Stem rust resistance and yield performance of irrigated Zimbabwean spring wheat
The strategic importance of wheat in Sub-Saharan Africa and Zimbabwe is under threat of the Ug99 stem rust race group. Since first detection of Ug99 (TTKSK according to the North American nomenclature system using five sets of four gene differential lines (www.fao.org, accessed 27/3/2018)) in 1998/9, 13 races have been detected in 13 countries by 2016. PTKST, TTKSF and TTKSF+Sr9h have been confirmed in Zimbabwe since 2009 while South Africa has TTKSP as a fourth race. Ug99 is virulent to a broad spectrum of resistance genes including Sr9h, Sr24, Sr31, Sr36 and SrTmp that are found in Southern Africa wheat germplasm. Genotyping of Zimbabwean lines by CenGen (2012) and Limagrain (2015) showed a frequency of 38.7-44.4% of lines possessing Sr2 alone or in combination with Sr31. Sr24 alone or in combination with unknown Sr genes constituted 10.2-11.1% whereas Sr31 alone or in combination constituted 53.1%. Sr36 had a frequency of 2% and Lr19/Sr25 alone or in combination with Sr2 was recorded in 39.9% entries. Continuous genotyping, pathotyping and annual rust surveys are important components of an updated wheat database. For example, the lack of early confirmation of stem rust occurrence led to deregistration of a new variety “Busi” in 2002. Breeding for durable resistance is the most effective, efficient, environmentally friendly and sustainable way of managing the threat of stem rust. Resistance breeding can aim for adult plant resistance (APR) or all stage resistance (ASR) or a combination of the two. Four APR genes have been catalogued and these confer resistance to stem, leaf and stripe rust and powdery mildew. Any one of Sr2/Yr30/Lr27, Lr34/Yr18/Sr57, Lr46/Yr29/Sr58 and Lr67/Yr46/Sr55 must be used as the foundation for gene pyramiding. A successful disease breeding programme requires investment in effective breeding procedures, marker assisted breeding, gene stewardship, pathogenicity surveys and analysis to keep track of pathogen dynamics. Although global initiatives offer a platform for germplasm and knowledge sharing, in-country expertise and ongoing rust programmes are required in at-risk regions. Results from this study indicate PTKST with virulence on Sr31 is a major threat to wheat production in Zimbabwe with 63.3% of germplasm containing either Sr31 or Sr24. Compared to TTKSF, TTKSF+Sr9h and TTKSP, PTKST was the most virulent race with 40.8% of lines being susceptible in the greenhouse. At seedling stage, 59.2% of lines were resistant to all four Southern Africa Ug99 races. At adult stage, 42.9% of the lines were resistant or moderately resistant to PTKST in the greenhouse and a 55.1% resistance frequency was recorded in the field. PTKST resulted in 29% and 21% mean yield loss in susceptible varieties (SC1 and SC3, respectively) at Greytown, South Africa over two seasons. ASR offers better protection against Ug99 with SC8 recording the lowest yield loss of 6.4%. This was lower than losses in the APR lines Kingbird (10.1%), W1406 (19.5%) and W6979 (15.4%). Molecular markers can be used to complement phenotypic markers in characterizing wheat lines. A total of 89.5% of lines genotyped to have Sr2 also showed Sr2 flecking at adult stage when inoculated with PTKST. SC2, SC8, SC30, SC35 and SC36 had best protection as measured by infection type at seedling stage and modified Cobb field scores at adult stage against PTKST showing the value of gene pyramids. These lines have Sr2, Sr31 and other Sr genes. SC1 confirmed to have only Sr31 was susceptible to PTKST in both seedling and adult stages and needs to be replaced as a commercial variety. To test the viability of using marker assisted backcrossing, Sr25, Sr26 and Sr39 were successfully transferred to, and tracked in the development of adapted genotypes. Generally, it is difficult to predict phenotypic responses of varieties due to complexity in gene interactions and environmental influences. In the current project an AMMI model and GGE biplots identified desirable and stable genotypes and demarcated most favourable environments, important considerations for strategic decision making in wheat breeding and variety release in Zimbabwe.