Agriculture Reference
In-Depth Information
3.2
Modes of action
Perhaps the greatest recent advances in biological disease control have been concerned
with understanding modes of action. This refl ects the huge developments in molecu-
lar biology of bacteria, fungi and plants providing the tools to dissect the many types
of interactions that can occur, particularly through the use of mutants and genetically
marked strains of microorganism as well as gene expression studies. General modes of
action include competition, antibiosis, parasitism, induced resistance and plant-growth
promotion along with highly specialised mechanisms such as that associated with hypo-
virulence. Commonly, biological disease control by a single BCA can involve a num-
ber of modes of action and no one mode of action is necessarily mutually exclusive to
another. This holds true for BCAs active in the phyllosphere, spermosphere, rhizosphere
and post-harvest environments.
3.2.1
Competition for space and nutrients
One of the classic demonstrations of competition for space, infection sites or nutrients as
a mode of action concerns the control of fi reblight caused by the pathogenic bacterium,
Erwinia amylovora
by the non-pathogenic bacterium
Pseudomonas fl uorescens
A506
(Lindow & Leveau, 2002). By spraying fl owers of apple and pear with
P. fl uorescens
A506
just as they open, the BCA colonises the fl owers, utilises the available nutrients, and
prevents multiplication of small numbers of
E. amylovora
that might then
encounter the
fl owers, thereby preventing infection by pre-emptive exclusion. Pre-emptive colonisation
of necrotic leaf tissues by the fungus
Ulocladium atrum
to control the fungal pathogen,
Botrytis cinerea,
is another case of this type of mode of action which may involve both
competition for infection sites and nutrients, resulting in reduced pathogen sporulation
(Kessel
et al
., 2005). Competition for infection sites and/or nutrients, in or on roots, has
also been recorded. For example, between non-pathogenic and pathogenic strains of
Fusarium oxysporum
on tomato (Olivain & Alabouvette, 1999; Bolwerk
et al
., 2005;
Olivain
et al
., 2006) and between non-pathogenic strains of
Rhizoctonia
and the pathogen
Rhizoctonia solani
on several plant species (Herr, 1995).
Pseudomonas
spp. have also
been shown to act in part by competition for space and nutrients during the colonization
of tomato roots by
Fusarium oxysporum
f. sp.
radicis-lycopersici
(Bolwerk
et al
., 2003)
and may involve secretion of a site-specifi c recombinase (Dekkers
et al
., 2000). There is
also evidence that ectomycorrhizal fungi, by way of their physical sheathing of the root,
may also occupy pathogen infection sites, thereby preventing infection (Whipps, 2004).
Competition for nutrients alone, ranging from simple carbon- and nitrogen-containing
compounds to complex plant residues, in a range of situations is also a commonly reported
mode of action for both bacterial and fungal BCAs. For example, the yeasts
Cryptococcus
laurentii
BSR-Y22 and
Sporobolomyces roseus
FS43-238 controlled
Botrytis cinerea
in apple wounds by competing for fructose, glucose and sucrose (Filonow, 1998) and
Candida guillermondii
competed for nitrates during control of
Penicillium expansum
on
apple (Scherm
et al
., 2003). Competition in soil for carbon in the form of glucose has
also been shown to be involved in the suppression of pathogenic
F. oxysporum
by non-
pathogenic strains of the same species (Larkin & Fravel, 1999). A related mode of action
concerns the ability of some bacteria and fungi to metabolise organic compounds released
