Runyambo Irakiza

Genetic variation and host-parasite specificity of Striga resistance and tolerance in rice: the need for predictive breeding.

Summary The parasitic weeds Striga asiatica and Striga hermonthica cause devastating yield losses to upland rice in Africa. Little is known about genetic variation in host resistance and tolerance across rice genotypes, in relation to virulence differences across Striga species and ecotypes. Diverse rice genotypes were phenotyped for the above traits in S. asiatica‐ (Tanzania) and S. hermonthica‐infested fields (Kenya and Uganda) and under controlled conditions. New rice genotypes with either ecotype‐specific or broad‐spectrum resistance were identified. Resistance identified in the field was confirmed under controlled conditions, providing evidence that resistance was largely genetically determined. Striga‐resistant genotypes contributed to yield security under Striga‐infested conditions, although grain yield was also determined by the genotype‐specific yield potential and tolerance. Tolerance, the physiological mechanism mitigating Striga effects on host growth and physiology, was unrelated to resistance, implying that any combination of high, medium or low levels of these traits can be found across rice genotypes. Striga virulence varies across species and ecotypes. The extent of Striga‐induced host damage results from the interaction between parasite virulence and genetically determined levels of host–plant resistance and tolerance. These novel findings support the need for predictive breeding strategies based on knowledge of host resistance and parasite virulence.

Labor-Saving Weed Technologies for Lowland Rice Farmers in sub-Saharan Africa

Time requirements, weed control efficacy, and yield effects of three labor-saving weed technologies were tested against hand weeding during three seasons in 2012 and 2013. The technologies included two hand-operated mechanical weeders, the straight-spike and the twisted-spike floating weeder, and the PRE application of oxadiazon. The straight-spike floating weeder reduced weeding time by 32 to 49%, the twisted-spike floating weeder reduced weeding time by 32 to 56%, and the application of herbicide required 88 to 97% less time than hand weeding. Herbicide application provided the best weed control in two of the three seasons. No differences in weed control efficacy were observed between mechanical and hand weeding. Yield differences were only observed in season 3 with higher rice yields after PRE application of oxadiazon compared with other weed management treatments. Nomenclature: Oxadiazon; rice, Oryza sativa L. Los requisitos de tiempo, la eficacia del control de malezas, y los efectos en el rendimiento de tres tecnologías para reducir las labores manual fueron evaluados en comparación con la deshierba manual durante tres temporadas productivas en 2012 y 2013. Las tecnologías incluyeron dos cultivadores mecánicos operados manualmente, el cultivador flotante de picos rectos, el cultivador flotante de picos curvados, y la aplicación PRE de oxadiazon. El cultivador flotante de picos rectos redujo el tiempo de deshierba de 32 a 49%, el cultivador flotante de picos curvados redujo el tiempo de deshierba de 32 a 56%, y la aplicación de herbicidas requirió 88 a 97% menos tiempo de deshierba manual. La aplicación de herbicidas brindó el mejor control de malezas en dos de las tres temporadas. No se observaron diferencia en la eficacia en el control de malezas entre la deshierba mecánica y la manual. Las diferencias en el rendimiento fueron solamente observadas en la tercera temporada con mayores rendimientos del arroz después de la aplicación PRE de oxadiazon al compararse con los otros tratamientos de manejo de malezas.