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only to be antagonistic to pathogens
in vitro
but to be effective also in the
rhizosphere environment (Whipps, 2001).
At the start of a programme to identify biocontrol agents that destroy
white rot sclerotia and to investigate factors affecting their activity, Clarkson
et
al.
(2002) identified 65 fungal isolates that could destroy white rot sclerotia on
agar. Fifteen of these degraded sclerotia in soil and 16 controlled white rot
disease in seedling assays. From these tests two isolates of
T. viride
were selected
as most promising. In field trials on heavily infected soil, infected bulbs were
reduced from 75 to about 50% by the application of these - cultured in bran
and suspended in a gel - below seeds at sowing and to the base of plants during
growth. Further studies on these antagonists established how soil
temperatures, water potential (see Fig 5.18) and soil type influenced sclerotial
destruction. Sclerotia were destroyed by the antagonists in all soil types and
they were clearly active in the range 15-18°C, temperatures optimal for white
rot infection of onions. However, the degree of white rot control in seedling
bioassays using the selected strains still varied with soil type and against
different isolates of the pathogen (Clarkson
et al.
, 2004).
Improved disease control amounting to its virtual elimination was achieved
in some glasshouse trials by combining
T. viride
inoculation with growing
seedlings in a combination of 50:50 soil:composted onion waste, or by com-
Fig. 5.18.
The effect of soil water potential on the degradation of white rot
(
Sclerotium cepivorum
) sclerotia by a strain of the antagonistic fungus
Trichoderma
viride
selected for its virulence. The bars around the points show standard errors,
and the line was fitted by regression (from Clarkson
et al
., 2004, Fig. 1a. Courtesy
of
Plant Pathology
).
