Agriculture Reference
In-Depth Information
13.4
Recent approaches for using phages in
plant pathology
Studies conducted since the early 1990s have helped to identify strategies for improving
phage effi cacy. Various approaches were attempted to improve the competitive advantage
of phage in the environment in order to improve effi cacy.
A major factor limiting the use of phages for control of plant diseases was the probabil-
ity of developing bacterial strains resistant to the phage. This risk was addressed in 1937
by Katznelson (1937) and later in two reviews on phages (Okabe & Goto, 1963; Vidaver,
1976). Jackson (1989) developed a strategy to prevent occurrence of phage-resistant
mutants. This involved preparing mixtures of host range mutant phages (h-mutants) that
lyse bacterial strains that are resistant to the parent phage (Adams, 1959), while main-
taining the ability to lyse the wild-type bacterium. Using this strategy, a mixture of four
phages including wild-type and h-mutant phages were applied twice weekly and provided
signifi cantly better disease control and produced greater yield of extra large fruits than the
standard copper-mancozeb (Flaherty et al., 2000).
An important and oftentimes neglected aspect of phage-based biological control
is pre-screening phages for their biocontrol value before application; that is, identify-
ing specifi c bacteriophages with particular characteristics that may prove effective in
control rather than arbitrarily selecting them based strictly on lytic activity for disease
control studies. Saccardi and co-workers (Zaccardelli et al., 1992; Saccardi et al., 1993)
selected from a collection of eight phages a lytic phage with the broadest host range to
use in studies. The proper assay for phage selection is critical. Although in vitro assays
are frequently used as a selection process for phages, these may not be good predic-
tors of biological control ability. Balogh (2006) found no correlation between in vitro
characteristics, such as antibacterial activity or phage multiplication rate, and disease
control effi cacy (unpublished results). On the other hand, he found that phages which
multiplied more effi ciently on their host in the phyllosphere, were also better in disease
control.
Timing of bacteriophage applications relative to the arrival of the pathogen infl uenced
effi cacy of disease control in several instances. Civerolo & Keil (1969) achieved a marked
reduction of peach bacterial spot only if phage treatment was applied one hour or one day
before inoculation with the pathogen. There was a slight disease reduction when phage
was applied one hour after inoculation and no effect if applied one day later. Civerolo
(1972) suggested that bacteria were inaccessible to phage in the intercellular spaces, or
there were not enough phages reaching the pathogen. Schnabel et al . (1999) achieved a
signifi cant reduction of fi re blight on apple blossoms when the phage mixture was applied
at the same time as the pathogen, Erwinia amylovora . In contrast, disease reduction was
not signifi cant when phages were applied a day before inoculation. Bergamin Filho &
Kimati (1981) investigated the effect of timing on the effi cacy of phage treatment in
greenhouse trials with two pathosystems: black rot of cabbage, caused by Xanthomonas
campestris pv. campestris and bacterial spot of pepper, caused by Xanthomonas camp-
estris pv. vesicatoria . Phage treatment was applied once varying from seven days before
to four days after pathogen inoculation. On cabbage, signifi cant disease reduction was
achieved if the phage treatment was applied three days before to one day after inocula-
tion, whereas on pepper from three days before to the day of inoculation. The greatest
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