Effects of Maize Chlorotic Mottle Virus and Potyvirus Resistance on Maize Lethal Necrosis Disease.

Maize lethal necrosis (MLN) is a viral disease caused by host co-infection by maize chlorotic mottle virus (MCMV) and a potyvirus, such as sugarcane mosaic virus (SCMV). The disease is most effectively managed by growing MLN-resistant varieties. However, the relative importance of MCMV and potyvirus resistance in managing this synergistic disease is poorly characterized. In this study, we evaluated the effects of SCMV and/or MCMV resistance on disease, virus titers, and synergism and explored expression patterns of known potyvirus resistance genes and MLN disease was significantly lower in both the MCMV-resistant and SCMV-resistant inbred lines compared with the susceptible control Oh28. Prior to 14 days postinoculation (dpi), MCMV titers in resistant lines N211 and KS23-6 were more than 100,000-fold lower than found in the susceptible Oh28. However, despite no visible symptoms, titer differences between MCMV-resistant and -susceptible lines were negligible by 14 dpi. In contrast, systemic SCMV titers in the potyvirus-resistant line, Pa405, ranged from 130,000-fold to 2 million-fold lower than susceptible Oh28 as disease progressed. Initial expression was up to 49,000-fold lower in Oh28 compared with other genotypes, whereas expression of was up to 4.5-fold lower. Measures of virus synergy indicate that whereas MCMV resistance is effective in early infection, strong potyvirus resistance is critical for reducing synergist effects of co-infection on MCMV titer. These results emphasize the importance of both potyvirus resistance and MCMV resistance in an effective breeding program for MLN management.