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RUHUNA JOURNAL OF SCIENCE
Vol. 2, September 2007, pp. 82-88
http://www.ruh.ac.lk/rjs/rjs.html
ISSN 1800-279X

Abstract. Root rot, caused by Fusarium solani f. sp. phaseoli, is one of the main root
diseases impacting production of common bean in Sri Lanka. Rhizobacteria were screened
in dual Petri plate assays to select antagonistic strains against F. solani f. sp. phaseoli. B.
subtilis CA32 effectively antagonized the pathogen. T. harzianum RU01 also showed the
antagonistic activity. The efficacy of the B. subtilis CA32 and the T. harzianum RU01 were
tested in greenhouse pot experiments against F. solani f. sp. phaseoli. Seed bacterization
with B. subtilis CA32 and T. harzianum RU01 significantly protected bean seedlings from
F. solani f. sp. phaseoli compared to the untreated control plants. Plant protection was more
pronounced in T. harzianum RU01 treated plants than bacterized plants. Enhanced root
growth was observed only T. harzianum RU01 treated plants, suggesting that the biotic
modifications of the mycorrhizosphere as a result of colonization with T. harzianum RU01.

Keywords: Rhizobacteria, Biological control, bean, antagonism

1 Introduction
Root rot of common bean (Phaseolus vulgaris L.) is a soil-borne disease that is incited by
several fungal pathogens including Fusarium spp., Pythium spp. and, Rhizoctonia solani. It
occurs in all bean-growing areas of the world. Root rot caused by F. solani f. sp. phaseoli is
a major concern in many bean growing areas in Sri Lanka leading to enormous crop losses.
The pathogen is known to be very persistent in soil and capable of surviving in infested
fields for very long period and is difficult to control. Fusarium root rot is characterized by
reddish-brown lesions along the tap roots and lower hypocotyls. Diseased areas of the plant
enlarge with age and gradually turn brown. Longitudinal cracks may develop in older
lesions and the cortical tissues be discolored and decayed. Root rots are particularly severe
under water-stress condition (Burke and Hall, 1991).

Disease management options include crop rotation, improving soil fertility levels, use of
resistant cultivars, use of fungicides and biological control. The impetus for developing
biological control agents has been the public perception of pesticide toxicity in the
environment. Residue-free produce has become a valuable commodity. Biological control is

Biological control of Fusarium solani f. sp.
phaseoli the causal agent of root rot of bean using

Bacillus subtilis CA32 and Trichoderma harzianum
RU01

Saman Abeysinghe

Department of Botany, Faculty of Science, University of Ruhuna, Matara, Sri Lanka
Correspondence: saman@bot.ruh.ac.lk

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compatible with pesticide-free agriculture and the environment. Roots of plants support the
growth of a complex of microorganisms that can have a profound effect on the growth and
survival of the plant. Among these organisms, arbuscular mycorrhizal fungi (AMF) are
known to improve the nutritional status of their host, provide alterations in the host’s
physiology and exudation from roots (Filton et al., 2006). There is accumulating evidence
that AMF can reduce disease incidence and propagule number of several soil-borne
pathogens including Fusarium (Filton et al., 2006). Apart from the AMF, other microbial
biological control agents such as rhizospheric bacteria (Handelsman & Stabb, 1996) and
certain saprophytic fungi (Steyaert et al., 2003) have been identified as promising
candidates to combat with various deleterious soil-borne pathogens on different crops.
Certain biological control agents protect plants under field condition and commercial
formulations of these organisms are now available (Spadaro & Gullino, 2005). However,
control of F. solani f. sp. phaseoli by using bacterial biological control agents and
Trichoderma spp. do not exist. At present, there is very limited knowledge and experience
regarding the biological control of soil-borne diseases in Sri Lanka. Therefore, the main
objective of the current study was to screen rhizospheric bacteria and test Trichoderma
isolate RU01 for in vitro antagonism against F. solani f. sp. phaseoli and evaluate their
efficacy under greenhouse conditions in order to select potential biological control agents
which could be used in the field.

