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Several etiological and epidemiological studies have been undertaken to determine the
disease causal agent and the mechanism of spread of Cassava brown streak disease
(CBSD). Until recently, two distinct potyviruses have been reported to cause the disease.
These are Cassava brown streak virus (originated in Tanzania but most widely spread)
and Ugandan Cassava brown streak virus (reported in Uganda and a few areas in
Tanzania). Limited knowledge on the transmission mechanisms of the virus curtailed the
designing of practical CBSD management techniques. Transmission by the whitefly
vector, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae), and dissemination of
virus-infected cuttings are the reported mechanisms through which Cassava brown
streak virus (CBSV) is mostly spread. However, the occurrence and subsequent spread
of the disease in originally un-infected stock and in absence of B. tabaci is not
uncommon. Thus, the need to explore further, other transmission mechanisms and their
efficiency was paramount. In the current study, CBSV was successfully transmitted
through a series of non-vector techniques. Subsequent detection and confirmation of
CBSV infections were done by RT-PCR using coat protein gene-specific CBSV primers.
In replicated screen-house experiments, transmission of CBSV was achieved through
cutting tools (22 %) using susceptible cassava cv. Albert as test plants. Up to 54 %
transmission efficiency was achieved through sap inoculation of CBSV from infected
cassava to CBSV-free cv. Mreteta. Grafting CBSV-free susceptible scions onto CBSV infected rootstocks was the most efficient CBSV transmission technique with up to 100
% of scions infected within 4-weeks. The infected plants developed characteristic foliar
vein chlorosis and blotches on the previously symptomless CBSV-free scions. The virus
was not transmitted from infected root debris to cassava seedlings or virus-free cuttings.
The findings suggest that the non-vector methods, such as sap transmission, cutting tools
and leaf harvesting, could contribute significantly to CBSV spread in field and non-field
conditions, such as in propagation nurseries or cassava leaf handling for food. Moreover,
grafting was justified to be an effective technique to quickly test for susceptibility or
resistance of any newly bred cultivar for CBSD resistance |
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