growth. To
inhibit phytopathogens, the microbial
biocontrol agents use different mechanisms
such as the production of cell wall-degrading
enzymes, parasitism, antibiosis, and the
induction of host resistance (Palaniyandi et al.,
2013). In this regard, previous findings
showed the ability of actinomycetes to
synthesize hydrolytic enzymes including
chitinases, proteases, amylases, lipases,
cellulases, or β-1,3 glucanase (Anusree &
Bhai, 2017; Jayamurthy et al., 2014). In the
same sense, actinomycetes are described as
natural antibiotic producers and can inhibit
various soilborne pathogens such as Fusarium
oxysporum f. sp. cubense (Getha &
Vikineswary, 2002), Verticillium dahliae
(Aouar et al., 2012), and Rhizoctonia solani
(Patil et al., 2010). Regarding plant growth
promotion, the efficiency of various
microorganisms, including actinomycetes,
were widely reported (Himaman et al., 2016;
Gopalakrishnan et al., 2014; Salla et al., 2014;
Peralta et al., 2012). These agents may
promote their host growth either directly by
the synthesis of phytohormones (Goudjal et
al., 2013), nitrogen fixation (Bibha et al.,
2017), siderophores, phosphate and zinc
solubilization, indole acetic acid, extracellular
enzyme lipase (Anusree & Bhai, 2017); or
indirectly through the suppression of their
associated plant pathogens (Jose & Jha, 2016).
Ayed et al., 2021
87
5. Conclusion
Screened composts were found to be a
potentially important source for the isolation of
potent actinomycetes, acting both as biocontrol
agents and as biofertilizers. Nevertheless,
more investigations are needed to more
understand the mechanisms of action, to
identify secondary metabolites involved in the
growth promotion and the stem rot biocontrol,
and to identify the potent isolates; which can
be used for the development of eco-friendly
and economically feasible biofertilizers and
bio-fungicides.
Acknowledgments
This work was funded by the Ministry of
Higher Education and Scientific Research in
Tunisia through the budget assigned to
UR13AGR09-Integrated Horticultural
Production in the Tunisian Centre-East, The
Regional Research Centre on Horticulture and
Organic Agriculture of Chott-Meriem,
University of Sousse, Tunisia.
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