Abstract:
Rice yellow mottle virus (RYMV), genus Sobemovirus is a highly variable pathogen that is very infectious to rice plant. This variability hinders rice breeding for durable resistance to the virus and effective deployment of improved cultivars in Nigeria. This study aimed at characterizing RYMV isolates population in Nigeria. Disease survey study in 13 states (Ebonyi, Enugu, Ekiti, Ondo, Lagos, Kano, Kogi, Ogun, Oyo, Niger, Benue, Taraba and Edo) in Nigeria revealed RYMV disease incidence of 5-60 % in farmers’ fields and serological and biological indexing confirmed 90% of collected leaf samples positive to RYMV with 65% from rice varieties and 35% from weeds. The weed having 35% RYMV positive by serological indexing suggests reservoir host of RYMV in 13 states in Nigeria. Pathotyping 50 RYMV isolates against 10 differential varieties revealed two major RYMV pathotypes (RNgP1 and RNgP2). RNgP1pathotype was made up of 45 virulent (Vr) RYMV isolates while RNgP2 pathotype constituted 5 mildly virulent (MVr) RYMV isolates. Out of 10 rice varieties tested 6 rice varieties (V4=Gigante; V5=TOG5681; V6=FARO 52; V7=NERICA 19; V8=NERICA 42; V9=FARO 61) showed high level of resistance to all the 50 RYMV isolates while the rest varieties (V1=IR64; V2=Bouake189; V3=BG 90-1; V10=NERICA 8) were susceptible. Serological study using same 50 RYMV isolates revealed two major serogroups (RNgS1 and RNgS2), and four subgroups (RNgS1a, RNgS1b, RNgS2a and RNgS2b). RNgS1b serogroup comprised of only highly virulent RYMV isolates while RNgS1a, RNgS2a and RNgS2b serogroups were made up of mixture of both highly virulent and moderately virulent RYMV isolates. Molecular typing revealed three major (RNgMt1, RNgMt2 and RNgMt3) genotypes among 50 RYMV isolates. RNgMt2 genotype was made up two subgroup genotypes (RNgMt2a and RNgMt2b). RNgMt1 genotype has 13 virulent (Vr) RYMV isolates, RNgMt2a constituted 15, while RNgMt2b has 15 and 7 RYMV isolates were RNgMt3 genotype. Out of 45 RYMV isolates confirmed virulent (Vr) by pathotyping analysis, 92% and 86% were further confirmed virulent (Vr) by both serotyping and molecular typing analysis respectively thus establishing their correlation. Pathotyping, serotyping and molecular-typing studies revealed the distribution and population structure of RYMV isolates in 13 states of Nigeria. The phylogeny of RYMV isolates, using pathological, serological and molecular methods, should be useful for its surveillance in rice growing regions, in epidemiological studies to assess its identity and interaction as well as assist breeding programs aiming at the effective development of cultivars with durable resistance to RYMV disease.
