Linkage and Association of Genes for Morphological Traits and Scab Resistance In US Soft Red Winter Wheat Cultivars.

Two recombinant inbred line (RIL) mapping populations were derived from US soft red winter wheat (Triticum aestivum) cultivars Ernie and Massey having moderate scab (Fusarium graminearum) resistance.  Scab resistance QTL have been mapped onto chromosomes 4B and 4D of several resistant sources where two major dwarfing genes Rht1 and Rht2 reside. The dwarf gene Rht8 is linked to photoperiod sensitive gene Ppd-D1 on chromosome 2D. Two hundred thirty-four RILs derived from the cross of Ernie/MO94-317 (EM) segregate for the awn suppressor gene B1 on chromosome 5AL, Rht1, Rht2, and Ppd-D1.  In a Becker/Massey (BM) population, 152 RILs segregate for genes Rht1, Rht2, Rht8, and Ppd-D1.  Heading and flowering dates, scab incidence, severity, and index were evaluated among RILs in these two populations. Groups of RILs with only one gene difference were compared to determine their effects on scab resistance. Two RILs with three dwarfing genes from BM are very short and have significantly (P< 0.05) higher scab incidence and severity than single gene dwarf RILs in field tests but not in greenhouse tests.  Nine double gene dwarf RILs from EM were significantly (P< 0.05) shorter in height and had higher scab incidence and severity than single gene dwarf RILs.  RILs having gene Ppd-D1a headed 2 to 5 days earlier in both populations.  Some RILs with Rht2 did not differ significantly (P< 0.05) from RILs with Rht1 for scab incidence or severity.  Among RILs lacking dwarfing genes, the presence of Ppd-D1a significantly (P< 0.05) reduced heading and/or flowering dates. Scab incidence was significantly (P< 0.05) higher in BM than EM in field tests. A consistent trend was not observed for effects of dwarfing genes, B1, or Ppd-D1 genes on scab resistance.  This may be due to the effects or interactions of other genes and/or morphological traits with scab resistance loci in these genetic backgrounds.  Isogenic lines can be developed and tested further to determine whether such trait associations are due to linkage or pleiotropy.