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Identification of Fungal rDNA Associated with Soil Suppressiveness Against Heterodera schachtii Using Oligonucleotide Fingerprinting

To understand the nature of a soil with suppressiveness against Heterodera schachtii, an rDNA analysis was used to identify fungi associated with H. schachtii cysts obtained from soils possessing various levels of suppressiveness. Because H. schachtii cysts isolated from these suppressive soils can transfer this beneficial property to nonsuppressive soils, analysis of the microorganisms associated with the cysts should lead to the identification of the causal organisms. Five soil treatments, generated by mixing different amounts of suppressive and fumigation-induced nonsuppressive soils, were infested with second-stage juveniles of H. schachtii and cropped with mustard-greens. Fungi were identified through an rDNA analysis termed oligonucleotide fingerprinting of ribosomal RNA genes (OFRG). Cysts obtained from soil mixtures consisting of 10 and 100% suppressive soil predominantly contained fungal rDNA with high sequence identity to Dactylella oviparasitica. The dominant fungal rDNA in the cysts isolated from the soil mixtures composed of 0.1 and 1% suppressive soil had high sequence identity to Fusarium oxysporum. Polymerase chain reaction (PCR) amplifications performed with sequence-selective primers corroborated the treatment-specific distribution of rDNA clones obtained by the OFRG analysis. When these sequence-selective PCR primers were used to examine H. schachtii cysts from biocidal soil treatments that produced various levels of suppressiveness, only the D. oviparasitica-like rDNA was consistently identified in the highly suppressive soils.