Cassava mosaic virus
Cassava mosaic virus is the common name used to refer to any of eleven different species of plant pathogenic virus in the genus Begomovirus. African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), and South African cassava mosaic virus (SACMV) are distinct species of circular single-stranded DNA viruses which are transmitted by whiteflies and primarily infect cassava plants; these have thus far only been reported from Africa. Related species of viruses (Indian cassava mosaic virus, ICMV) are found in India and neighbouring islands (Sri Lankan cassava mosaic virus, SLCMV), though cassava is cultivated in Latin America as well as Southeast Asia. Nine species of cassava-infecting geminiviruses have been identified between Africa and India based on genomic sequencing and phylogenetic analysis. This number is likely to grow due to a high rate of natural transformation associated with CMV. The viruses are members of the family Geminiviridae and the genus Begomovirus. The first report of cassava mosaic disease (CMD) was from East Africa in 1894. Since then, epidemics have occurred throughout the African continent, resulting in great economic loss and devastating famine. In 1971, a resistant line of cassava, the predominant host of the pathogen, was established and used by the International Institute of Tropical Agriculture in Nigeria. This resistance worked as an effective control for many years. However, in the late 20th century, a more virulent virus broke out in Uganda and quickly spread to East and Central Africa. This highly virulent strain was later discovered to be a chimaera of two distinct Begomovirus species. CMD is primarily managed through phytosanitation practices as well as the use of conventional resistance breeding. Additionally, vector management and cross-protection help to minimize transmission and symptom development. Though management practices are useful, the viruses’ high rate of recombination and co-infection capabilities have caused CMD to be one of the most detrimental diseases affecting food supply in Africa. Hosts and symptoms Cassava originated in South America and was introduced to Africa in relatively recent times. It is known to be a very drought-tolerant crop with the ability to yield even when planted in poor soils. When cassava was first grown in Africa, it was used for subsidiary purposes though it is now considered to be one of the most important food staple crops on the continent. Its production is moving toward an industrialized system in which plant material is used for a variety of products including starch, flour, and animal feed. As cassava is vegetatively propagated, it is particularly vulnerable to viruses and thus Cassava geminiviruses lead to great economic loss each year. When these infect a host plant, the plant’s defense system is triggered. Plants use gene silencing to suppress viral replication, though begomoviruses have evolved a counter-acting suppressor protein against this natural host defense. Because different species of Begomovirus produce different variants of this suppressor protein, co-infection by multiple species typically leads to more severe disease symptoms. Initially following infection of a cassava geminivirus in cassava, systemic symptoms develop. These symptoms include