Cassava Root Necrosis Disease (CRND): A New Crop Disease Spreading in Western Democratic Republic of Congo and in Some Central African Countries

Cassava is consumed in the Democratic Republic of Congo (DRC) as a staple food for the majority of the Congolese population. This crop is used in several forms- as fufu, chikwangue and pondu; cassava leaves are the most consumed vegetable in the country. In 2002, cassava root symptoms similar to cassava brown streak disease (CBSD) were reported for the first time in western DRC. PCR assays, using primers specific to Cassava brown streak virus (CBSV), failed to detect or identify any viral pathogens in diseased cassava samples from western DRC. Therefore, next generation sequencing (NGS) techniques were used as they are able to sequence full organism genomes and are widely used for the identification of pathogens responsible for new diseases. The main objective of this study was to identify the pathogens causing root necrosis in western DRC. Whatman®FTA™ cards were used to collect 12 cassava leaf samples from plants with symptoms indicative of very severe root necrosis, as well as two asymptomatic samples. These 12 samples were sent to Australia at the University of Western Australia in Perth for next generation sequencing (NGS) using the Illumina HiSeq platform. Additional bioinformatics tools included Geneious, CLC workbench, ParaKraken and Kaijou software for short DNA sequences. No viruses (including CBSV) were found in any of the DRC samples. These preliminary results confirm all the previous negative results obtained using PCR and CBSV primers. However, NGS analyses did reveal the presence of a number of bacterial and fungal taxa. These will require further investigation and tests such as the Koch Postulates, to establish their specific pathogenic role in cassava. This is the first scientific evidence that no currently known virus is responsible for the disease which had been referred to previously as ‘CBSD-like disease’. Consequently, the disease found in DRC cassava samples has been designated ‘Cassava Root Necrosis Disease’ or CRND.