Fusarium graminearum causes seed decay and damping-off of soybean. This study evaluated the effect of inoculum density of F. graminearum, temperature, and fungicide seed treatments on disease development. To determine the optimum conditions for disease development, individual soybean seed was inoculated with 100 μl of a suspension of 2.5 × 102, 2.5 × 103, 2.5 × 104, or 2.5 × 105 macroconidia/ml in a rolled-towel assay at temperatures of 18, 22, and 25°C. Inoculum concentrations of 2.5 × 104 macroconidia/ml or higher were necessary for optimum disease development at all temperatures. The efficacy of captan, fludioxonil, mefenoxam + fludioxonil, azoxystrobin, trifloxystrobin, and pyraclostrobin as seed treatments was then evaluated with the same assay at 2.5 × 104 and 2.5 × 105 macroconidia/ml. Seed treated with captan at 61.9 g a.i. or fludioxonil at 2.5 or 5.0 g a.i. per 100 kg developed smaller lesions than other seed treatments and the nontreated control. Based on these results, there are limited choices in fungicide seed treatments for managing this seedling disease, and it is possible that shifts in seed treatment products may have played a role in the recent emergence of this soybean pathogen.

Fusarium graminearum, a necrotroph, has emerged as an important soybean [Glycine max (L.) Merr.] seedling pathogen in Ohio. An effective management strategy for seedling pathogens is host resistance; thus, the objectives of this research were to identify and characterize sources of resistance to F. graminearum. Twenty‐four soybean genotypes were screened for resistance using a rolled‐towel assay; seeds were inoculated with 2.5 × 104 macroconidia mL−1. A disease severity index (DSI) was calculated on the basis of the ratio of lesion length to total plant length at 7 days after inoculation. Five genotypes had high levels of resistance to F. graminearum, with DSIs ranging from 24 to 41.5%. These included the cultivar Conrad, which has high levels of partial resistance to the hemibiotroph, Phytophthora sojae. A population of 262 F6:8 recombinant inbred lines derived from a cross of Conrad × Sloan (susceptible) was then evaluated for resistance. The same rolled towel method was used, and resistance to F. graminearum segregated as a quantitative trait. The DSI ranged from 22 to 100% and the broad‐sense heritability estimate was 0.72. Four putative quantitative trait loci (QTLs) were identified from Conrad on chromosomes 8, 13, 15, and 16, and one putative QTL from Sloan mapped to chromosome 19. The putative QTLs identified in this population were not the same as those that confer resistance to P. sojae, which suggests that different loci are required for resistance to these two different types of seedling pathogens.

Phytophthora sojae Kaufm. and Gerd., Pythium irregulare Busiman, and Fusarium graminearum Schwabe [teleomorph: Gibberella zeae (Schwien.) Petch] are important pathogens of soybean [Glycine max (L.) Merr.] and are all capable of causing seed rot, damping‐off, and root rot. The objective of this study was to identify quantitative trait loci (QTL) for resistance to Py. irregulare and to refine previously mapped QTL for resistance to P. sojae and F. graminearum in a larger, more advanced ‘Conrad’ × ‘Sloan’ F9:11 recombinant inbred line population. The population was mapped with 1032 single nucleotide polymorphisms from the SoySNP6K BeadChip and 31 polymerase chain reaction–based molecular markers. Families were evaluated for resistance response to three isolates of P. sojae, one isolate of Py. irregulare, and one isolate of F. graminearum. A total of 10, 2, and 3 QTL and suggestive QTL were found that confer resistance to P. sojae, Py. irregulare, and F. graminearum, respectively. Individual QTL explained 2 to 13.6% of the phenotypic variance. Quantitative trait loci for resistance toward both Py. irregulare and F. graminearum colocalized on chromosome 19. This resistance was contributed by Sloan and was juxtaposed to a QTL for P. sojae with resistance contributed from Conrad. Alleles for resistance to different pathogens contributed from different parents in the same region; the number of unique QTL for each pathogen and the lack of correlation of resistance suggest that different mechanisms are involved in resistance toward P. sojae, Py. irregulare, and F. graminearum.

Pythium irregulare Buisman is an important seed and seedling pathogen of many hosts including soybean [Glycine max (L.) Merr.] and has been increasingly linked to reports of stand losses. The objectives of this research were to identify sources of resistance to P. irregulare, characterize the type of resistance identified, and genetically map the resistance. Sixty‐five soybean genotypes were evaluated for resistance to two isolates of P. irregulare, and approximately one‐third had moderate to high levels of resistance. The PI 424354 had the highest levels of resistance to P. irregulare, and resistance was mapped in two three‐way cross populations: ‘OHS 303’ (moderately susceptible) × (‘Williams’ (moderately susceptible) × PI 424354) and ‘Dennison’ (moderately susceptible) × (Williams × PI 424354). Resistance quantitative trait loci (QTL) with alleles from PI 424354 were mapped on chromosomes 1 and 6 in OHS 303 × (Williams × PI 424354) using composite interval mapping (CIM) for root weights and root rot scores. Resistance QTL from PI 424354 were also mapped on chromosomes 8 and 11 with CIM for root weight data in the Dennison × (Williams × PI 424354) population and chromosome 13 using root rot scores. Each QTL explained 7.9 to 17.8% of the phenotypic variation. These are the first reported putative QTL for resistance to P. irregulare in soybean.