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 cassava production in the coastal areas of southern Tanzania. Symptoms include foliar chlorosis and sometimes stem lesions. The disease also a€ects the tuberous roots which develop a yellow/brown, dry, corky necrosis within the starch-bearing tissues, some- times accompanied by pitting and distortion, that is visible externally. The foliar symptoms of the disease often do not greatly a€ect plant growth, although the most sensitive cultivars may be stunted and defoliated. The main impact of the disease on the crop is by causing root necrosis. Field experiments were conducted at two sites in Tanzania to determine the e€ect of the disease on yield and quality of the roots. Cassava brown streak disease (CBSD) decreased root weight and patches of root necrosis made roots unmarketable, although the una€ected parts might still have been suitable for home consumption. The disease therefore has two e€ects, one on total root yield and one on root quality, which a€ects marketability. The ®eld trials showed that CBSD can decrease root weight in the most sensitive cultivars by up to 70%. The length of time between the appearance of foliar symptoms and the development of root necrosis is a varietal characteristic. In the most susceptible culti- vars, root necrosis may appear within 6 months of planting cuttings derived from symptomatic mother plants. A local cultivar known as cv. Nachinyaya exhibited a form of tolerance to CBSD in which foliar symptoms appeared but the development of root nec- rosis was delayed allowing the full yield potential to be realized.