Specific detection of Phialophora gregata and Plectosporium tabacinum in infected soybean plants using polymerase chain reaction

The fungal deuteromycetes Phialophora gregata and Plectosporium tabacinum are associated with soybean plants. P. gregata causes brown stem rot (BSR) of soybean, whereas P. tabacinum is a frequent cohabitant of soybean stems. The role of P. tabacinum in soybean growth and in the development of BSR is not known. Traditional methods of isolating and differentiating these two fungi require up to 3 weeks to complete. In order to effectively study the interactions among P. gregata, P. tabacinum, and the soybean plant, we developed specific primers for P. tabacinum based on its rDNA internal transcribed spacer sequences. In combination with specific primers developed previously for P. gregata, the specific primer pairs were used successfully in polymerase chain reactions to detect the targeted fungi in both artificially inoculated and naturally infected soybean plants. Using this technique, we examined 130 soybean plants collected from natural field environments for the presence or absence of P. gregata and P. tabacinum. Statistical analyses of the results showed that the frequency of co-occurrence of P. gregata and P. tabacinum in soybean plants was significantly less than expected if the two fungi would occur independently, suggesting that one of the fungi may be inhibitory to the other fungus.