Cassava brown streak virus disease

Cassava brown streak virus disease (CBSD) is a damaging disease of cassava plants, and is especially troublesome in East Africa. It was first identified in 1936 in Tanzania, and has spread to other coastal areas of East Africa, from Kenya to Mozambique. Recently, it was found that two distinct viruses are responsible for the disease: cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). Both have (+)ss RNA genomes, belong to the genus Ipomovirus in the family Potyviridae, and produce generally similar symptoms in infected plants. Root rot renders the cassava tuber inedible, resulting in severe loss of economic value; therefore, current research focuses on achieving cultivars that do not develop the necrotic rot. This disease is considered to be the biggest threat to food security in coastal East Africa and around the eastern lakes. CBSD is characterized by severe chlorosis and necrosis on infected leaves, giving them a yellowish, mottled appearance. Chlorosis may be associated with the veins, spanning from the mid vein, secondary and tertiary veins, or rather in blotches unconnected to veins. Leaf symptoms vary greatly depending on a variety of factors. The growing conditions (i.e. altitude, rainfall quantity), plant age, and the virus species account for these differences. Brown streaks may appear on the stems of the cassava plant. Also, a dry brown-black necrotic rot of the cassava tuber exists, which may progress from a small lesion to the whole root. Finally, the roots can become constricted due to the tuber rot, stunting growth. Typically the affected plants do not possess all of these characteristics, but those that are severely affected may. Farmers may be unaware of their infected cassava crops until they are harvested and see the tuber lesions because leaves are asymptomatic. The cassava mosaic virus (CMV) is another cassava virus that exhibits foliar symptoms similar to CBSD, but they are more obvious. UCBSV has milder symptoms than CBSV, and lower pathogenicity. After a period of ambiguity among researchers, the consensus is that the most likely candidate of CBSD vector is Bemisia tabaci biotype B, the silverleaf whitefly. It is also sometimes referred to as Bemisia argentifolii. There is a close association between surging whitefly populations and CBSD incidence. This whitefly species is also considered to be the vector of CMV. It is suggested, however, that B. tabaci whiteflies transmit CBSVs less effectively than CMVs. Also, the CBSD retention period in B. tabaci may not exceed more than 24 h, but more research is needed to confirm this. The adult B. tabaci lives an average of sixteen days, and the maturation process from egg to adult is thirty days. Eggs may be deposited haphazardly or in a spiral fashion on the leaf undersides. Both juvenile and adult whiteflies feed on the phloem of the leaves by inserting a sucker mouth part into the leaf, thereby transmitting the virus to the plant. Saliva containing toxins is also injected into the cassava plant while whiteflies feed, disturbing plant growth and ultimately reducing yield. Seedlings are particularly affected. After its first identification in 1936, CBSD was almost totally eliminated in Uganda due to program efforts, and there were relatively small yield losses in affected areas. The disease was restricted to < 1000 m above sea level along coastal Kenya to Mozambique, and the shores of Lake Malawi. However, as of the year 2000, CBSD has spread rapidly throughout Eastern Africa.Midaltitude levels (1200–1500 m above sea level) now accommodates CBSD, as it has been reported in Uganda, Democratic Republic Congo, and around Tanzanian lake zones. As of 2009, CBSD outbreaks were most prevalent in south - central Uganda and in Mara Region. There have also been reports of CBSD in Rwanda and Burundi. The incidence of CBSD is greatest in Uganda where there is resistance to CMV in cassava and other locations in general. Recent surveys demonstrated that of the 23 districts in Uganda surveyed, 70% had CMD-resistant cassava varieties, and all are vulnerable to CBSD – causing viruses. These varieties also hosted as many as 200 adult whiteflies on the top five cassava leaves. Predictions cannot be made about the spreading pattern. Because the disease does not fan out from only one source point, but rather appears in remote areas, or “hot spots,” models are difficult to devise. This challenge may arise from the movement of cuttings from infected regions and/or an abundance of whiteflies in a particular area.