Genetic analysis of stem rust resistance among Ethiopian grown wheat lines.
Wheat (Triticum aestivum L.) is one of the major food crops in the world. Ethiopia is the second largest wheat producer in sub-Saharan Africa. However, wheat production in Ethiopia is constrained by many biotic and abiotic factors, and socioeconomic constraints. Among the biotic stresses are the rust diseases: stem rust caused by Puccinia graminis f.sp. tritici, leaf rust (P. triticina Eriks) and stripe rust (P. striiformis Westend. f.sp. tritici)). Stem rust is considered to be the most destructive disease of wheat in the main wheat growing regions of Ethiopia. Losses may reach 100% on susceptible wheat cultivars when conditions are favorable for disease development. Use of resistant cultivars is the most effective, economical and environmentally safe control measure, especially for the resource poor farmers. Due to the frequent emergence of new stem rust races through mutation, migration and recombination of exsisting virulence genes, efforts to identify potentially new sources of effective resistance genes are of the highest importance followed by their incorporation into a desirable genetic background. The objectives of the study were 1) to identify the primary threats to wheat production, farmers’ selection criteria for wheat varieties, and disease management practices with emphasis on wheat rusts in the Arsi, Bale and West Shewa administrative zones of Ethiopia; 2) to identify possible sources of stem rust resistance among Ethiopian wheat lines; 3) to determine the levels of heterosis and combining ability, and to identify the best parents and crosses for breeding to stem rust resistance, high grain yield and desirable agronomic traits; 4) to introgress durable resistance genes from known resistance sources into farmers’- preferred and locally adapted but stem rust susceptible, improved wheat varieties. A participatory rural appraisal (PRA) research was conducted involving 270 farmers in six districts of three administrative zones in Ethiopia. The participating farmers listed and prioritized their wheat production constraints. Wheat rust diseases, the high costs of fertilizers, lack of access to seeds of improved varieties and high seed prices were the major constraints reported by the respondents. The most important traits that farmers sought in wheat varieties were disease resistance and high grain yield. Estimated yield losses due to stem rust disease were more than 60% in all the surveyed areas. Fungicide application was the main disease management practice used by the majority of respondent farmers. Field and greenhouse experiments were conducted to identify possible sources of stem rust resistance among Ethiopian wheat lines. Two hundred fifty two wheat genotypes were evaluated for their resistance to stem rust at the seedling stage. Ninety one lines that exhibited intermediate and susceptible seedling reactions were field tested for their slow rusting characteristics. Among the 91, 38 lines that had high to moderate level of slow rusting were advanced to further field evaluation. Ten lines (H04-2, 204408-3, 214551-1, 231545-1, 7041-1, 7514-1, 226385-1, 226815-1, 7579-1, and 222495-1) were identified as good slow rusting lines while seven (237886-1, 227059-1, 203763-1, 226275-1, 227068-2, 226278-1 and 7994-1) were identified as moderately slow rusting lines. Fifteen wheat hybrids were developed through a half diallel mating design involving six parents. The F1’s and their parents were field evaluated for their stem rust reaction and agronomic performances at the Debere-Zeit Agricultural Research Center in Ethiopia, which is a well known hot spot area for stem rust. The analysis of variance revealed that tested genotypes had considerable genetic variability for all characters studied. The maximum positive mid-parent (31.45%) and betterparent heterosis (25.38%) were observed for grain yield. Plant height and days to maturity had maximum negative mid-parent heterosis levels of -11.01% and -8.02%, respectively. The majority of the crosses expressed negative heterosis over their mid-parent for AUDPC, indicating these crosses manifested resistance against stem rust. Significant general combining ability (GCA) effects were observed for all the characters studied. Furthermore, significant specific combining ability (SCA) effects were detected for most of the traits. Non-additive gene action was predominant for grain yield, thousand kernel weight and plant height. Additive gene action played a greater role in the inheritance of AUDPC, kernels per spike, number of tillers per plant and days to maturity. The study identified parental lines with good GCA effects for most of the characters, especially H04-2, Digelu and Danda’a. Crosses 231545- 1 x H04-2, 7041-1 x H04-2, Digelu x Kubsa and Danda’a x Kubsa had significantly negative SCA effects for AUDPC. Progenies of these crosses will be selected in an ongoing stem rust resistance breeding program. In general, H04-2 and Danda’a were good general combiners for most of the important studied characters. Crosses that involved these lines performed well for most of the traits. Hence, Lines H04-2 and Danda’a could be exploited in wheat breeding programs to develop stem rust resistant and high yielding wheat cultivars. Stem rust resistance genes were introgressed into locally adapted, high yielding susceptible wheat varieties, Kubsa (HAR1685) and Galama (HAR604), from two sources of adult plant resistance, Pavon 76 and Kenya Plume, using the single backcross-selected bulk breeding approach. The resistance sources were crossed with the adapted high yielding varieties and a single backcross was made with the recurrent parent. The resulting BC1 populations were selfed until the F3. Bulk selection was practiced from BC1- F3. The F3 populations, along with the recurrent parents, were evaluated in a replicated trial at Debre-Zeit Agricultural Research Center under high stem rust pressure to determine the genetic improvement attained in the populations for stem rust resistance and agronomic traits. All F3 populations, except the cross of Galama x Kenya Plume, were better performing for stem rust resistance and most agronomic traits studied when compared to the recurrent parents. The F3 progenies of Kubsa x Pavon 76 had superior mean values and high genetic gains for most agronomic attributes and stem rust resistance. These progenies will be advanced and selected in subsequent generations to develop locally adapted pure line wheat varieties with improved stem rust resistance and farmers’-preferred agronomic traits. Overall, the present study attempted to understand farmers’ wheat varietal preferences, farmers’ wheat production constraints, identified slow rusting wheat lines among the Ethiopian bread wheat germplasm, identified promising lines and F1 hybrids with good combining ability for breeding towards stem rust resistance and high yields. Durable stem rust resistance genes were incorporated into locally adapted susceptible wheat varieties for further selection and future release to enhance wheat productivity in Ethiopia.