Abstract:
Screenhouse and field experiments were conducted between 2008 and 2010 to quantify yield loss due to Rice yellow mottle virus (RYMV) in some selected rice cultivars; determine the epidemiology of RYMV in relation to weather factors on the incidence of RYMV and insect vector population composition and identification studies; identify its alternative hosts; determine serotypes of the pathogen; and molecular characterization of the strains and their distribution in the northern states of Nigeria. The screenhouse experiment was arranged in a strip plot design with three replicates. Significant interaction between the effects of inoculation regimes and rice cultivars were found between the disease, growth and yield parameters. The critical periods of RYMV infection for most of the rice cultivars used was within 4 – 8 Weeks after sowing (WAS). Paddy yield losses of 6.59 – 79.11 % and 3.65 – 81.67 % were recorded on the test cultivars in rainy and dry seasons, respectively. Keeping the plants RYMV- free for up to 10 WAS resulted in grain yield almost similar to control. The epidemiology experiment was laid out in a randomized complete block design (RCBD) with three replicates. Significant positive and negative correlations were recorded between RYMV incidence and severity index, some insect vectors and weather factors. Insect species collected in and around the experimental field, included Oxya sp., Locris rubra Fabriculus,, Chnootriba similis Thunberg, Chaetocnema sp., Cheilomenes sp., Zonocerus variegatus L., Paratettix sp., Conocephalus sp., Nezara viridula L. and Dactylispa sp. they transmitted the virus to different degree. Serological studies showed that some weed species with or without symptoms collected, across RYMV infected areas in Northern Nigeria tested positive to RYMV. These included Imperata cylindrica L., Erograstic tenella L., E. Ciliaris L., Digitaria horizontalis Wild., Eleusine indica (L) Gaertner., E. corocana L., Cynodon dactylon (L) Pers., Panicum sp., Dactyloctenium aegyptium L., Brachiaria lata (Schumach) C.E. Hubbard ex Robyns., Axonopus compressus (Sw.) P. Beauv., Cyperus rotundus L., C. difformis L., Oryza barthii A. Chev., O. longistaminata A. Chev.& Roehr. and Pennisetum pedicellatum Trin. Six serological profiles (Sep.) or serotypes were identified (Sep A to F) in this study. Sep A was found in Gombe, Kaduna, Kano, Niger, Sokoto and Zamfara states, Sep B was found in Borno, Gombe, Kaduna, and Kano, states, Sep C was found in Gombe state, Sep D was found in Borno, Sokoto and Zamfara states, Sep E was found in Borno and Zamfara states and Sep F was found in Kano, Niger and Sokoto states. The molecular characterization of RYMV isolates revealed that more than two strains of the virus were present in northern Nigeria. Strain S1 is widespread in Borno, Gombe, Kaduna, Kano, Niger, Sokoto and Zamfara states. By contrast, strain S2 was only detected in Gombe, Kaduna and Niger states. Strains SX occurred in swampy sites in Borno, Gombe, Kaduna, Kano and Zamfara states. The information provided on the vulnerable phase of the plant growth on yield loss to the pathogen can be useful in integrated pest and production management for rice in RYMV endemic areas. The relationship established between RYMV incidences, insect vector population and variation in weather factors data could be used in RYMV disease magnitude prediction. In addition, Knowledge of weed hosts, potential insect vectors in and around RYMV infected fields, and distinction between isolates/strains is critical and would provide useful information in pathogenicity and breeding studies of RYMV in Northern Nigeria.
EPIDEMIOLOGY, MOLECULAR CHARACTERIZATION AND YIELD LOSS ASSESSMENT OF RICE DUE TO RICE YELLOW MOTTLE SOBEMOVIRUS IN NORTHERN NIGERIA
