Soil microbial biomass influence on growth and biocontrol efficacy of Trichoderma harzianum

Hyphal growth and biocontrol efficacy of Trichoderma harzianum may depend on its interactions with biotic components of the soil environment. Effects of soil microbial biomass on growth and biocontrol efficacy of the green fluorescent protein transformant T. harzianum ThzID1-M3 were investigated using different levels of soil microbial biomass (153, 328, or 517 (mu)g biomass carbon/g of dry soil). Hyphal growth of T. harzianum was significantly inhibited in soil containing 328 or 517 (mu)g biomass carbon/g of dry soil compared with soil containing 153 (mu)g biomass carbon/g. However, when ThzID1-M3 was added to soil as an alginate pellet formulation, recoverable populations of ThzID1-M3 varied, with the highest populations in soil containing 517 (mu)g biomass carbon/g. When sclerotia of Sclerotinia sclerotiorum were added to soils (10 sclerotia per 150 g soil) with ThzID1-M3 (20 pellets per 150 g soil), colonization of sclerotia by ThzID1-M3 was significantly lower in the soil containing the highest level of biomass. Addition of alginate pellets of ThzID1-M3 to soils (10 pellets per 50 g) resulted in increased indigenous microbial populations (total fungi, bacteria, fluorescent Pseudomonas spp., and actinomycetes). Our results suggest that higher levels of microbial soil biomass result in increased interactions between introduced T. harzianum and soil microorganisms, and further that microbial competition in soil favors a shift from hyphal growth to sporulation in T. harzianum, potentially reducing its biocontrol efficacy.