Plant Physiology (1999)
Selection of fungi for high rhizosphere colonization in vitro, and subsequent
ability to colonize roots in the field.
Thomas Björkman, Lisa M. Blanchard and Gary E. Harman
Department of Horticultural Sciences, New York State Agricultural Experiment
Station, Cornell University, Geneva, NY 14456-0462
ABSTRACT To be effective biocontrol agents on roots, fungi need
to colonize quickly and abundantly under a wide range of conditions. The
ability do so is called rhizosphere competence. A strain of the ubiquitous
rhizosphere-inhabiting fungus, Trichoderma harzianum, was selected
in vitro for high populations when grown on pea roots. In agricultural
fields, T-22 colonized maize roots at population densities much higher
than did native strains. It was also resistant to edaphic conditions that
reduce colonization by indigenous Trichoderma species. Well-drained
sand, stone or gravel soils supported lower populations of wild Trichoderma
than did loams (100.5 vs 10 3.3 cfu/g), but populations
of T-22 were high in all soils (>104 cfu/g). In a multivariate
analysis of soil characteristics affecting colonization, only soils with
low Ca and low pH had reduced populations. No other measured soil characteristics
were correlated with colonization. When sown in the field at different
initial soil temperatures ranging from 10 to 27°C, T-22 populations
were unaffected by temperature, having a population from 104.6
to 105.4 cfu/g. Indigenous strains were present at 103
cfu/g in cold soils, peaked at 104 at 15°C, and declined
in later sowings due to higher biological competition. Differences in microbial
competition had little effect. Roots were equally colonized in the differing
soil microflora established over 18 years at the Rodale Farming Systems
Trial. Invading an existing soil microbial community is the most difficult
thing to achieve with a biocontrol organism. These data show that T-22
is the first to consistently do so.
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