Endemism and Reemergence Potential of the Ipomovirus Sweet Potato Mild Mottle Virus (FamilyPotyviridae) in Eastern Africa: Half a Century of Mystery

Abstract

Viruses have the ability to frequently colonize new hosts and ecological niches because of their inherently high genetic and evolutionary plasticity. However, a virus may emerge and remain of no or less economic importance until changes in viral and/or environmental factors dictate its epidemiological status. An example is sweet potato mild mottle virus (SPMMV), which was first reported in the 1970s on sweetpotatoes in eastern Africa, has remained endemic in the region and poorly understood, yet accounting for 60-95% losses especially in mixed infections. Unlike other sweetpotato viruses which have a global incidence, SPMMV has never been confirmed outside eastern Africa. This implicates the region as its center of origin, but does not fully account for SPMMV’s exclusive geographic delimitation to eastern Africa. Despite its importance, several mysteries and research gaps surround SPMMV, which decelerate efforts for effective virus disease management in sweetpotato. The aim of this review is to articulate research gaps, propose pivotal scientific directions and stimulate knowledge generation for better management of virus diseases in sweetpotato. Vector-mediated transmission of SPMMV remains enigmatic. Here we postulate testable hypotheses to explain SPMMV transmission. Comparisons between SPMMV and cassava brown streak ipomoviruses demonstrate epidemiological “hallmarks” for monitoring SPMMV. Evolutionary forces on SPMMV coupled with the virus’ broad host range imply a ‘silent build up’ of better fit variants in a changing climate, and this could explode into a worse disease conundrum. These information gaps need urgent filling to ease future management of virus disease emergences in sweetpotato.

Cassava brown streak virus evolves with a nucleotide-substitution rate that is typical for the family Potyviridae

“…Cassava was introduced into Africa by the Portuguese in the late 16th century and CBSD was first reported around Lake Malawi region in the 1930s ( Storey, 1936 ; Nichols, 1950 ; Tomlinson et al, 2018 ). Our results support the commonly held assumption that CBSV is likely the result of an indigenous African potyvirus host-shifting into cassava ( Robson et al, 2023 ), and it is still an emerging cassava virus ( Jacobson et al, 2018 ; Jones, 2021 ; Tugume et al, 2023 ). This has implication on disease management as virulence and spread of newly introduced pathogens is more detrimental to sustainable food security ( Jones, 2021 ).…”