Efficiency of non-vector methods of Cassava brown streak virus transmission to susceptible Cassava plants
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.
Keywords: Cassava, Cassava brown streak, Disease, grafting, potyviruses, non-vector transmission, RT-PCR
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