Agriculture Reference
In-Depth Information
tecnobucazole, and the emergence percentage and early plant size from seeds
and onion sets decreases as the concentration applied in seed-coats and set dips
increases. The best compromise between these reductions in vigour and
control of white rot was to use 0.4 g of tecnobucazole per 10
5
seeds and to dip
onion sets in a 1.25 g/l solution for 20 min (Wood
et al
., 2002). Experiments
combining tecnobucazole seed treatment with the biological control agent
Trichoderma viride
have given enhanced control of white rot compared with
either treatment individually, showing that the two approaches to disease
control are not necessarily incompatible (Clarkson
et al
., 2006).
In the 1980s the dicarboximide fungicide iprodione was effective for the
control of white rot and was used in targeted application methods, including
seed-coating, stem base sprays, incorporation below the seed at sowing and
incorporation in compost for seedling production (Entwistle, 1990). After
several years, iprodione ceased to be effective for white rot control. The same
had occurred some years previously with dichloran. The cause of these failures
was the selection and build-up in soil of bacteria that rapidly degrade the
fungicides, a phenomenon termed 'enhanced degradation' (Entwistle, 1990).
Enhanced degradation has proved long-lasting and irreversible on a practical
field scale.
Biological control
The increasing concerns about chemicals in the environment, plus problems of
loss of fungicidal activity through the build-up of resistance and of enhanced
degradation in soil, have encouraged efforts to develop alternative, biologically
based systems to control root disease. There are many examples of bacteria and
fungi exerting control against rhizosphere pathogens through a number of
mechanisms, including competition and exclusion, antibiosis, parasitizing the
pathogen (hyperparasatism) and inducing host resistance (Whipps, 2001).
Quite a number of such fungal antagonists have been identified, including
strains of
Sporidesmium sclerotivorum
, which destroy white rot sclerotia, and
Trichoderma harzianum
, which attacks both the hyphae and sclerotia of white
rot. The latter fungus has been applied to a growing crop in trials in Egypt, and
has reduced disease (Entwistle, 1990). Cultures of
T. koningii
applied around
onion seeds at a rate equivalent to 1590 kg/ha reduced the proportion of
seedlings infected with white rot by about 70% (Metcalf
et al.
, 2004). Compost
inoculation with
T. viride
, or dipping transplant roots in a suspension of the
fungus, reduced pink root disease in leeks (Biesiada
et al.
, 2004). On the other
hand, applying a culture of
T. harzianum
below garlic cloves at planting or
dipping cloves in a suspension of the bacterial antagonist
Bacillus subtilis
had
only a slight effect on white rot disease (Melero-Vara
et al.
, 2000).
The general experience with microbial antagonists has been that the control
achieved is erratic and unpredictable under field conditions. Improvement in this
situation is only likely to come about through the better understanding of
biological processes in the rhizosphere. Successful biocontrol agents need not
