| Summary: | The Oomycete Pythium comprises one of the most important groups of seedling pathogens affecting soybean, causing both pre- and post-emergence damping off. Numerous species of Pythium have been identified and found to be pathogenic on a wide range of hosts. Recent research on Pythium sp. infecting soybean has been limited to regions other than the Northern Great Plains and has not included North Dakota. In addition, little research has been conducted on the pathogenicity of various Pythium species on soybean or associations between Pythium communities and soil properties. Therefore, the objectives of this research were to isolate and identify the Pythium sp. infecting soybean in North Dakota, test their pathogenicity and assess if any associations between Pythium sp. and soil properties exist. Identification of the Pythium sp. was achieved using molecular techniques and morphological features. A total of 26 described Pythium sp. and several unknown species were recovered from soybean roots collected from 138 fields between 2011 and 2012. The majority of Pythium species (P. attrantheridium, P. debaryanum, P. diclinum, P. dissotocum, P. heterothallicum, P. hypogynum, P. inflatum, P. intermedium, P. irregulare, P. kashmirense, P. lutarium, P. minus, P. oopapillum, P. perplexum, P. terrestris, P. viniferum, P. violae,and an unknown Pythium sp.) caused pre-emergence damping off on soybean seedlings with less than 50% emergence and survival. In contrast, P. orthogonon, P. nunn, and P. rostratifingens had approximately 80% or greater emergence and survival of soybean seedlings. The negative and positive controls had 100% and 0% emergence and survival of soybean seedlings. Associations between soil properties and three Pythium groups were performed using logistic regression analysis. Logistic regression analysis determined that the presence of group one characterized by P. ultimum was correlated with zinc levels. Group two was characterized by P. kashmirense and an unknown Pythium sp. and was correlated with cation exchange capacity (CEC) values. Group three was characterized by P. irregulare and P. heterothallicum and was correlated with calcium carbonate exchange and CEC.
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