The Effect of Rhizosphere Competence on Colonization of Sweet Corn Roots by Biocontrol Fungi in Differing Soils

Abstract

To be effective, biocontrol agents, fungi need to colonize roots under a wide range of conditions. The ability do so is called rhizosphere competence. A common beneficial fungus, Trichoderma harzianum, has been bred to produce a new strain, T-22, that has exceptionally high rhizosphere competence. In field experiments, we have demonstrated that T-22 was resistant to edaphic conditions that reduce colonization by indigenous Trichoderma species, so that it can provide protection against root pathogens. 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 103 in cold soils, peaked at 104 at 15 °C, declining in later sowings due to higher biological competition. Differences in microbial competition had little effect. Roots were equally colonized in the differing soil microflora in three management systems 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.