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.