Characterization and Interactions of Pythium and Fusarium spp. associated with Soybean on Seed Decay and Root rot

Seedling diseases are among the most widely distributed diseases of soybean and affect yields through reductions in seed germination, seedling emergence and plant growth (Rupe and Hartman, 1999; Coker et al.,2001). In Arkansas, Pythium spp. and Fusarium spp. are the two main pathogens associated with seed decay and root rot in soybean (Killebrew et. al., 1993; Kirkpatrick, et al., 2006; Bates et al., 2008; Rosso, 2007). The objectives of this research were (1) to determine the dynamics of seed and root colonization on soybean, (2) to identify and characterize the effect of Pythium and Fusarium spp. on soybean seeds and roots; (3) to evaluate the interactions of the two dominant species recovered, P. sylvaticum and F. oxysporum, on disease development; and (4) to establish the role of seed exudates on soybean cultivar resistance to P. aphanidermatum.;In naturally infested soil, Pythium spp. were early colonizers of soybean seeds with the onset of colonization highly dependent on temperature. Fusarium spp. were rarely recovered from seeds, but colonized tap roots beginning 72 hours after planting and lateral roots as they developed. Pythium spp. also colonized the roots. Pythium sylvaticum and F. oxysporum were the most frequently recovered species from soybean seeds and roots followed by P. irregulare and P. dissotocum and F. equiseti and F. tricinctum. Interactions of P. sylvaticum with F. oxysporum and with other Pythium species resulted in three disease responses: the same, more or less disease than the most pathogenic isolate in the interaction. Disease development whether with single isolates or in co-infestations was highly dependent on the isolates involved and temperature. Seed exudate composition of the Pythium susceptible cultivar Hutcheson had significantly higher levels of sugars than the Pythium resistant cultivar Archer which had a higher concentration of isoflavones than Hutcheson. Some of these sugars increased growth and oospore germination of P. aphanidermatum in vitro, while concentrations as low as 0.01M of the isoflavone genistein reduced in vitro growth. Results suggest that seed exudates play a role in susceptibility to Pythium seed rot caused by P. aphanidermatum in soybean.;Since effective control strategies depend upon a more thorough understanding of the host-pathogen interaction, future research will focus on determining the underlying mechanism/s of Pythium and Fusarium interactions associated with disease expression and in a further characterization of seed exudates to evaluate how exudates initiate and regulate Pythium response to soybeans.