Brown streak virus disease is the most important biotic constraint to cassa
va production in the coastal areas of southern Tanzania. Symptoms include f
oliar chlorosis and sometimes stem lesions. The disease also affects the tu
berous roots which develop a yellow/brown, dry, corky necrosis within the s
tarch-bearing tissues, sometimes accompanied by pitting and distortion, tha
t is visible externally. The foliar symptoms of the disease often do not gr
eatly affect plant growth, although the most sensitive cultivars may be stu
nted and defoliated. The main impact of the disease on the crop is by causi
ng root necrosis. Field experiments were conducted at two sites in Tanzania
to determine the effect of the disease on yield and quality of the roots.
Cassava brown streak disease (CBSD) decreased root weight and patches of ro
ot necrosis made roots unmarketable, although the unaffected parts might st
ill have been suitable for home consumption. The disease therefore has two
effects, one on total root yield and one on root quality, which affects mar
ketability. The field trials showed that CBSD can decrease root weight in t
he most sensitive cultivars by up to 70%. The length of time between the ap
pearance of foliar symptoms and the development of root necrosis is a varie
tal characteristic. In the most susceptible cultivars, root necrosis may ap
pear within 6 months of planting cuttings derived from symptomatic mother p
lants. A local cultivar known as cv. Nachinyaya exhibited a form of toleran
ce to CBSD in which foliar symptoms appeared but the development of root ne
crosis was delayed allowing the full yield potential to be realized.