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DEVELOPMENT OF SORGHUM [Sorghum bicolor (L) Moench] FOR RESISTANCE TO GRAIN MOLD IN SENEGAL

This thesis, presented by Mr Cyril DIATTA, is submitted to the university of Ghana, legon in partial fulfilment of the requirements for the award of Doctor of philosophy degree in plant breeding.

ABSTRACT
Sorghum cultivation in Senegal is faced with several constraints that limit production and yield. To address these constraints, several improved varieties have been developed and released by the the Senegalese Institute of Agricultural Research (ISRA). However, most of these released varities have been pooly adopted by farmers. Therefore, a Participatory Rural Appraisal (PRA) was conducted in the main sorghum growing area of Senegal to identify farmers’ production constraints, preferred plant traits and their perception on sorghum grain mold using focus group discussion and semi-structured interview. Furthermore a population of 450 F3 families from a cross between Nganda susceptible to grain mold and Sureño resistant to grain mold was evaluated for grain mold resistance in 2014 and 2015 at Bambey, Darou and Sinthiou Maleme under natural infestation. These 450 F3 families along with their two parents, Nganda and Sureño, were also genotyped with 201 SNP markers with the objective of identifying genomic regions (QTLs) associated with grain mold resistance and other agronomic traits. Additionally, 11 sorghum inbred lines from the working collection of ISRA were crossed to two cytoplasmic male sterile lines (CE310-31A and AVG-1) to generate 22 F1 hybrids using line x tester mating design. The 22 F1 hybrids along with their parents and one check were evaluated at Bambey, Darou and Sinthiou Maleme for heterosis and combining ability of the parents. The major constraints limiting sorghum production in Senegal were striga, pests, and low soil fertility. Among the diseases, grain mold was ranked second after damping-off. Farmers are aware on sorghum grain mold. The black, pink and white discolorations on the grain surface were used by farmers to identified the disease. Farmers are aware that rain after maturity, high moisture and insects increased the disease severity. The most important traits farmers desired in an improved variety were : medium to short cycle, medium plant height, big and open or semi-compact panicles, with big and white grain. Significant and negative associations were noted between grain mold related-traits and yield component traits. At Bambey 42 F3 families outperformed the resistant parent Sureño while at Sinthiou Maleme ; five (5) F3 families were superior to the resistant parent Sureño. Of the 450 F3 families evaluated in six environments, 32 F3 families exhibited low grain mold severity in three or two environments. None of the F3 families showed stable resistance to grain mold in all the six environments. A total of 30 QTLs for 10 traits explaining each between 0.1 and 22.7% of the phenotypic variance were identified. Four minor QTLs were identified for mold resistance at physiological maturity two each on SBI-04 and SBI-05 and one QTL for mold resistance after harvest was found on SBI-03. Among the QTLs identified for grain mold at physiological maturity, QPGMR_S5SBI04 and QPGMR_D5SBI05 on SBI04 and SBI05, respectively were reported for the first time in this study. The QTLs for mold resistance, time to flowering, panicle length, panicle width and thousand grain weight co-localized on SBI-05. QTLs for seven important traits on sorghum such as time to flowering, plant height, panicle length, panicle width, panicle weight, grain weight per plant, and thousand grain weight were also identified. Both additive and non-additive genes controlled most of the studied traits. The results on combining ability identified the parental line Dorado as the best combiner for both resistance to grain mold and yield and the parental lines F2-20, Macia, and Faourou as the best general combiners for grain mold resistance. Based on specific combining ability (SCA) effects for grain mold resistance and grain yield, the hybrid cross CE 310-31A x Sureño was recommended for further testing and release. The best mid-parent heterosis for grain yield and resistance to grain mold were seen in CE310-31A x F2-20, CE310-31A x Sureño and AVG-1 x Dorado. Based on their mean performance and heterosis, the hybrid crosses CE310-31A x Dorado, CE310-31A x Darou, CE310-31A x Sureño, AVG-1 x Dorado, AVG-1 x Darou, AVG-1 x Nguinthe, and AVG-1 x Macia were retained for further multi-location trial and release.


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