Identification of Soybean Genotypes Resistant to Fusarium graminearum and Genetic Mapping of Resistance Quantitative Trait Loci in the Cultivar Conrad

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 x 10(4) 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 F-6:8 recombinant inbred lines derived from a cross of Conrad x 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.