Characterization of Pythium spp. associated with asymptomatic soybeans in southeastern Pennsylvania

Soybean production in Pennsylvania has increased substantially over the past twenty years and is a highly valued field crop together with corn. Soilborne pathogens such as Pythium spp. can contribute to soybean stand establishment issues, particularly under the conservation tillage practices that are common in the state. In this study, we collected soil samples from eight asymptomatic soybean-corn rotation fields across six counties in southeastern Pennsylvania between May and June, 2012. Pythium species were isolated via baiting, and tested for aggressiveness on both soybean and corn using laboratory pathogenicity assays. In addition to our culture-based survey, we also assessed the use of cytochrome oxidase subunit 1 (COI) pyrosequencing as a culture-independent method for measuring Pythium species diversity from environmental samples. Diversity estimates were consistent between the culture-based and pyrosequencing datasets; however, important methodological biases inherent to culture-independent methods may have led to some differences. Our results show that several Pythium species previously characterized as soybean and/or corn pathogens are present in southeastern Pennsylvania, including P. irregulare, P. sylvaticum, and P. ultimum var. sporangiiferum, with isolates showing aggressive phenotypes in lab assays.