Studies of Sorghum (Sorghum bicolor (L) Moench) resistance to downy mildew (Peronosclerospora sorghi) disease in Uganda
Sorghum (Sorghum bicolor [L.] Moench) is a multi-purpose crop ranked fifth as the World’s major cereal after Maize, Wheat, Rice and Barley in terms of worldwide production and acreage. In Uganda, it is the third most important staple cereal food crop after maize and millet occupying over 400,000 ha of the total arable land with a production of 580,000 metric tonnes. Despite the economic importance of Sorghum, its production is severely constrained by a downy mildew disease. Sorghum downy mildew (SDM) is caused by a seed-borne obligate fungus Peronosclerospora sorghi, which limits the production and productivity of the crop. It affects sorghum plants from seedling till harvesting stage, exhibiting both localized and systemic infections with recurrent epidemics. Economic losses reaching 100 % have been reported depending on the cultivar type, source of planting material (seeds), environment, and developmental stage of the host plant. There is currently no SDM resistant cultivar released in Uganda though the disease is endemic. Host plant resistance in disease management is sustainable and environmentally safe method for limiting inoculum build-up, spread and development of disease. The objectives of this study were to: (i) map the prevalence and distribution of sorghum downy mildew (SDM) disease and sample collection of P. sorghi biotypes; (ii) characterize the biotypes of Peronosclerospora sorghi in sorghum (iii) identify sources of resistance to downy mildew disease and (iv) determine the genetics of sorghum resistance to downy mildew disease. Field surveys were carried in 13 major sorghum-growing districts across 10 agro-ecological zones. Results from the survey showed a significant (P< 0.001) mean incidence of 49.4% and 78.9% for Pader and Namutumba, respectively. Arua, Namutumba and Pallisa were identified as SDM disease hot spots with a severity score above 3.0. Morphological and molecular characterizations of 195 P. sorghi isolates in Uganda were carried out. Cluster analysis of 195 P. sorghi isolates based on their morphological structures showed three significantly distinct morphological groups, however, seven distinct clusters were identified at the molecular level. Mantel test revealed no association between genetic differentiation and geographical distance and therefore implied that breeding different resistant sorghum cultivars for the different agro-ecologies in Uganda is not necessary. A total of 100 sorghum genotypes, including five introductions from USDA-USA were screened at Makerere University Agricultural Research Institute Kabanyolo (MUARIK) and Abi-Zonal Agricultural Research and Development Institute (Abi-ZARDI) research station at Arua. Out of the 100 screened genotypes, two resistant and four moderately resistant genotypes were identified, indicating variation in SDM disease pressure across the two locations. Subsequently, six male lines were crossed with three female lines using North Carolina II mating design to generate 18 hybrids and advanced to F2 seeds. The F2 hybrids together with the 9 parental lines were evaluated at two different locations (Arua and Kabanyolo) to study the heritability and gene action controlling resistance to P. sorghi. General combining ability (GCA) and specific combining ability (SCA) across the two locations showed significant differences (P < 0.001 - P < 0.05) for four traits viz., Area under disease progress curve (AUDPC), plant height, 1000 seed weight and grain yield. Parents P1 656061 and P1 533831 were the best combiners for improvement in SDM resistance, early maturity, 1000 seed weight and grain yield, this implied that they could be utilized as parents in sorghum breeding. Four crosses P1 669760 x P1 533831, P1 656010 x P1 655990, SESO 1 x E 40, and SESO 1 x Epuripur significantly improved resistance to SDM while three crosses P1 656010 x E40, SESO x E 40, and SESO 1 x P1 656061 improved yield component (grain yield and 1000 seed weight). Both additive and non-additive gene action effects were involved in traits for AUDPC, plant height, 1000 seed weight and grain yield. This study found sources of resistance to downy mildew disease in Uganda germplasm collection as well as the introduced lines from USDA. Two resistant parents identified as best combiners for SDM and grain yield are recommend as parents in a sorghum breeding programme. The seven crosses aforementioned are recommended as progenies in a breeding programme targeting yield improvement and SDM resistance.