A Forest Soil Suppressive to Phytophthora cinnamomi and Conducive to Phytophthora cryptogea. II. Suppression of Sporulation.

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Abstract
Extracts of soil from a wet sclerophyll eucalypt forest suppressed the formation of Phytophthora cinnamomi sporangia but facilitated production of P. cryptogea sporangia and zoospores. This suppressive action was shown to be associated in part with the microbial components of the extract and in part with the soluble components. The microbial population of the suppressive soil contained significantly greater numbers of actinomycetes than the conducive soil tested at the same time. All Streptomyes isolates from the suppressive soil were shown to be antagonistic to certain of the aerobic bacteria from the same soil and to directly inhibit mycelial growth of P. cinnamomi. The soluble component of the soil extract inhibited growth of P. cinnamomi to a greater extent than P. Cryptogea. Interactions between soil microorganisms and inhibitory compounds in the soil are considered to be the cause of the suppressive action of the soil against P. cinnamomi whilst facilitating sporulation of P. Cryptogea.