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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Tracking code 0087 2-24-99