2 ADDITIONAL LOCI UNDERLYING DURABLE FIELD-RESISTANCE TO SOYBEAN SUDDEN-DEATH SYNDROME (SDS)

Severe losses of soybean [Glycine mar (L.) Merr.] grain yield occur as a result of the disease sudden death syndrome (SDS), caused by Fusari um solani (Mart.) Sacc, f. sp. phaseoli (Burk.) Snyd. & Hans., type A, Selection for resistance to SDS is currently the most efficient means of yield protection. This study was undertaken within adapted soybean germplasm to identify and characterize loci underlying useful held re sistance to SDS. One hundred eleven polymorphic DNA markers Here compa red with SDS disease response among 100 recombinant inbred lines deriv ed from a cross between a durably SDS resistant cultivar, 'Forrest', a nd a SDS susceptible cultivar, 'Essex'. SDS disease incidence (DI) and disease severity (DS) were determined in replicated, F.solani infeste d field-test sites during 4 yr encompassing Eve locations. Four separa te chromosomal segments were strongly associated with mean SDS DI acro ss 5 locations (P < 0.001). Tn a previous report using the same geneti c materials tested in the same environments two of these quantitative trait loci (QTL) had been identified. With the further analysis Kith 4 0 additional markers, tao more QTL were detected. The two new QTL were stably associated with SDS resistance within each of five F. solani i nfested locations (P < 0.007). These two loci were identified by RAPD markers OI03(450) and OG13(490) and by OE04(450) and OE02(1000). The a lleles that conferred resistance were both derived from Forrest. Joint ly, the four QTL accounted for about 65% of total phenotypic variabili ty in mean DI and 50% in mean DS. DNA markers can be used to define al leles conferring resistance to SDS. Selection far the SDS resistance Q TL may allow efficient selection of resistant genotypes with good yiel d potential in F. solani infected fields.