Resistance to Stenocarpella maydis and Fusarium graminearum cob rot pathogens in tropical maize
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Maize cob rots are among the most important impediments in the sub-saharan Africa, causing yield losses and reduced grain quality as a result of mycotoxins. The mycotoxins are carcinogenic and affect the health of both humans and animals. Developing varieties resistant to cob rots is a practical and economic strategy that provides cheaper protection against yield loss and poor grain quality. The objectives of this study were to i) Assess farmers’ perceptions, knowledge occurrence and control of Stenocarpella maydis and Fusarium graminearum maize cob rots; ii) investigate the appropriateness of multiple infection as a selection and breeding strategy for multiple resistance to F. graminearum and S. maydis cob rots; iii) Evaluate the influence of cob and kernel characteristics on the response to F. graminearum and S. maydis disease reaction and to evaluate the prospects of direct selection for high yielding hybrids resistant to F. graminearum and S. maydis and; vi) Map quantitative trait loci (QTL) associated with resistance to S. maydis and F. graminearum and to analyze the possibilities of utilizing these QTL for marker assisted selection. In order to promote adoption of improved materials, there is need to understudy the factors that affect adoption of seed and incorporate these into variety development. In objective 1 of this thesis, a focus group discussion (FGD) was conducted in four districts (Iganga, Kapchorwa, Kayunga and Masindi) of Uganda to assess farmers’ perceptions on maize cob rots and to investigate the possibilities of breeding farmer-preferred cob rot resistant varieties. Semi- structured questionnaires were administered to selected seed merchants to verify accuracy of farmer reporting. The study shows that absolute resistance to cob rot may be associated to undesirable traits such as small seeds, late maturity and low yields. Yield followed by earliness were the most preferred farmer agronomic traits, obtaining a farmer-preference mean derived score of 4.5 and 3.75 respectively from the total of 5. Thus selection for farmer-preferred cob rot resistant varieties should maintain high yield and /or earliness or strike a balance with cob rot resistance. To investigate the appropriateness of multiple infection, as a selection and breeding strategy for multiple resistance to F. graminearum and S. maydis, twelve tropical inbred lines with varying resistance to both pathogens were mated in a full diallel and the progeny and their parents evaluated for reaction to single or multiple infection. Under multiple inoculation, S. maydis suppressed colonization of cobs by F. graminearum. General combining ability estimates (GCA) indicated that inbred WL 118-10 effectively transmitted resistance to both diseases. Hybrids resistant to S. maydis were also resistant to F.graminearum, but the reverse was not true. This suggests that indirect selection for F. graminearum resistant can be done by screening and selecting for resistance to S. maydis. Overall, the suppression of F. graminearum by S. maydis shows that multiple infection cannot be used as an appropriate breeding strategy to obtain multiple resistance. Single infection of F. graminearum and S. maydis separately is therefore the best breeding strategy. In the third objective this thesis evaluated the influence of cob and kernel characteristics on efficiency of screening processes and genotype reaction to cob rots. Correlation of husk cover to disease severity of both pathogens, achieved under toothpick inoculation across locations was significant (P< 0.01) implying that rain-water influences disease development. Accordingly, screening for cob rots caused by F. graminearum and S. maydis should preferably test genotypes in high rain fall prone environments. The low correlation (r values less than 0.45) obtained from kernel and plant growth attributes such as kernel texture, husk cover and drooping to F. graminearum and S. maydis incidence and severity across locations as well as inconsistent r values within locations, suggest that indirect selection may only be used as supplementary tool and not a substitute for direct selection. Moreover, resistance to cob rots in maize is associated with undesirable traits such as low yields. However, in this thesis, the overall responses of genotypes to yield and the correlation of yield to S. maydis and F. graminearum disease development (r ≤ 0.30) indicated that the prospects of breeding for high yielding and resistant cob rot varieties exist. A three way hybrid cross of (CML 506 x WL 118-10) x CZL 8 was predicted as an appropriate candidate with multiple resistance to both pathogens (F. graminearum and S. maydis) and with desirable yield. In the final result chapter of this thesis, efforts were made to map quantitative trait loci to underpin the use of marker assisted selection (MAS) in resistace breeding. In this study a total of seven QTL were mapped. Stable QTL mapped were Fg 4,2 (R2= 0.22) and Sm 4,1 (R2=0.16) associated with resistance to F. graminearum and S. maydis respectively on chromosome 4. The other one was Fg 5 (R2= 0.30) on chromosome 5 and was associated with resistance to F. graminearum. A QTL with pleiotropic effect was detected on chromosome 1, 22 cM from marker umc1269, with R2= 0.13 and 0.22 for resistance to S. maydis and F. graminearum respectively. The additive effects ranged from -0.14 to -0.35 for all mapped associated QTL to both pathogens. All the mapped QTL were more than 5 cM from the nearest molecular marker utilized in the study. Therefore, there is need to utilise the maize genomic map to identify and test several markers near the mapped QTL, in order to locate more reliable molecular markers for MAS.