Synergistic QTL Stacks in Soybean for SDS Resistance.

Soybean (Glycine max (L. Merr.)) resistance to any population of  Heterodera glycines (I.), or Fusarium virguliforme (Akoi, O’Donnell, Homma & Lattanzi) required a functional allele at Rhg1/Rfs2. H. glycines, the soybean cyst nematode (SCN) was an ancient, endemic, pest of soybean whereas F. virguliforme causal agent of sudden death syndrome (SDS), was a recent, regional, pest. SDS has arisen in the south Midwest and spread north as far as Canada in just the last 30 years and has been linked to global warming. Novel tools underlying the plant response to FRR have been developed for soybean over the past 2 decades. About 18 resistance loci have been identified over the past 2 decades.  To be selected 8-10 loci per cross must be stacked and this is a hard task for plant breeders since not all the QTL work well together. Recent analyses of NILs, SSRs and SNPs suggest some QTL have clustered positive and negative alleles making them hard to use. The other problem is the large number  of loci. The reason for the large number of loci is that the resistance mechanisms that have evolved in plants provide only partial protection. This may be because the plants have to contend with a group of fungi with a wide host range and flexible hemibiotrophic lifestyle. Consequently, full resistances are rare, or absent, among legumes, brassicas, cucurbits and solanaceous plants. This presentation focus’ on the use of plant genomics resources to aid breeding selections and lead to a better understanding of the disease process and resistance to both types of Fusarial rots with special emphasis on soybean sudden death syndrome (SDS). SDS is a combination of two disease effects caused by translocated toxins, rotted roots and toxin restricted roots. The leaf effects of toxins have several causal agents with different target proteins. Effects on the roots include variations in infection severity and infection frequency. Breeding for FRR resistance has begun with some exciting new tools for pathogen quantification in roots. QTL stacks of 6 have been used for 15 years. QTL stacks of 12-18 have been made and tested so that QTL and their interactions can be ranked in terms of effectiveness. Therefore, we are at a time of opportunity for new breeding initiatives and this presentation aims to inform those new programs of the core discoveries from the past 20 years.