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Influence of Pseudomonas fluorescens on Hyphal Growth and Biocontrol Activity of Trichoderma harzianum in the Spermosphere and Rhizosphere of Pea. L. M. Dandurand,Plant Pathology Division, University of Idaho, Moscow 83843; G. R. Knudsen, Plant Pathology Division, University of Idaho, Moscow 83843. Phytopathology 83:265-270. Accepted for publication 12 November 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-265.

Trichoderma harzianum isolate ThzID1 was grown in liquid culture, was formulated with alginate and polyethylene glycol 8000, and was milled into fine granules (average diameter 500 μm). Granules contained chlamydospores, conidia, and hyphal fragments. Viability of the encapsulated fungus remained high for at least 6 mo when stored at 5 C (i.e., >90% of the granules produced hyphal growth when incubated on agar); viability was reduced significantly when granules were stored at 22 C. Application of the granular formulation of T. harzianum to pea seeds reduced root rot by Aphanomyces euteiches f. sp. pisi in growth-chamber experiments and also increased plant top weights compared to noncoated seeds. Seed treatment with slurries of Pseudomonas fluorescens strain 2-79RN10, which produces a phenazine antibiotic, also reduced Aphanomyces root rot but to a lesser extent than did T. harzianum ThzID1. Disease suppression was not significantly different when seeds were treated with a combination of T. harzianum and 2-79RN10 compared to treatment with T. harzianum alone. Root rot was not reduced by the mutant P. fluorescens strain 2-79-B46, which lacks phenazine. Treatment with T. harzianum plus 2-79-B46 resulted in the same level of disease control achieved by T. harzianum alone. These results suggest that the biocontrol mechanism of P. fluorescens 2-79RN10 neither inhibited nor enhanced the biocontrol activity of T. harzianum ThzID1. In other experiments, density of T. harzianum hyphae originating from coated pea seeds in soil was not affected by the addition of 2-79RN10, but when 2-79-B46 was added, density was greater after 5 days. The colony radius of T. harzianum was initially enhanced (at 3 days) by the addition of either strain, but the effect diminished by day 5. The same treatments were then applied to peas and to glass beads of equivalent size, and similar effects of the added bacterial strains were observed on both substrates, suggesting that the growth enhancement of T. harzianum in the presence of bacteria was not the direct result of stimulation of seed exudation by the bacteria. Our results provide a potentially improved formulation methodology for coating seeds with biocontrol organisms and methods for evaluating the compatibility of fungal and bacterial biocontrol agents applied to seeds.