2 Materials & Methods

Biological control agents used in this study
Bacterial cultures

Bacteria used in this study were isolated from Capsicum annuum L. rhizosphere as
described in Abeysinghe, (2007). Bacterial isolates were maintained in 80% glycerol (v/v) at
-80 0C. In order to culture these bacteria, a loopful of inoculum was streaked on CAA plates
(Bactocassamino acid 5g (Difco, Detroit, MI), K2HPO4 0.18 g, MgSO4.7H2O 0.25 g, 18 g of
bactoagar, water 1 l). After incubation for 24 h at room temperature, single colonies were
streaked on fresh CAA plates.

Trichoderma harzianum RU01

Trichoderma spp. was isolated from soil samples obtained from a commercial chili field at
Angunukolapalasse by using dilution Petri plate technique on Trichoderma Selective
Medium (Askew and Laing, 1993) and maintained on PDA. Identification was performed
by using colony characteristics described in Gams & Bissett (1998). T. harzianum RU01
was selected for further studies because of its antagonistic activity against different fungal
pathogens.

Screening of antagonistic bacteria against F. solani in Petri plate assay

All bacterial isolates were initially screened for the ability to inhibit fungal growth on CAA
plates. Single colonies were selected and patched along the perimeters of plates and
incubated overnight at 28 oC. The following day each plate was inoculated at the centre with

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a 5 mm diameter plug of the growing fungus. The plates were incubated at 28oC for 2 -3
days and observed for inhibition of fungal growth. The bacteria positive for antagonism
were selected. Zones of inhibition indicated antifungal activity and the strains were ranked
according to the inhibition zone. An inhibition zone was defined as the distance between the
leading edge of the fungal growth and the closest edge of the bacteria. Inhibition was
expressed relative to a control strain spotted on the same plate.

Interaction between T. harzianum and F. solani in Petri plate assay

Five millimeter-diameter discs of T. harzianum isolate were removed from the edge of
colonies of 4-day old PDA cultures and placed on one side of a Petri dishes containing PDA
medium. Similar dishes of F. solani isolate grown in the same manner were placed on the
opposite side of Petri plates and made three replicates. Cultures were observed daily and
recorded for antagonism or parasitism of Trichoderma isolate against F. solani.

Isolation of pathogen

Phaseolus vulgaris plants showing root rot were collected from commercial bean fields at
Balangoda, in the Sabaragamuwa Province of Sri Lanka. Infected parts of the plants were
excised with a sterile scalpel and, surface sterilized with 3% (w/w) NaOCl for 2 min.
Sterilized pieces were washed twice with sterile water for 60s and, cut into small pieces (1
cm length) and transferred on to antibiotic amended PDA plates. Plates were incubated at
room temperature for 48 h, and white mycelium growth from the infected stem pieces were
transferred to new PDA plates. After incubation for 5 days, a single spore was isolated and
cultured on new PDA plates. The pathogen was identified as Fusarium solani f. sp.
phaseoli, based on the characteristics described by Booth (1977). Koch’s postulates were
demonstrated for the pathogen and confirmed as the causal agent of root rot of P. vulgaris.

Bacterial innoculum preparation for seed bacterization

Among the bacterial isolates the most antagonistic bacterial isolate, according to the in vitro
Petri plate assay, Bacillus subtilis CA32 was selected for greenhouse pot experiments. Fresh
cells were obtained from stock cultures stored at – 80 oC and grown in CAA broth overnight
at room temperature in a shaker. From this 100 mL of CAA broth in a 250 mL flask was
inoculated and incubated for 48 h at room temperature in a rotary shaker (100 round/min).
The bacterial culture was centrifuged (6000 g for 10 min at 4 oC) and the supernatant was
discarded. The cell pellet was resuspended in sterile 0.85% NaCl and centrifuged again
under the same conditions. The supernatant was discarded and washed bacterial cells were
resuspended in sterile distilled water (SDW). The concentration of cells in the suspension
was spectrophotometricaly adjusted to 108 CFU/mL and used for seed bacterization.

