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Incidence of Rice yellow mottle virus (RYMV) on rice plants (ofada) grown in two local government areas (LGAs) of Ogun State had been evaluated during a two year field survey. Six month old rice plants were observed for symptom expression and leaf samples collected for serological indexing. Of the 60 leaf samples collected for both years, 96.7 % were presented with symptoms and severity ranged from 1 to 5 in 2012. Symptoms of brown blotches, chlorosis, leaf yellowing and necrosis were observed on rice plants from which samples were collected at Ewekoro LGA. Similar symptoms were also seen on rice plants at Obafemi Owode. Disease incidence due to RYMV was 7.7 % at Ewekoro in 2012 while it was 13.3 % for plants surveyed at Obafemi owode in the same year. In 2013, incidence varied from 30.0 to 90.0 % and the higher incidence was observed on samples at Obafemi owode. Serodiagnosis of Grasshopper (Zonocerus variegatus) and Locust (Locusta migratoides) trapped at the rice fields during the survey revealed that they were not implicated in virus transmission as their absorbance values at spectrophotometric wavelength of 405 nm were not up to one and half times the values of the healthy controls.Keywords: rice, ELISA, incidence, virus symptom, survey Correspondence: sdara@hotmail.com IntroductionWith increase in world population, there is emphasis on increasing level of food production, among which is rice. Rice (Oryza spp.) is grown widely in many parts of the world. It is the major food source for about 40 % of the world's human population (Ortiz, 2011) Production of rice in Nigeria is being threatened by infections from various pathogens such as Rice yellow mottle virus (RYMV) and others such as Maize streak virus (MSV) (Alegbejo, 2013). The virus, Rice yellow mottle virus (RYMV) is a non-enveloped isometric virus belonging to the genus Sobemovirus. Its particle size is 25 nm in diameter with conspicuous capsomere arrangement. The biochemical properties of RYMV include its virions containing 23 % nucleic acid, 77 % protein and 0 % lipid. It is a RNA virus, single stranded and its total genome size is 4.2 kb (kilobase).

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A field survey was conducted in eight local government areas (LGA) of Ogun state, Nigeria to assess the incidence of viral diseases of yams in the areas. Leaf samples were collected from 90 yam plants which were either symptomatic or asymptomatic. These were bulked into 45 during serological tests and the viruses indexed include yam mosaic virus (YMV); Dioscorea alata bacilliform virus (DaBV) and cucumber mosaic virus (CMV). DaBV was the most prevalent virus on the field with incidence of 48.9% (22/45) followed by YMV which occurred in 42.2% (19/45). CMV had the lowest percentage of incidence; 2.2% (1/45). Of all the LGAs visited, Abeokuta north and Abeokuta south had the highest incidence of YMV and DaBV, respectively. Mixed virus infections were also detected.

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