Fungal Populations in Container Media Amended with Composted Hardwood Bark Suppressive and Conducive to Rhizoctonia Damping-Off

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Abstract
Sample fungal populations (200-300 isolates each) were isolated from container media suppressive and conducive to Rhizoctonia amended with hardwood [largely Quercus spp.] bark composts (CHB) produced in windrows (field compost) or in a reactor (bin compost). Sample populations from suppressive and conductive batches of container media were compared to determine the relationship between fungi and disease suppression. Although the total numbers of fungi and pH of the media containing CHB from the 2 compost sources differed, these differences could not account for variations in suppressiveness. Fungal populations isolated from both suppressive and conducive media were dominated by hyphomycetes, ascomycetes and zygomycetes with 34 taxa accounting for > 80% of over 2000 isolations. Quantitative differences in the relative abundance fo taxa differentiated suppressive and conducive batches. Principal components analysis demonstrated relationships among species and disease suppressiveness. High relative densities of Trichoderma hamatum characterized populations isolated from suppressive container media amended with field compost, whereas T. harzianum was the predominant species in suppressive media amended with bin composts. Conducive media, however, had high populations of Penicillium verrucosum var. cyclopium or varieites of Geomyces pannorum. In addition, Trichoderma spp. were the most abundant taxa in populations isolated from radish rhizospheres and inocula of Rhizoctonia planted and incubated in suppressive media. Associations between populations of Trichoderma spp. and suppression suggested that antagonism toward R. solani by these fungi may have accounted for the reduction in disease. Disease suppression was not associated with a single fungal taxon. Furthermore, quantitative differences in sample populations isolated from suppressive and conducive container media indicated that the lack of suppression in some media was due to factors that limited development of high populations of Trichoderma or interfered with the antagonistic activity of these fungi.