Inoculum production of Trichoderma harzianum

For seed inoculation, Trichoderma was grown on PDA in Petri plates for seven days at
room temperature under dark, to allow profuse sporulation. Sterile distilled water was added
to each plate and a conidial suspension was obtained by scraping the colony surface with a

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sterile spatula and filter through cheese cloth. Conidial suspension was adjusted to 106
spores/mL and mixed (0.01%) Tween-20 as a wetting agent.

F. solani inoculum and experimental set-up

The pathogen inoculum was produced on potato dextrose agar (PDA). The plates were
inoculated with an agar plug (5 mm in diameter) containing actively growing F. solani
mycelium, and incubated under fluorescence for 10 days at room temperature. Spores were
washed from the plates with sterile distilled water and the concentration was adjusted to 106
conidia/ml with a haemocytometer. Ten days after planting when the primary leaves were
fully expanded, the best seedlings were selected by thinning to three plants per pot. Five
milliliters of spore suspension was applied by pipette just below the collar region around the
hypocotyls of each plant. Disease severity was assessed 21 days after inoculation. Seedlings
were removed from the pots and excess soil clumps were removed by gently shaking and by
dipping the roots in water. Roots were dried with a paper towel and rated immediately for
symptoms of root rot. The severity of root rot was visually scored by assessing necrotic
lesions on the roots and hypocotyls using a rating scale of 0-5 described according to Filion
et al., (2003). Treatments consisting of 3 seedlings/pot and 5 replicates were included per
treatment. The trial was conducted twice and the experiment was arranged in a completely
randomized block design.

3 Results
Pathogen identification and characteristics

Colonies grown on PDA or CAA became brown 7 day after incubation, and produced
macro and micro conidia. The conidia were approximately 0.4 mm diameter. The
pathogenicity tests for the F. solani isolate was examined with different local bean cultivars.
Based on the Koch’s postulates Fusarium isolate was pathogenic to all tested local bean
cultivars to different levels (data not shown).

Identification and characterization of Trichoderma spp.

Trichoderma spp. grew rapidly at room temperature on PDA. Cultures at first were white
and cottony then turned to bright green, finally they became dark green. Chlamydospores
were intercalary and/or terminal, globose and smooth walled. Single phialides arose laterally
on the conidiophores in clusters. Therefore Trichoderma isolate was identified as T.
harzianum and named as RU01.

In vitro antagonism tests

T. harzianum RU01 against F. solani

Antagonistic properties of T. harzianum RU01 was tested using dual Petri plate method.
Trichoderma inhibited the mycelial growth of F. solani but could not overgrow the
pathogen until 3 to 4 days. However, several days later F. solani over grew the Trichoderma
mycelia. Furthermore, conidia production decreased compared to the control plates (data not

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shown). T. harzianum hyphae coiled around hyphae of F. solani causing vacuolization and
disintegration of F. solani hyphae indicating strong antagonistic activity of the T. harzianum
isolate RU01.

Screening of rhizobacteria against F. solani

From the preliminary round of screening for antagonism in Petri plate assay, only four
bacterial isolates were selected for further study. Among these isolates, B. subtilis CA32
was selected for greenhouse pot experiments because CA32 was the most antagonistic
isolate in terms of inhibition zones in the plate assay (Table I).

Table 1: Mycelial inhibition of F. solani f. sp. phaseoli in CAA by bacterial isolates

Isolate Mycelial inhibition mean (%)x

Bacillus subtilis CA32 55.50 a

Pseudomonas fluorescens CA05 32.00 b

Pseudomonas putida CA28 30.32 b

Bacillus subtilis CA16 28.70 b

Control 0 c
X The percent mycelial inhibition data were analyzed using analysis of variance (ANOVA) and a mean
separation was done by Fisher’s least significant difference at P < 0.05. Three replication plates were
included per bacterial isolates and the data were pooled from two separate experiments. Mycelial
inhibition means with the same letter are not significantly different from each other (95% confident
limit).

Plant protection ability of biological control agents under greenhouse conditions

All bean seedlings grown in soil inoculated with a conidial suspension of F. solani f. sp.
phaseoli showed red lesions on hypocotyls and tap roots characteristically distinctive of
Fusarium root rot. In contrast, all plants from seeds bacterized with CA32 or treated with T.
harzianum RU01 conidia showed significantly less number of lesions and low disease
severity. The noninfested controls showed no symptoms. However, T. harzianum treated
plants were well protected from the pathogen infection than CA32 treated plants (Table 2).

