Occurrence of cassava mosaic disease and characterization of casual viruses in Mozambique

Abstract

Cassava mosaic disease (CMD) is one of the major constraints to cassava production in Mozambique. In this research, two studies were carried out to monitor the incidence and severity of CMD and its causal viruses in the country. Field surveys were conducted in seven major cassava-producing provinces in 2013 and 2015. The disease was found in all seven provinces. Incidence ranged from 14.4% to 81.3% and 19.3% to 40.8% in 2013 and 2015, respectively. In comparison to previous surveys conducted in 2003, 2005 and 2006 results of this study suggest that the disease is generally decreasing. This trend could be attributed to increased use of disease-free planting material of local or improved CMD resistant varieties, as well as increased farmer knowledge on how to manage the disease. The second objective of the study was to investigate the genetic diversity of cassava mosaic begomoviruses (CMBs) based on partial genomes analyses of 45 isolates. Collection of CMD symptomatic leaves was done in May 2016 followed by molecular analysis using PCR-specific primers. PCR findings revealed the presence of East Africa cassava mosaic virus - Uganda Variant (EACMV-UG2) in Maputo province, southern Mozambique. This is the first report of EACMV-UG2in Maputo. PCR also detected East Africa cassava mosaic virus (EACMV), Africa cassava mosaic virus (ACMV) as well as mixed infections of the two viruses in the Southern provinces of Mozambique, except in Maputo; EACMV was the most widely distributed virus. Results from Sanger sequencing also revealed the presence of South Africa cassava mosaic virus (SACMV), East Africa cassava mosaic Cameron virus (EACMCV), East Africa cassava mosaic Kenyan virus (EACMKV) in addition to ACMV. This is the first detection of EACMKV in Mozambique. The virus isolates clustered together with reference isolates for the four species derived from GenBank in phylogenetic analyses. These results indicate a relatively high level of diversity of CMBs affecting cassava in Mozambique. The existence of different CMBs poses a threat to the sustainable production of cassava and consequently food security for cassava-dependent rural households. The high viral genetic diversity calls for more surveillance and due diligence in quarantine or restriction of material movement at entry points, coupled with cassava breeding programmes that should continuously develop new resistant varieties to viral diseases.