Diseases causing substantial economic losses in the northern grains region of Australia include the destructive soil-borne pathogens, root-lesion nematodes (Pratylenchus thornei and P. neglectus) and the stubble-borne fungal pathogen causing yellow spot (Pyrenophora tritici-repentis). Very effective sources of resistance to these diseases have been identified in synthetic hexaploid wheat lines. The genetics and nature of inheritance of disease resistance in these synthetic hexaploid wheat lines was investigated to give an insight into the most effective breeding strategy for durable resistance to multiple pathogens.
The inheritance of P. thornei resistance was investigated in five elite synthetic hexaploid wheat lines (Yallaroi/AUS24152, TAMD870167/AUS18913, CPI 133842, CPI 133859, and CPI 133872) using a half-diallel crossing design. The
combining ability of resistance genes in the synthetic hexaploid wheat lines was compared with the performance of GS50a, the previous best source of P. thornei resistance used in Australian wheat breeding programs. Inheritance of P. thornei resistance was identified as polygenic and additive in gene action. General combining ability of the parents was more important in the inheritance of P. thornei resistance than specific combining ability. All synthetic hexaploid wheat lines investigated possessed better general combining ability for P. thornei resistance than GS50a. Thus, novel sources of resistance were identified in the synthetic hexaploid wheat lines that will provide alternative and more effective sources of resistance to be utilised in wheat breeding programs. CPI 133872 was identified as the synthetic hexaploid with the best general combining ability for P. thornei resistance,
indicating that use of this parent in breeding programs should prove to be an effective approach in breeding wheat with resistance to P. thornei. The presence of transgressive segregants in the resistant x resistant crosses between the synthetic hexaploid parents indicated that the synthetic hexaploid lines contain unique sources of P. thornei resistance, which have the potential to be exploited by pyramiding the resistance genes to obtain durable resistance to P. thornei.
The inheritance of disease resistance in the synthetic hexaploid wheat, CPI 133872, was investigated in more detail through the molecular characterisation of the resistance genes. A CPI 133872 x Janz doubled haploid population was phenotyped for resistance to P. thornei, P. neglectus, and yellow spot. A framework mapping strategy and quantitative trait loci (QTL) analysis was then used to
determine the nature of the genetics of multiple disease resistance in the mapping population. All three of the diseases under investigation were quantitative traits that involved additive inheritance of resistance genes. QTL for P. thornei resistance were detected on chromosome arms 2DS, 6AS, 6AL, and 6DS. One QTL on chromosome 2DS was detected for P. neglectus resistance. Further mapping of chromosome 2D is required to determine if one gene is conferring dual resistance to both species of root-lesion nematode, or if there are two closely linked resistance genes that individually confer resistance to a single nematode species. QTL for yellow spot resistance were detected on chromosomes 3AL, 3DL, and 5BL. Resistance to root-lesion nematodes and yellow spot in CPI 13872 was inherited from both the tetraploid durum (AABB) and diploid Aegilops tauschii (DD) wild progenitor of the synthetic hexaploid wheat lines.
The development of molecular markers that are closely associated with the respective resistance genes would accelerate the selection of desirable disease-resistance gene combinations. Incorporation of durable disease resistance into wheat cultivars of high agronomic quality will allow newly developed wheat varieties to withstand a range of pathogen threats.