Soil Suppressiveness to a Plant PathogenicPythiumspecies

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Abstract
Attempts to induce soil suppressiveness to a plant pathogenic Pythium sp. by adding mycoparasitic Trichoderma spp. (previously reported to induce suppressiveness to Rhizoctonia solani) were not successful. Weekly addition of dried ground bean leaf material to Nunn sandy loam soil (pH 7.3 at 26.degree. C) at weekly intervals over a 6-wk period decreased propagule population density of the plant pathogenic Pythium sp. Suppressiveness was not observed in otherwise similarly treated soil incubated at 19.degree. C or adjusted to pH 5.0. The decline in propagule population density of the plant pathogenic Pythium sp. was associated with an increase in population density of an unidentified mycoparasitic species of Pythium. The latter induced early lysis of germinating sporangia of the plant pathogenic Pythium sp. in vitro and on membrane filters laid on soil infected with the mycoparasitic isolates. Addition of the antagonist to soil suppressed the competitive saprophytic ability of the plant pathogenic Pythium sp. and also its capacity to induce preemergence damping-off of cucumber. Therefore, it induced both pathogen and disease suppressiveness. Since the antagonist was not pathogenic to plants commonly used in agriculture, it has potential for use as a biological control agent against diseases induced by plant pathogenic species of Pythium.