In general, viruses of Potyviridae (potyvirids) have genomes ∼10 kb in length that encode a polyprotein that undergoes proteolytic processing to generate 10 mature products ( Revers and García 2015). These are positive-sense, single-stranded RNA viruses belonging to the genus Ipomovirus of the family Potyviridae. 2001).ĬBSD can be caused by individual or simultaneous infection with Cassava brown streak virus (CBSV) or Ugandan cassava brown streak virus (UCBSV) ( Hillocks et al. Losses of up to 70% have been reported, highlighting the current impact of CBSD and potential damage should the epidemic continue its spread toward the populous, high cassava production countries of West Africa ( Hillocks et al. The disease is named for brown streaking of cassava stems but also causes leaf chlorosis, reductions in storage root yield, and necrosis within the storage roots ( Fig. tabaci, but distributed over long distances by transport of infected planting materials. Historically, the disease was confined to the coastal lowlands of eastern sub-Saharan Africa however, during the past two decades, CBSD has spread to East and Central Africa's Great Lakes region ( Storey 1936). Like CMD, cassava brown streak disease (CBSD) has a long history in sub-Saharan Africa and was first reported in 1935 ( Patil et al. (For a more detailed summary of CMD and associated resistance, see Akano et al. CMD2 and CMD3 both map to the same arm of chromosome 12. CMD1 is thought to be polygenic, whereas CMD2 segregates as a dominant single locus. None of the underlying genetic determinants have been identified. Three types of genetic resistance have been described for CMD, called CMD1, CMD2, and CMD3. To date, nine species account for CMD found in Africa, whereas two other species are present in Asia ( Rey and Vanderschuren 2017). Geminiviruses are often found as mixed infections in plants, which when coupled with a propensity for recombination allows for the emergence of new virus species and variants ( Hanley-Bowdoin et al. The genomes of CMGs are small, circular, and bipartite with separately packaged DNA-A and DNA-B molecules (each ∼2.7 kb) that together have eight to nine genes encoding multifunctional proteins. CMGs are single-stranded DNA viruses of the genus Begomovirus, family Geminiviridae. The causative agents of CMD are cassava mosaic geminiviruses (CMGs), which are vectored by the whitefly Bemisia tabaci ( Legg et al. ( C) Cross-section of a cassava storage root revealing brown necrotic tissue caused by cassava brown streak disease. ( B) Cassava leaves showing developmental defects associated with cassava mosaic disease. ( A) Healthy cassava grown in a test plot. In this article, we provide an overview of the major microbial diseases of cassava, discuss current and potential future efforts to engineer new sources of resistance, and conclude with a discussion of the regulatory hurdles that face biotechnology.Įffects of viral diseases on cassava. Of note are cassava bacterial blight (CBB), cassava mosaic disease (CMD), and cassava brown streak disease (CBSD), all of which can cause catastrophic losses for growers. In particular, bacterial and viral diseases can cause severe yield losses. Cassava has seen strong demand since 2000 and production has increased consistently year-over-year, but potential yields are hampered by susceptibility to biotic and abiotic stresses. Cassava root starch also has use in the pharmaceutical, textile, paper, and biofuel industries. In parts of sub-Saharan Africa, cassava is particularly crucial for food security. Today, cassava is grown in the tropics of South America, Africa, and Asia with an estimated 800 million people relying on it as a staple source of calories. Manihot esculenta Crantz (cassava) is a food crop originating from South America grown primarily for its starchy storage roots.
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