Rice Virus Diseases: Epidemiology and Management Strategies

Abstract

ABSTRACT There are now over 30 viruses reported to infect rice through experimental tests and in nature. However, only 25 are of any direct economic impact to rice production. And most of them are transmitted by either plant or leaf hoppers. The rice grassy stunt tenuivirus (RGSV), rice hoja blanca tenuivirus (RHBV), rice ragged stunt phytoreovirus (RRSV), rice wilted stunt virus (RWSV) (a relative of Rice grassy stunt tenuivirus (RGSV)), rice stripe tenuivirus (RSV), rice black streaked dwarf fijivirus (RBSDV) are plant hopper borne, while rice bunchy stunt pytoreovirus (RBSV), rice dwarf phytoreovirus (RDV), rice gall dwarf phytoreovirus (RGDV), rice transitory yellowing rhabdovirus (RTYV), rice yellow dwarf mycoplasma-like organisms (RYDMLO's), rice waika machlovirus (RWV), rice orange leaf virus (ROLV), maize streak geminivirus (MSV) strain A, and rice tungro machlovirus/badnavirus (RTV) are transmitted by leafhoppers. The African specific rice yellow mottle sobemovirus (RYMV) is transmitted by the Chrysomelid beetles, whereas rice chlorotic streak virus (RCSV) is transmitted by the rice mealybug. The rice guillame luteovirus (RGV) is aphidborne while rice necrosis mosaic luteovirus (RNMV) and rice necrosis furovirus (RNV) are transmitted by the fungus Polymyxa graminis. Two viruses, rice wrinkled stunt disease (RWSD) and witches' broom disease (WBD) are seedborne. The rice crinkle disease (RCD) is transmitted through soil while rice mosaic virus (RMV) is so far associated with contact transmission alone. While the majority of these viruses occur in countries of Asia and the Americas, only 5 (rice stripe necrosis furovirus, rice crinkle disease, maize streak geminivirus strain A, African cereal streak virus, and rice yellow mottle sobemo-virus) are known to infect rice in nature in Africa and its neighbouring islands. The underlying causes of epidemics of these viruses and their pests are primarily based on the intensification of rice cultivation in these areas with the introduction of new high yielding varieties in conjunction with increased use of irrigation facility, fertilizer, pesticides, changing cultural practices, crop monoculture, and obvious mechanical contamination. It is worthwhile to note that in Asia and the Americas it is the japonica rice types that are mostly susceptible to the viruses whereas in Africa and the neighbouring islands it is the indica rice types that are highly susceptible to the rice yellow mottle sobemovirus. The management strategies for rice virus diseases involve the use of screening and breeding for varietal resistance, chemical treatment, plant quarantine, efficient cultural practices, integrated pest management, training for disease management, genetically engineered resistance and constant collaboration with other laboratories working to achieve the same goal of disease management.