| Producing a successful
crop depends on a variety of factors — including the genetics
of the crop plant, the availability of nutrients and the impact of microorganisms
(fungi and bacteria) in the soil. Some microorganisms are beneficial
and promote plant growth while others cause diseases.
In a technique similar to Integrated Pest Management, above-ground populations
of disease-causing phytopathogens can be inhibited and controlled through
the use of other beneficial microorganisms. This process is referred
to as biocontrol.
Naturally occurring, plant growth promoting microorganisms provide a
range of functions within the soil, including supplying minerals and
other nutrients to the plant in usable forms, producing phytohormones
that can encourage plant growth and stimulation of the systemic resistance
mechanisms of the plant (the plant’s defence systems).
Some assist the plant indirectly by preventing pathogens from interfering
with plant growth and development. Beneficial organisms may do this
either by out-competing the pathogen for resources or root spaces, or
by releasing antibiotics or other compounds that inhibit the growth
of pathogenic strains.
As we have seen with cotton diseases such as Fusarium wilt and Verticillium
wilt, phytopathogens are capable of reducing crop yields. In the worst
cases the crop may not be viable. Fusarium wilt — one of the industry’s
‘worst-case’ fungal diseases — is now affecting most
cotton growing districts and has sent some growers out of cotton production.
The cotton industry has relied on plant breeding as the primary defence
against diseases. Chemicals such as fungicides are not effective against
all diseases, may not always be economical and are often hazardous and
persistent in the environment. Some of the soil-borne diseases are not
able to be controlled by chemical means. Fungal spores, for example,
are very resilient and well dispersed through the soil. Biological control
agents may be an environmentally friendly alternative.
BIOCONTROL COLLECTION
Nearly 10 years ago, NSW Agriculture researchers, Drs Stephen Allen
and Subbu Putcha, began collecting endophytic and rhizosphere (root
zone) microorganisms from cotton plants. This collection was expected
to contain potential biocontrol agents for controlling cotton diseases,
particularly fungal diseases.
Initial research identified bacteria able to suppress the growth of
a selection of diseases harmful to the cotton industry (Verticillium
dahliae, Theilaviopsis basicola, Rhizoctonia solani and Pythium ultimum).
About 29 per cent of the collection was able to suppress the growth
of one or more of the pathogens in the laboratory. Several hundred of
the isolates were tested in glasshouse experiments and promising strains
were further evaluated in the field.
Dr Putcha went on to develop a rapid, in-field screening method to identify
the most promising biocontrol agents. Of the 180 bacterial strains examined,
two strains of bacteria were identified as being potentially useful.
USEFUL BACTERIA
One was a Pseudomonas species and the other was of the Bacillus genus.
Representatives of these genera have been previously used in commercialised
biocontrol products. The strain of Bacillus isolated was shown to be
potentially effective for managing Fusarium wilt.
Subbu tested this strain over a number of years in field trials, mainly
on the Darling Downs. The trials included the use of a foliar spray,
2-hydroxy benzoic acid (HBA), which is an activator of the plant’s
defence system, and Subbu determined that the combination of soil-applied
bacteria and foliar acid spray was most effective in controlling Fusarium
wilt.
In the 2000–01 season, small and large scale trials were attempted
in Queensland. A commercially produced formulation of Subbu’s
Bacillus species biocontrol agent was used in large-scale trials.
Subbu finished working on biocontrol at the end of June 2001. NSW Agriculture,
lead by Dallas Gibb, in conjunction with Queensland Department of Primary
Industries (Joe Kochman and Anthony Mitchell), are continuing large-scale
trials this season. The trials cover about 120 hectares and are located
in fields infected with Fusarium on the Darling Downs.
Figures 1 to 3, from the 2000–01 season, show the performance
of the Bacillus strain as a biocontrol agent against Fusarium. Figure
1 shows the in-field performance of the NSW Agriculture’s Bacillus
strain, which was isolated by Subbu, compared to a commercial biocontrol
product named Kodiak.
Kodiak is a product registered for the control of the Fusarium fungus
in the US, and contains a Bacillus subtilis strain of bacteria to inhibit
the fungus. In this small-scale trial, the two strains of Bacillus,
applied along with an in-furrow injection of HBA and foliar applications
of HBA throughout the growing period, showed no significant difference
in performance and both showed results that were significantly better
than the control.
In the large-scale biocontrol trials during 2000–01, results were
inconsistent for a number of reasons, including the difficulty of finding
large-scale fields with a reasonably even distribution of Fusarium,
some problems with trial set up and low infection levels during the
past season.
There were no significant differences between control and biocontrol
treatments in much of the analysed data. But this may be due to the
lack of substantial levels of Fusarium wilt throughout last season.
Figure 2 is an example where there was a significant difference between
control and biocontrol treatments in plant count data at one trial site.
At this site, 40 per cent of plants were severely infected and there
was a significant increase in plant stands in the biocontrol treatments.
This season the large-scale field trials have been designed to obtain
statistically valid data and should provide better data on the effectiveness
of the biocontrol agents.
Figure 3 shows the performance of the foliar spray HBA against Fusarium
wilt in a small-scale trial in 2000–01. Last year’s result
suggests that the spray alone works well — but this contrasts
with previous years’ data. The relative efficacy of acid spray
alone, Bacillus strain alone and spray plus bacterial application are
being further assessed in large-scale trials this season.
IMPORTANT ROLE?
Biocontrol has a potentially important part to play in disease control,
particularly in the short to medium term until plant breeders are able
to produce resistant varieties. Further research into useful strains
of bacteria for biocontrol needs to continue.
It may be possible to genetically engineer strains to over emphasise
the anti-pathogenic traits or to transfer traits between strains. It
may even be possible to create a bacterial biocontrol agent capable
of defending cotton against the major fungal diseases affecting the
industry. It is also important to begin looking at the ecological impact
of introduced organisms, particularly as pure strains in concentrated
forms in the soil environment.
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