Association mapping for resistance to Turcicum leaf blight (Exserohilum turcicum) in sorghum

Abstract

Turcicum Leaf Blight (TLB) disease, caused by a fungal pathogen Exserohilum turcicum is one of the major diseases affecting sorghum and maize in the warm and humid parts of the world including Uganda. TLB causes leaf necrosis or chlorosis that reduces the photosynthetic surface area resulting in yield reduction. The use of TLB resistant sorghum varieties in an integrated disease management strategy is a sustainable option for increasing sorghum production. This study seeks to identify genotypes with resistance to TLB in the Multi-parent Advanced Generation Inter-Cross (MAGIC) population and map genomic regions associated with TLB resistance. A total of 198 sorghum MAGIC lines and 12 checks were planted in Ngetta ZARDI and MUARIK in Uganda under natural conditions for two seasons of 2019A and 2019B using a randomized complete block design. Data was collected on disease severity at 54, 61, 68, 75 and 82 days after planting (DAP) respectively and these data points were used to calculate the area under disease progress curve (AUDPC). Important agronomic traits including plant height, panicle width, panicle length stay green, days to 50% flowering and grain yield were also collected. Analysis of variance (ANOVA) was performed using R software. A highly significant (P<0.001) difference was observed among the MAGIC lines for TLB severity at 54, 61, 68, 75 and 82 DAP and AUDPC respectively, indicating high genetic variance among the MAGIC population for TLB resistance. In addition, a highly significant (P<0.001) Genotype x Environment (Season and Location) effects were observed among the MAGIC lines for TLB severity at 54, 61, 68, 75 and 82 DAP and AUDPC suggesting that MAGIC population performed differently in the different testing locations across seasons. Ngetta ZARDI recorded the highest disease severity in both seasons. This study identified new sources of resistance to TLB disease in the MAGIC population including 2003_PP34FG_S7_337, 2003_PP34FG_S7_866, 2003_PP34FG_S7_331 and 2003_PP34FG_S7_429 with high yield potential and is recommended as parents in a sorghum breeding programme and additional testing for release as varieties. A genome-wide association study analysis (GWAS) detected eight significant association signals for TLB resistance on chromosome five. Markers S1_413262695, S1_413272788, S1_537057636, S1_156097306 and S1_156097317 were within 220 kb near positional candidate genes including NBS-LRR, NB-ARC and a remorin gene involved in disease response in plants. This study gave insights into the basis of TLB resistance in sorghum adding on knowledge like the REM39 whereby there are few evidence of role for remorin in plant-fungal interactions. The SNP markers identified in this study can be converted to KASP markers and validated for use in marker-assisted selection for TLB into well adapted backgrounds.