Genetic analysis of dual resistance to anthracnose and turcicum leaf blight in sorghum

Sorghum (Sorghum bicolor L. Moench (2n=20)) and fungal pathogens have continuously co-evolved in a battle for growth and survival. In this rivalry, sorghum evolved a stunning array of structural and gene-based defences designed to combat diverse pathogens and so did pathogens by developing new races. Anthracnose, caused by Colletotrichum sublineolum, and turcicum leaf blight, caused by Exserohilum turcicum, is two major foliar diseases that limit sorghum productivity in Sub-Saharan Africa. Deployment of resistant varieties is the most cost effective way to manage both diseases especially when integrated with appropriate agronomic practices. There are very limited studies of such phenomena in sorghum, a crop affected with unusually large number of diseases and pests. The main objective of this thesis was to contribute to the knowledge of dual resistance to anthracnose and turcicum leaf blight in sorghum in East and Central Africa. The specific objectives included: (1) establishing the reaction of sorghum lines to dual infection by both pathogenes in Sudan and Uganda; (2) identifying gene action conditioning resistance to both pathogenes in sorghum; and (3) identifying simple sequence repeats that co-segregate with anthracnose and turcicum leaf blight resistance loci. Four field and two greenhouse experiments were conducted in sorghum growing regions of Sudan and Uganda that are centers of diversity. The results showed that Wad Medani in Sudan and Makerere University Agricultural Research Institute, Kabanyolo (MUARIK) in Uganda were hot spot sites for both leaf anthracnose and turcicum leaf blight epidemics. The results showed a varied response of test genotypes under field and greenhouse conditions across locations in both Sudan and Uganda. Cultivars, Jesu91-104DL and Butana (farmer preferred varieties in either country) were tolerant to both diseases. The advanced breeding line MUTLB1003 exhibited high levels of resistance to anthracnose and moderate reaction to turcicum leaf blight. Genotype main effect plus genotype-by-environment interaction analysis revealed that breeding for leaf anthracnose resistant genotypes was equally effective in all environments while it was not the same for turcicum leaf blight. Some of the genotypes in the study were resistant to dual infection and are thus suitable for multiple trait breeding. Segregating progeny generated through half diallel mating of six parents indigenous to East and Central Africa i.e. HD1, Epuripuri, Sekedo, GA06/106, GA06/18 and MUC007/009, with varied reaction to leaf blight and anthracnose was used to study the mode of inheritance of resistance to foliar anthracnose and turcicum leaf blight. The results showed a negative but significant correlation between anthracnose and turcicum leaf blight disease occurence. Thus, suggesting that loci conditioning resistance to anthracnose were different xv from those that affect turcicum leaf blight. Dominant and additive variance components were almost equal indicating the significant role of both additive and non-additive genetic variance towards anthracnose resistance as supported by the Baker’s ratio of 0.4. The contribution of additive gene effects towards turcicum leaf blight resistance was greater than non-additive gene effects as supported by the Baker’s ratio of 0.6. This study showed that sorghum genotypes studied such as GA06/18 had resistant alleles to both diseases. Two crosses GA06/106 x Epuripuri and MUK007/009 x Epuripuri clearly demonstrated that they were good starting populations for TLB and anthracnose resistance breeding programmes. Mapping of resistance to anthracnose and turcicum leaf blight was undertaken in 126 F8:9 sorghum recombinant inbred lines derived from a cross between MUC007/009 and Epuripuri under Uganda and Sudan field conditions. Transgressive segregation was observed in RILs indicating that both parents carried minor alleles for resistance. High polymorphic information content, gene diversity and allele frequency were observed suggesting that all of polymorphic SSR markers evaluated could contribute substantive information to breeding research and construction of genetic map of sorghum. The information gained from this study can be used in deploying marker-assisted selection for dual diseases infection of anthracnose and turcicum leaf blight.