Flow Cytometric Analysis of Phialophora gregata Isolated from Soybean Plants Resistant and Susceptible to Brown Stem Rot

Flow cytometry was used to characterize isolates of Phialophora gregata, the causal organism of brown stem rot (BSR) in soybean, using the fluorescence intensity of YOYO-1 stained conidia. The conidia were derived from isolates of P. gregata isolated from fieldgrown soybean vascular tissue from two locations in Wisconsin. At each location, four soybean cultivars were grown. Two cultivars were susceptible to BSR and two cultivars were resistant to BSR. Ninety-six isolates, one per plant, were characterized for DNA content. The stained conidia were analyzed with an EPICS XL (Coulter Corporation, Hialeah, Florida, USA) flow cytometer cell sorter with CICERO software (San Rafael, CA, USA). The isolates differed in their mean DNA content, ranging from 78.0 to 116.8 arbitrary units. Statistical analysis showed a significant difference in the DNA content variability of the isolates between the two locations. A notable difference in DNA content variability was observed between the isolates collected from the resistant cultivars between the two locations. However, no significant variation occurred in the DNA content of the isolates collected from susceptible cultivars within or between locations. The observed variation among the isolates is hypothesized to be due to the occurrence of separate populations of P. gregata and selection for mean DNA content that could colonize specific plant genotypes.