Table 2. Effect of application of B. subtilis CA32 and T. harzianum to the seeds of P.
vulgaris on infection of F. solani f. sp. phaseoli in pot experiments

Disease severity x Treatment

Trial I y Trial II Trial III

Healthy control 0 a 0 a 0 a

Disease control 4.2 b 4.5 b 3.8 b

B. Subtilis CA32 2.4 c 2.2 c 2.6 c

T. harzianum 1.6 c 1.8 c 2.0 c
X Disease severity was assessed using a rating scale of 0-5, where 0 = no disease symptoms, 1 =
slightly brown <50% surface discoloration of the hypocotyl, 2 = >50% surface discoloration, 3 =

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discolored hypocotyls and extensive root pruning, 4 = darkly discolored hypocotyls and root
completely collapsed and severe root pruning, and 5 = dead or dying plant.
Y Values followed by a different letter within columns are significantly different according to the LSD
test at P<0.05 using Duncan’s multiple range test.

The length and fresh weight of root was significantly increased (P < 0.05) in plants
colonized by T. harizanum than CA32 treated plants as compared with nontreated control
seedlings (Table 3).

Table 3. Effect of biological control agents on P. vulgaris root development

Growth parameters Treatment

Root length (cm) Root fresh weight (g)

Healthy control 5.2±1.25 ax 9.25±1.25 a

Disease control 3.2±1.87 c 5.28±0.98 c

B. subtilis CA32 5.6±1.92 a 10.25±1.75 a

T. harzianum 9.8±2.12 b 15.35±2.02 b
X values are the means of three replicates ± standard error of the mean. Values followed by a
different letter within columns are significantly different using Duncan’s multiple range test
(P<0.05).

4 Discussion
Although the interactions between many bacteria and fungi have been studied those
involving Fusarium solani f. sp. phaseoli have received less attention. This study presents
the data of screening of bacteria isolated from C. annuum rhizosphere and Trichoderma
harzianum against Fusarium solani f. sp. phaseoli, one of the major causal agents of root rot
of bean in Sri Lanka. Among 53 bacterial isolates only 4 isolates were able to antagonize F.
solani in dual Petri plate assay. B. subtilis CA32 was the strongest antagonistic isolate
(Table 1). More over, Trichoderma harzianum RU01 also inhibits the growth of F. solani
and reduces the production of conidia indicating antagonistic properties of T. harzianum
RU01 against the pathogen. However, spore production inhibition did not quantify. The in
vitro culture of F. solani and T. harzianum RU01 in culture media led to a variety of
interactions. F. solani growth was generally inhibited; the host cell contents disorganized
and the hyphae were intensively parasitized by T. harzianum RU01. Similar reactions have
been reported on other fungal pathogens (Hanson & Howell, 2004) but no reports on F.
solani.

The results reported here suggest that the presence of either B. subtilis CA32 or T.
harzianum RU01 in the rhizosphere significantly reduces the root rot caused by F. solani in
bean plants. The reduction might be related to the decline of the population density of F.
solani in soil and also due to alterations caused by T. harzianum RU01 in the F. solani
hyphae as observed in vitro. However, the protection exerted by the T. harzianum RU01
against F. solani was pronounced than B. subtilis CA32. This difference may be due to the
more than one mode of mechanisms exerted by the T. harzianum RU01 which may have an

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additive effect in plant protection. Most likely the enhanced growth of root system by T.
harzianum as evidenced by increased biomass may be positively acted in this respect. More
over T. harzianum is a well-known producer of cell wall-degrading enzymes and the
antibiotics thus could act synergistically with other mechanisms (Vinale et al., 2006).

In conclusion, the present study clearly demonstrated that antagonistic bacterial strain B.
subtilis CA32 and T. harzianum RU01 can be used as biological control agents in order to
protect bean plants from F. solani f. sp. phaseoli under greenhouse conditions. The combine
use of these biocontrol agents and the evaluation of the biological control efficacy under
field conditions are underway.

References
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