Seedling disease of soybean is an economically important disease that reduces crop stands and may reduce yield. Symptoms of seedling disease include pre- and post-emergence damping-off and root rot. Many species of Pythium and Fusarium cause seedling disease and are frequently recovered together from diseased soybean seedlings. Host resistance as a strategy to manage multiple pathogens has only briefly been explored in soybean and the interaction between these pathogens is unclear. To better understand the soybean-Pythium pathosystem, lab and growth chamber assays were conducted to accomplish the following objectives, 1) identify with resistance to seed and root rot caused by multiple Pythium species in the parental lines of the SoyNAM populations, 2) map quantitative trait loci (QTL) contributing to seed rot caused by multiple Pythium species in a SoyNAM population, and 3) evaluate the effect of co-inoculation with Pythium and Fusarium species on soybean seedling disease development.

A seed and root rot assay was used to evaluate traits associated with seed and root rot. The SoyNAM parents were screened for resistance to Pythium lutarium, P. oopapillum, P. sylvaticum, and P. torulosum. Of the 40 SoyNAM parents, Magellan, Maverick, CLOJ095-4-6, and CLOJ173-6-8 were resistant to P. lutarium, P. sylvaticum, and P. torulosum. The remaining parents were resistant to two, one, or none of the species they were screened against. A correlation analysis was performed to compare seed and root rot severity, and a weak to insignificant correlation was found suggesting that resistance to seed and root rot are not necessarily the same.

The 140 recombinant inbred lines (RILs) of SoyNAM population 29 (PI 427.136 x IA3023) were evaluated for seed rot resistance to P. lutarium, P. oopapillum, and P. sylvaticum. Assessments included seed rot severity (SRS), percent rotted seeds (ROTS), and adjusted germination (GERM). Inclusive composite interval mapping was performed to identify quantitative trait loci (QTL) contributing to seed rot resistance to three Pythium species. When the log of the odds (LOD) threshold was set using 1000 permutations, no QTL were identified for resistance. When the LOD was manually set at 3, several QTL were identified for resistance to each Pythium species, however the explained phenotypic variance (PVE) for all QTL was below 2.3%. Further screening with a modified assay may be necessary to identify QTL for resistance to seed rot.

The interaction between P. irregulare or P. sylvaticum with either Fusarium oxysporum or F. graminearum on soybean, was evaluated in a cup assay under controlled environmental conditions. Each interaction was evaluated in a separate experiment that was repeated three times. Controls included a non-inoculated control, and a Pythium-only and Fusarium-only treatment, the species of which corresponded to the interaction being tested. Disease development parameters, disease severity (DS), wet root weight (WRW), and emergence (EME) were assessed at five sampling times, 2, 4, 6, 8, and 10 days after planting (DAP). Generally, no significant differences in DS, WRW, and EME was detected among the Pythium-only and Pythium-Fusarium treatments in all of the experiments. Less disease development occurred in the F. oxysporum treatments while disease in the F. graminearum treatment was comparable to the Pythium-only treatments. Over time, DS and WRW increased, and EME remained constant in the Pythium-Fusarium treatments. Overall, under the conditions used in this study, there was no evidence that co-inoculation with Pythium and Fusarium species affected the development of seedling disease in soybean.