Evaluation and Deployment of Rice Genotypes Resistant to Rice Yellow Mottle Virus Disease in the Southern Highlands of Tanzania

Abstract

Rice yellow mottle virus (RYMV) is the most important biotic stress undermining productivity of the crop envisaged to spur economic growth and improvement of livelihoods in Tanzania. In efforts to exploit economic and ecofriendly natural host resistance for sustained disease management, reactions of 60 promising rice genotypes selected from various germplasm to a mixture of isolates of the virus under artificial inoculation in the Southern highlands zone represented by Kyela district were studied. Twenty of the most promising entries were further verified on farm at two lowland rainfed and two upland ecologies in the district. Resistance or susceptibility of the genotypes was determined by severity of symptoms as scored on a standard international evaluation scale ranging from 1 to 9 and plant height reduction measured as the difference in height between inoculated and uninoculated plants. Results indicated that RYMV isolates used differed by their virulence and the varieties by their vertical resistance. Observed reactions ranged from highly susceptible to highly resistant. About 50% of the genotypes showed a high level of resistance while 20% were highly susceptible. The intermediate reactions were 0% resistant, 10% susceptible and 20% moderately resistant. Susceptible cultivars recorded up to 75% reduction in height and death of plants whereas those resistant had mild symptoms and negligible plant height reduction. Highly resistant genotypes were immune to infection by the virus across all locations. Five of the highly resistant cultivars namely Salama M55, M57, M19, M35, IITA 235 were acceptable to farmers in terms of cycle length, plant and grain types. Further research to enable official release for commercial use and wide scale accessibility of seed to farmers was deemed necessary to ease damage from RYMV. The varieties can also be deployed as sources of resistance in cross breeding programmes to improve the local susceptible cultivars whose background is already acceptable to farmers. Exhaustive screening of germplasm collections to identify diverse sources of novel genes for resistance was also recommended together with verification of high resistance of genotypes across regions.