Synergistic crop virus disease complexes in Sub-saharan Africa: causes, consequences and control

Many plant virus diseases of major economic importance in sub-Saharan Africa are caused by disease complexes resulting from synergistic interaction of two or more viral agents known to enhance disease severity inflicting heavier crop losses. The most destructive of these are maize lethal necrosis disease (MLND), sweet potato virus disease (SPVD), cassava mosaic disease (CMD), cowpea mosaic disease (CPMD), groundnut rosette disease (GRD) and tobacco bushy top disease (TBTD). MLND, SPVD, CMD and CPMD are caused by synergistic interaction of two independent viruses in mixed infection, whereas GRD and TBTD represent helper-dependent synergism in which the multiple agents involved in the disease complexes which include an umbravirus, a polerovirus and a satellite RNA synergistically interact with each other for their survival and spread. Mixed virus infections can cause disease synergism due to viral suppression of RNA silencing of host defense, an increase in viral replication, enhanced viral movement or any combination of these. Each disease complex has its own characteristics, and a variety of factors affecting its epidemiology must be considered when devising diagnostic tools and management options. All the causal viruses are transmitted by insect vectors such as aphids, whiteflies, thrips, or beetles while some are transmitted by seeds or vegetative propagation. Although the diagnosis of the multiple agents is more complicated than those with single infections, multiplex methods primarily based on serology, PCR and next generation sequencing are available and widely used. This paper briefly addresses the etiology, symptoms, economic importance, synergistic mechanisms, diagnosis, field spread and management practices of these disease complexes and discusses future research needs.