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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.〔(【引用サイトリンク】 New Spread of Cassava Brown Streak Virus Disease and its Implications for the Movement of Cassava Germplasm in the East and Central African Region )〕 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.
==Symptoms==
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.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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