Effects of brown streak virus disease on yield and quality of cassava in Tanzania

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.