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Influence of an Introduced Composite of Microorganisms on Infection of Tobacco by Phytophthora parasitica var. nicotianae. J. T. English, Former graduate research assistant, Plant Pathology Department, University of Florida, Gainesville 32611, Current address: Department of Plant Pathology, University of California, Davis 95616; D. J. Mitchell, professor, Plant Pathology Department, University of Florida, Gainesville 32611. Phytopathology 78:1484-1490. Accepted for publication 13 July 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1484.

A composite of organisms that colonized tobacco roots rapidly in field soil was evaluated for its ability to compete with Phytophthora parasitica var. nicotianae for occupation of sites susceptible to infection within root systems of tobacco. The composite comprised organisms derived from the microbial community within the rhizosphere of tobacco. Numbers of root infections by the pathogen after 2 wk of tobacco growth in infested soil were not reduced within soils amended with the composite composed of propagules of Trichoderma harzianum, Aspergillus carbonarius, Aspergillus terreus, Penicillium steckii, and Pseudomonas putida. As determined from plate counts, amendment of soils with the composite was associated with increased densities of fungi and fluorescent Pseudomonas spp. around tobacco roots as compared to densities around roots in nonamended soils. However, no differences in density or patchiness (assessed by Lloyd’s indices) of hyphae on root surfaces were observed. Survival of the pathogen in nonrhizosphere soil was not influenced by the introduced organisms. Amendment of pathogen-infested soils with the composite was associated with significant decreases in mortalities of tobacco after 90 days of plant growth in a glasshouse.