Resistance in Maize ( Zea mays ) to Isolates of Puccinia sorghi from Eastern Australia

The causal agent of maize common rust (CR), , has increased in incidence and severity in Australia in recent years, prompting the assessment of sources of resistance and a preliminary survey of the diversity of populations. The maize commercial hybrids tested carried no resistance to 14 isolates of and had infection types comparable with that of a susceptible check. The resistance gene that remained effective in the United States for 35 years was ineffective against 7 of the 14 isolates. Maize lines carrying known "resistance to " ( ) genes were inoculated with the five isolates considered most diverse based on year of collection (2018 or 2019), location (Queensland or Victoria), and host from which they were isolated (maize or sweet corn). Lines carrying the resistance genes , and were resistant to all five isolates and to isolates collected in many agroecological regions. These lines were recommended as donors of effective resistance for maize breeding programs in Australia. Lines carrying no known resistance or resistance genes , and (conferring resistance to ) were susceptible to all five isolates. Differential lines carrying resistance genes , or were either resistant or susceptible depending upon the isolate used, showing that the isolates varied in virulence for these genes. Urediniospore production was reduced on adult compared with juvenile plants, presumably due to changes in plant physiology associated with age or the presence of adult plant resistance.