RESISTANCE TO SOYBEAN SUDDEN-DEATH SYNDROME AND ROOT COLONIZATION BY FUSARIUM-SOLANI F SP. GLYCINE IN NEAR-ISOGENIC LINES

One genomic region in soybean [Glycine max (L.) Merr.] 'Essex' and thr ee in 'Forrest' underlie field resistance to sudden death syndrome (SD S) leaf scorch in their recombinant inbred line (RIL) progeny. Root in fection by Fusarium solani (Mart.) Sacc. f. sp. glycine precedes the l eaf scorch caused by SDS. Forrest shows rate-reducing resistance to bo th root colonization by F. solani and leaf scorch, whereas Essex does not. Our objective was to determine whether genomic regions that under lie resistance to the SDS leaf scorch also caused resistance to root c olonization by F. solani in near-isogeneic lines (NILs). The NILs were derived from individual plants selected from within one recombinant-i nbred line (ExF34). ExF34 was heterogenous within regions of linkage g roup C2 and G, each of which encompassed quantitative trait loci (QTL) for resistance to SDS. Using DNA markers, four genotypic classes coul d be identified. The QTL effects were compared with two SDS disease pa rameters, leaf scorch measured as SDS disease index (DX) at the R6 gro wth stage and root colonization by F. solani measured as infection sev erity (IS) at the R5.5 and R8 growth stages. The Forrest allele of the genomic region on linkage group G was consistently associated with de creased DX (P < 0.05-0.0004) and IS (P < 0.05-0.0017). The Essex allel e of the genomic region on linkage group C2 caused a decrease in DX (P > 0.05) and IS in the susceptible pair of the four ML clases st R5.5 but not R8. Therefore, the QTL on linkage group G and C2 confer separa te components of resistance to SDS that are suitable for selection in a resistance gene pyramid.