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

Abstract

The causal agent of maize common rust (CR), Puccinia sorghi Schwein., 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 P. sorghi populations. The maize commercial hybrids tested carried no resistance to 14 isolates of P. sorghi and had infection types (IT) comparable to that of a susceptible check. The resistance gene Rp1_D that remained effective in the US for 35 years, was ineffective against seven of the 14 isolates. Maize lines carrying known Rp genes were inoculated with the five isolates considered most diverse based on year of collection (2018/2019), location (Queensland/Victoria), and host from which they were isolated (maize/sweet corn). Lines carrying the resistance genes RpG, Rp5, Rp1_E, Rp1_I, Rp1_L, RpGDJ, RpGJF and Rp5GCJ were resistant to all five isolates and to isolates collected in many agro-ecological regions. These lines were recommended as donors of effective resistance for maize breeding programs in Australia. Lines carrying no known resistance or resistance genes Rp8_A, Rp8_B, Rp1_J, Rp1_M, Rp7 and Rpp9 (conferring resistance to P. polysora Underw.) were susceptible to all five isolates. Differential lines carrying the resistance genes Rp1_B, Rp1_C, Rp1_D, Rp1_F, Rp1_K, Rp3_D, or Rp4_A 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 to juvenile plants, presumably due to changes in plant physiology associated with age or the presence of adult plant resistance.