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The name brown streak was given to the disease from the brown lesions which sometimes appear on young green stems. These were the first symptoms of the disease to be recognised; however stem lesions are not the most characteristic symptom of infection and occur only infrequently.
Unlike symptoms induced by the majority of plant viruses, those of CBSD in cassava normally affect mature or nearly mature leaves but not expanding, immature leaves. They consist of a characteristic yellow or necrotic vein banding which may enlarge and coalesce to form comparatively large, yellow patches. Tuberous root symptoms may also be present: these consist of dark-brown necrotic areas within the tuber and reduction in root size; lesions in roots can result in post-harvest spoilage of the crop. Leaf and/or stem symptoms can occur without the development of tuber symptoms; thus, of plants with above-ground symptoms surveyed in southern Tanzania, 21% failed to develop root necrosis (Hillocks et al., 1996).
The symptoms of the disease vary greatly with variety and environmental conditions, making diagnosis difficult, particularly when plants are infected both with CBSD and cassava mosaic disease.
The virus species UCBSV and CBSV cannot be differentiated according to symptoms induced in cassava. However several cassava cultivars are not susceptible to UCBSV (Winter et al., 2010).
CBSD has until recently received such limited research attention that current control recommendations are largely based on general pathological principles and experience rather than on extensive research (Hillocks, 1997).
Cassava is usually propagated vegetatively from hardwood stem cuttings, and CBSD is perpetuated and disseminated in this way. There is a considerable traffic in plant material within and sometimes between countries and inevitably CBSVs are disseminated in this way. Until recently, selected 'virus-free' planting material was seldom available, until the Gates-funded Great Lakes Cassava Initiative in the six countries of Kenya, Uganda, Tanzania, Burundi, Rwanda and eastern parts of DRC, except when produced specially for official introductions using meristem-tip therapy and/or heat therapy in order to meet quarantine requirements. It is important that such requirements are strictly enforced because of the present limited distribution of CBSD in parts of eastern, central and southern Africa.
A basic approach to control should be the use of virus-free planting material (Thresh et al., 1994). With CBSD there are difficulties in selecting virus-free material because the symptoms of infection can be vague and indistinct. However, effective means of detection are now available and being utilized. Furthermore, it is likely that considerable improvement could be made in the present unsatisfactory situation by selecting cuttings for propagation only from plants that have been inspected during growth and also at harvest and found to be free of leaf, stem and root symptoms. Foundation stocks could be established in this way for subsequent multiplication and distribution to farmers. There are also advantages in farmers selecting propagating material only from unaffected plants at the time they collect cuttings. Unfortunately, however, farmers are not familiar with the whole range of CBSD symptoms, and cuttings may be collected at times when the source plants are almost leafless or severely affected by the cassava green mite (Mononychellus tanajoa), cassava mealybug (Phenacoccus manihoti), leaf spot or bacterial blight. Further studies are required on the most effective means of selection and on the ways in which simple procedures can be introduced to farmers.
The differences between varieties in their sensitivity to infection have long been exploited by farmers in coastal Tanzania and elsewhere who tend to discard those severely affected by CBSD and retain those that grow and yield satisfactorily even when infected. There is certainly a high turnover in cassava varieties as recorded in many parts of Africa and influenced by various pests and diseases (Nweke et al., 1994).
Early experience in Tanzania showed the scope for breeding resistant or tolerant varieties. Moreover, in crop improvement programmes there is a need to evaluate the response of introduced genotypes before they are released for use by farmers in areas where CBSD is prevalent.
Integrated Pest Management
There is a general acceptance of the need to develop overall IPM programmes for the whole range of cassava pests and diseases (Thresh et al., 1994). However, little progress has been made in developing such programmes, even though there are likely to be important interactions between CBSD and other diseases, and also with cassava mealybug and green mite.
CBSD was formerly of limited importance in Africa as a whole because of its restricted distribution along lowland coastal areas of Kenya and Tanzania and in parts of Malawi (Legg and Raya, 1998; Hillocks et al., 2001; Gondwe et al., 2003; Shaba et al., 2003). However, the disease was later found to be prevalent in northern Mozambique (Hillocks et al., 2002). Since 2004 it has been causing increasingly severe problems in Uganda and the Lake Victoria zone of Tanzania. It has also been detected recently in Rwanda, Burundi and parts of DRC.
There is only limited evidence as to the effects of CBSD on vegetative growth and on the yield of tuberous roots (Bock, 1994; Hillocks et al., 2001). Varieties differ greatly in their sensitivity and response to infection. The growth and yield of sensitive varieties are severely affected as there is extensive dieback of the stems, and the tuberous roots develop extensive necrosis and rot to such an extent that they are virtually worthless. Thus, field trials have shown that CBSV can decrease root yields of the most sensitive varieties by 70% and induce necrosis of roots which renders them unsalable (Hillocks et al., 2001).Tolerant varieties are much less severely affected and there is little effect on root yield or quality; cv. Nachinyaya, a local cultivar in coastal areas of southern Tanzania has a form of tolerance in which leaf symptoms are produced but the development of root necrosis is so delayed that the full potential yield is obtainable (Hillocks et al., 2001).
The results of three surveys conducted in Malawi during 2001-2002 showed that CBSD was widely distributed and the severity was high in low-lying areas, particularly along the shores of Lake Malawi, with farmers estimating losses as high as 60%. In a study of the economic impact of CBSD on small-scale farmers in Malawi, Gondwe et al. (2003) reported yield losses of 18-25%.