Abstract
Rice yellow mottle virus (RYMV) is a major biotic constraint for rice production in Africa. The resistance-breaking ability of Tanzanian RYMV strains and phylotypes (S4lm (Tz526), S4lv (Tz516), S4ug (Tz508), S5 (Tz429, Tz445), S6c (Tz486) and S6w (Tz539)) were tested by inoculating rice cultivars with RYMV1 resistant alleles (Gigante (rymv1-2), Tog12387 (rymv1-3), Tog5681 (rymv1-3), Tog5438 (rymv1-4), Tog5672 (rymv1-4+rymv2) and Tog5674 (rymv 1-5)) in a screen house. The results revealed multiple resistance-breaking strains and phylotypes on resistant cultivars Gigante, Tog12387, Tog5438 and Tog5681. However, the resistance breakdown was highly variable depending on the strain used, and disease severity ranged from 11% - 75.3%. The virulence potential of RYMV phylotype S4lm (Tz526) was similar to phylotype S6w (Tz539). The impact of strains and phylotypes on yield and its components in rice cultivars revealed highly significant differences (P ≤ 0.001). The lowest percent plant height reduction (2.8%), number of tillers per plant (2.5%), 1000 grain weight (2.7%), spikelet sterility (3.5%) and yield (5%) was recorded in rice cultivar Gigante inoculated with RYMV phylotype S6c (Tz486). Phylotype S6c (Tz486) despite being less virulent compared to other strains, its virus titer in rice cultivar Gigante (1.833) was higher than S5 (Tz429, Tz445) inoculated on Tog5674 (0.171, 0.207) and S6w (Tz539) inoculated on Tog5681 (0.283). The resistant-breaking strain S5 (Tz445) multiplied in resistant rice cultivar Tog5674 without inducing visible symptoms but showed positive reaction to ELISA with low virus titer. The strain S5 overcame wide range of resistant alleles including rymv1-2, rymv1-3, rymv1-4 and rymv1-5 resistance, with exception of rymv1-4 + rymv2. The current results gave a new perspective for future identification of resistance-breaking mutations through sequencing of the RYMV genome in infected rice cultivars and mutagenesis of an infectious viral clone useful for future RYMV resistant breeding programs.
Highlights
High genetic diversity of Rice yellow mottle virus (RYMV) and evolution of resistant-breaking strains and phylotypes are challenges in developing durable disease resistant varieties
The resistance breakdown was highly variable depending on the strain used, and disease severity ranged from 11% - 75.3%
The results indicate that RYMV1 gene resistance breakdown was highly variable depending on the strain used, and disease severity ranged from 11% - 75.3%
Summary
High genetic diversity of RYMV and evolution of resistant-breaking strains and phylotypes are challenges in developing durable disease resistant varieties. Management of RYMV has relied on the use of resistant varieties. Insect vector control and prophylactic measures such as high surveillance of seedbeds, fields and weed reservoirs are used for management of RYMV despite time-consuming and variable efficiency [1] [2]. Most of resistance sources to RYMV obtained from Oryza glaberrima. Several highly resistant varieties with known genes of resistant to RYMV have been identified that include rice cultivars Gigante (rymv1-2) [3], Tog12387 (rymv1-3) [6], Tog5681 (rymv1-3) [4], Tog5438 (rymv1.4), Tog5672 (rymv1-4+rymv2) and Tog5674 (rymv1-5) [7]. Resistance conferred by RYMV2 and RYMV3 genes have currently been identified on Oryza glaberrima Tog7291 and Tog5307, respectively [2] [8]. Partial genetic resistance quantitative trait locus (QTLs) which is polygenic has been reported to be widespread in Oryza sativa subsp. The resistance has been estimated and measured the area under disease progress curve by assessing the disease severity at different number of days after inoculation [11]
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