Johnsongrass mosaic virus: resistance of maize in Australian and implications for preparedness for maize lethal necrosis

Abstract

Johnson grass mosaic virus (JGMV) is widely distributed in the major perennial host Johnson grass (Sorghum halepense) in southern and central Queensland and north-west New South Wales. Two strains of this aphid-transmitted potyvirus occur in Australia, the type strain (JGMV-JG) and the Krish -infecting strain (JGMV-K) which is able to infect grain sorghum hybrids/lines with the Krish resistance gene. Although the economic impact of JGMV is currently low, the virus has caused sporadic severe losses in sweet corn, maize and grain sorghum over 30 years. A secondary threat is posed by the interaction of mixed infections of JGMV (and other maize-infecting potyviruses) and the exotic maize chlorotic mottle virus (MCMV) which can lead to maize lethal necrosis (MLN). Severe losses due to MLN have been reported from eastern and central Africa, Ecuador and China. We used glasshouse and field studies to determine JGMV resistance status of commercially grown maize and sorghum. We used high pressure application of JGMV inoculum as a rapid and effective means for inoculating field plots. All ten commercial maize hybrids tested were resistant to both JGMV strains as were ten of 15 sweet corn hybrids. The resistance status of sweet corn and maize hybrids to JGMV was well correlated with published resistance levels to the related exotic maize dwarf mosaic virus (MDMV). All 15 sorghum hybrids were susceptible to JGMV-K while 10 of these hybrids were resistant to the type strain, suggesting the presence of the Krish resistance gene in their pedigree. We developed rapid LAMP assays for JGMV and MDMV to assist with monitoring for the exotic. The high resistance of maize hybrids to JGMV determined in this study provides confidence of a high level of protection against maize lethal necrosis disease in the event of an incursion of maize chlorotic mottle virus into Australia.