Characterization of Three Resistance-Breaking Isolates of Sugarcane Mosaic Virus from Rwanda and Implications for Maize Lethal Necrosis.

Maize lethal necrosis (MLN) is a devastating disease of maize caused by synergy between two viruses: maize chlorotic mottle virus (MCMV) and a potyvirus, often sugarcane mosaic virus (SCMV). Throughout the 2010s, severe MLN outbreaks occurred in East Africa, including Kenya, Rwanda, and Ethiopia. Previous studies have shown extensive sequence diversity among SCMV isolates collected from this region. In this study, we assessed how this diversity may alter virulence by screening a panel of maize near-isogenic lines containing different combinations of major potyvirus resistance loci with diverse SCMV isolates collected from Rwanda. We discovered that the three isolates tested overcame all three potyvirus resistance loci even when stacked, including one isolate that asymptomatically infected all resistant lines tested. To understand how SCMV virulence may contribute to MLN, each SCMV isolate was co-inoculated with MCMV on a panel of SCMV- and MCMV-resistant maize lines. No significant differences in MLN severity were observed for the isolates from Rwanda compared with the reference SCMV isolates, indicating that increased virulence in SCMV single infection did not necessarily correlate with increased MLN severity in co-infection with MCMV. At least two potyvirus resistance loci were needed to reduce MLN severity, and combining SCMV and MCMV resistance was most effective. Remarkably, in some cases, co-infection with MCMV facilitated SCMV infection of potyvirus-resistant lines that SCMV could not infect alone. These results underscore the challenges of developing durable MLN resistance and highlight the importance of incorporating strong, multigenic potyvirus resistance into MLN resistance breeding programs.