Agriculture Reference
In-Depth Information
has also been shown to be effective against a number of other pathogens (D'Allbra
and Mutto, 1986).
The bacterium
Pseudomonas chlororaphis
has recently been approved as a seed
treatment in Denmark for control of bunt on wheat. Isolates of the same bacterium
have been tested (along with strains of
P. fluorescens
and
Pantoea
sp.) for the
control of
Fusarium
and
Microdochium
seedling blights in Sweden (Johansson
et al.,
2003). There has also been interest shown in the possible use of fungal BCAs
to replace chemical seed treatments. Knudsen
et al.
(1995), for example, found that
strains of
Gliocladium roseum
were antagonistic to
Fusarium culmorum
on wheat
seeds and significantly reduced seedling blight in the field. Using an isolate of
the same fungal antagonist to treat the seeds, Burgess and Keane (1997) were able
to reduce the soft rot of chickpea seedlings caused by seed-borne
B. cinerea.
Commenting on their findings, the latter authors suggested that genetic manipulation
might enhance the efficiency of
G. r o s e u m
as a biocontrol agent. If genetic
engineering does provide an effective way of improving the efficacy of BCAs it
could greatly assist the move away from chemical control agents in sustainable
production systems. It is doubtful, however, if such genetically modified control
agents would find acceptance in organic agriculture. Amongst many other BCAs not
yet exploited commercially,
Coniothyrium minitans
can reduce
Sclerotinia
spp. on a
number of crops, including sunflower (Huang, 1980).
Further investigation of the interactions between the epidemiology of the
pathogen and the antagonist offers a fruitful field for further study. Attempts to
control the diseases of arable crops using this approach have generally given
disappointing results. Considerable research effort has, for example, been directed at
the development of a BCA for the control of take-all (Hornby, 1998), fluorescent
pseudomonads having been the most favoured organisms. By and large, however,
these attempts have met with very limited success. Part of the problem is that, of
necessity, the BCA has been introduced into an extremely complex ecosystem and
has had difficulty in maintaining itself in an environment that includes many of its
own competitors and antagonists.
In protected crops, biological control is made easier by the increased control
which the grower can have over the growing medium. The use of composted bark,
for example, has given excellent control of carnation wilt and certain other diseases -
the bark itself containing a complex of microorganisms antagonistic to the pathogen.
In this case, success does not depend on the introduction of a single BCA but on the
provision of an environment in which naturally occurring BCAs can flourish. Such
an approach is obviously more difficult in field crops, but an example is provided by
the old advice that grass clippings should be dug into the soil to control common
scab of potatoes (Actinomycetes developing on the clippings being antagonistic to
S.
scabies).
Cropping sequences can sometimes be modified so as to favour the development
of naturally-occurring microbial antagonists. The use of grass leys to build up the
root-colonizing saprophyte
Phialophora radicicola
(which successfully competes
for infection sites with
G. graminis)
has been advocated for the control of take-
all (Deacon, 1973), and the development under conditions of continuous cereal
