Survey of cassava mosaic and brown streak diseases and their vector whiteflies in Malawi

Cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are the two major threats to cassava production in eastern and southern Africa. There have been several interventions over the years in the fight against the two diseases. Whether these interventions have been effective or not can best be answered by continuous and periodic assessment of the diseases. This study was designed to determine the severity, incidence, and distribution of CMD and CBSD and their vector whiteflies in Malawi. Diagnostic surveys were carried out in 16 districts to assess CMD and CBSD parameters on the leaves in 2018. The magnitude of damage on cassava roots caused by CBSD was assessed in 12 districts. The study was designed also to identify alternative hosts for CMD and CBSD. The national mean CMD severity and incidence were 1.6 and 32%, respectively. The national mean CBSD severity and incidence were 1.3 and 75%, respectively. The highest number of whiteflies were recorded in Mangochi district with about 1.8 adult whiteflies per plant in farmer’s fields. There was positive correlation between disease incidence, severity and the number of adult whiteflies for both CMD and CBSD. CMD and CBSD incidence, severity and number of adult whiteflies negatively correlated with altitude. At national level, severity and incidence were higher for CMD than CBSD. CBSD foliar and root incidence and symptom severity were prominent along the shores of lake Malawi. In the contrary, CMD was not confined to the lakeshore areas. Karonga district had the highest CBSD root severity (2.1) and incidence (56%). The national mean percentage of cassava root necrosis was 5.3%. Based on these estimates, Malawi lost about US$ 42.7 million in 2018 due to CBSD root necrosis alone which could be extrapolated to the current cassava production systems. Therefore, it is imperative to improve disease management efforts while also considering the confirmed presence of alternative hosts for CMD in Malawi.