Benoit K. Bashizi

Cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are the two main viral diseases threatening cassava production in the Democratic Republic of the Congo (DRC). CMD can be effectively controlled using resistant varieties; however, currently, there are no cassava varieties that exhibit durable resistance to CBSD. As the deleterious effects of CBSD become more pronounced with the maturity of the cassava, we assessed the potential benefits of early harvesting on mitigating the impact of CBSD on the performance of two improved CMD-resistant cassava varieties, Nabana (MM 96/4653) and Sawasawa (MM 96/3920). The percentage yield reduction was calculated by comparing the yield of infected treatments to that of uninfected treatments. At 9 months after planting (MAP) and 12 MAP, overall fresh root yield from farmer-selected healthy planting material of varieties Nabana and Sawasawa was significantly higher than the yield of crops established using CBSD-infected cuttings of the same varieties. Fresh root yield losses due to CBSD for Nabana were 44% at 9 MAP and 86% at 12 MAP, while for Sawasawa, they were 40% at 9 MAP and 72% at 12 MAP. Healthy planting material of the two varieties yielded 5% more at 12 MAP than at 9 MAP, while infected planting material yielded 52.5% less at 12 MAP than at 9 MAP. These results highlight the benefits of using healthy planting material in conjunction with early harvesting to minimize losses among CBSD-susceptible cassava varieties. These results suggest the need for a robust seed system that can deliver disease-free planting material of market-preferred varieties to farmers, thereby promoting food security.

The present study characterizes Bemisia tabaci and Bemisia afer from cassava in eastern Democratic Republic of Congo (DRC). The Mitochondrial COI sequencing revealed the occurrence of six cassava B. tabaci mitotypes, which were designated into four haplogroups (SSA-ECA, SSA-CA, SSA2, and SSA-ESA) using KASP SNP genotyping. SSA-ECA (72%) was the most prevalent and occurred in the northern part of the surveyed area, in the Ituri and Nord/Sud-Kivu provinces, whilst SSA-CA (21%) was present in the south, primarily in Haut-Katanga. SSA-ECA was predominant in the areas of north-eastern DRC most severely affected by cassava brown streak disease and was also reported in the new outbreak area in Pweto territory, Haut-Katanga, in the south. Bemisia afer comprised two major clusters with 85.5% of samples in cluster one, while the rest were in cluster two, which has no reference sequence in GenBank. This study provides important information on the genetic diversity of B. tabaci and B. afer in eastern DRC. This knowledge will be used as a basis for further studies to understand and to identify the role of whitefly haplogroups, their population densities and consequences for virus epidemics and spread as well as leading to improved vector and virus management strategies.

Cassava plays a key role in ensuring food security and generating income for smallholder farmers throughout Central Africa, particularly in the Democratic Republic of Congo (DRC). This status is threatened, however, by cassava brown streak disease (CBSD), which has expanded its incidence and range in eastern DRC. The study described here comprises the first extensive assessment of temporal change in the occurrence of CBSD and its causal viruses in DRC, based on surveys conducted during 2016 and 2018. Cassava fields were inspected in Ituri, Nord-Kivu, Sud-Kivu, Tanganyika, and Haut-Katanga provinces within eastern DRC to record foliar incidence and severity of CBSD. Leaf samples were collected for virus detection and species-level identification. New occurrences of CBSD, confirmed by virus diagnostic tests, were recorded in two provinces (Haut-Katanga and Sud-Kivu) and nine previously unaffected territories, covering an area of >62,000 km², and at up to 900 km from locations of previously published reports of CBSD in DRC. Overall, average CBSD incidence within fields was 13.2% in 2016 and 16.1% in 2018. In the new spread zone of Haut-Katanga, incidence increased from 1.7 to 15.9%. CBSD is now present in provinces covering 321,000 km², which is approximately 14% of the total area of DRC. This represents a major expansion of the CBSD epidemic, which was only recorded from one province (Nord-Kivu) in 2012. Both Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus were detected in Ituri, Nord-Kivu, and Sud-Kivu, but only CBSV was detected in Haut-Katanga. Overall, these results confirm the increasing threat that CBSD poses to cassava production in DRC and describe an important expansion in the African pandemic of CBSD.