Agriculture Reference
In-Depth Information
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Days after application
Figure 13.4
The presence of the bacterial host increases phage persistence. Phage suspension was
applied to greenhouse tomatoes, which had been pre-inoculated with phage-sensitive bacterium (solid
circles) or were uninoculated (open circles).
This approach was investigated for treating both soil-borne and aerial diseases (Tanaka
et al.,
1990; Svircev
et al.,
2006). Tanaka
et al.
(1990) used an avirulent strain of
Ralstonia
solanacearum
and its phage that was active against both the virulent and avirulent strains
to reduce tobacco bacterial wilt incited by
R. solanacearum
. While the application of the
avirulent strain alone caused a signifi cant 59% reduction in the number of wilted plants,
the co-application of phage with the avirulent strain increased control signifi cantly, to
82%. A similar strategy was employed by Svircev
et al.
(2006) for controlling fi re blight
of pear. They selected phages based on the ability to lyse both the target organism, the
pathogen
Erwinia amylovora
, and also a closely related antagonistic phyllosphere bac-
terium,
Pantoea agglomerans
. When applied together with the phage,
P. agglomerans
served as a biological control agent, as well as a phage carrier, a vehicle of delivery and
medium of propagation on the leaf surface. While
P. agglomerans
alone signifi cantly
reduced disease, its combination with phage resulted in signifi cantly better disease con-
trol, which was comparable to streptomycin treatment.
13.5
Challenges in using phages for disease control
Biocontrol success with bacteriophages is to a large extent dependent on the popula-
tion dynamics of both the biological control agent and target bacterium (Johnson, 1994).
It is essential that the phage attaches to its host before being destroyed by the various
physical factors (Goodridge, 2004). The likelihood for phage-bacterium interactions
depends on several key factors: phage concentration at the site of interaction (i.e., phyllo-
sphere or rhizosphere), rates of virion degradation (phage vary in degradative properties),
timing of application to optimize effi cacy, phage infection and replication ability in the
target environment, concentration and accessibility of target bacteria, and the necessary
presence of adequate moisture as a medium for phage diffusion (Gill & Abedon, 2003).
Additionally, disease control effi cacy may be infl uenced by the relative fi tness of phage-
resistant bacterial mutants (Gill & Abedon, 2003) and the surrounding environment.
Phages have the potential for controlling plant pathogens in the rhizosphere or
phyllosphere. However, Gill & Abedon (2003) identifi ed factors that can hinder success
of disease control in the rhizosphere. The relatively low diffusion rate of phages through
