Risk Background

Cassava is host to a number of serious exotic diseases, including:

1. Cassava mosaic disease is caused by a group of cassava-infecting geminiviruses belonging to the family Geminiviridae. Cassava plants infected by Cassava mosaic geminiviruses (CMGs) express a range of symptoms. The most typical symptoms consist of a yellow or pale green chlorotic mosaic of leaves, commonly accompanied by distortion and crumpling. Infected plants usually express typical symptoms within 3–5 weeks after infection (Legg & Thresh 2003).CMGs are transmitted by the whitefly (Bemisia tabaci) Gennadius (Dubern 1994). Cassava mosaic disease is estimated to cause an average yield loss of 30-40% in infected plants (Legg & Thresh 2003).
   

2. Cassava brown streak disease (CBSD) is caused by Cassava brown streak virus belonging to the family Potyviridae. Symptoms of the disease include foliar chlorosis and stem lesions. The disease also affects the tuberous roots which develop a yellow, brown, dry, corky necrosis within the starch bearing tissues (Legg & Hillocks 2003).
   

3. Cassava witches’ broom is caused by a strain of the ‘Candidatus Phytoplasma asteris’ group. In cassava, symptoms include loss of plant vigour, stunting, excessive proliferation of branches – ‘witches' broom’, purple discolouration of branches, shortened internodes and small leaves. Above ground symptoms are accompanied by a reduction in tuber size with rough textured skin and brown streaking of the flesh.
   

4. The most important bacterial disease of cassava, the bacterial blight, is caused by Xanthomonas axonopodis pv. manihotis.