Reassessing the occurrence and genetic diversity of lethal necrosis and other maize and johnsongrass viruses in Ecuador

Maize lethal necrosis (MLN) disease, induced by coinfections involving maize chlorotic mottle virus (MCMV, genus Machlomovirus) and one or other of several cereal viruses belonging to the family Potyviridae, such as sugarcane mosaic virus (SCMV, genus Potyvirus), poses a significant threat to global corn production. This study monitored the occurrence of MCMV and SCMV over two years in field plots with high disease pressure, correlating it with vector populations. In addition, high-throughput sequencing (HTS) was used to explore the occurrence of additional viruses in maize (Zea mays L.) and johnsongrass (Sorghum halepense L.), which represents the most prevalent weed in maize plantings. The results revealed that the aphid Rhopalosiphum maidis (Fitch) (Hemiptera: Aphididae) and the thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) were the predominant virus vectors, with two peaks observed for aphids (April-May and August-October) and a single well-defined peak for thrips (May) during the two years. SCMV was the first virus detected early in the season and spread more rapidly, while MCMV was found in plots planted in March – April, correlating with exposure to high thrips populations in May. HTS identified MCMV, SCMV, and maize yellow mosaic virus (genus Polerovirus) as the most prevalent viruses in maize samples. In johnsongrass, HTS detected johnsongrass mosaic virus (genus Potyvirus), sorghum arundinaceaum-associated virus (genus Mastrevirus), and sorghum marafivirus (genus Marafivirus), all of which have not been previously reported in Ecuador.