Phenotypic Diversity of Puccinia helianthi (Sunflower Rust) in the United States from 2011 and 2012.

Puccinia helianthi, causal agent of sunflower rust, is a macrocyclic and autoecious pathogen. Widespread sexual reproduction of P. helianthi was documented in North Dakota and Nebraska for the first time in 2008 and has since frequently occurred. Concurrently, an increase in sunflower rust incidence, severity, and subsequent yield loss on sunflower has occurred since 2008. Rust can be managed with resistance genes but determination of virulence phenotypes is important for effective gene deployment and hybrid selection. However, the only P. helianthi virulence data available in the United States was generated prior to 2009 and consisted of aggregate virulence phenotypes determined on bulk field collections. The objective of this study was to determine the phenotypic diversity of P. helianthi in the United States. P. helianthi collections were made from cultivated, volunteer, and wild Helianthus spp. at 104 locations across seven U.S. states and one Canadian province in 2011 and 2012. Virulence phenotypes of 238 single-pustule isolates were determined on the internationally accepted differential set. In total, 29 races were identified, with races 300 and 304 occurring most frequently in 2011 and races 304 and 324 occurring most frequently in 2012. Differences in race prevalence occurred between survey years and across geography but were similar among host types. Four isolates virulent to all genes in the differential set (race 777) were identified. The resistance genes found in differential lines HA-R3 (R MC29 (R and R), and HA-R2 (R) conferred resistance to 96.6, 83.6, and 78.6% of the isolates tested, respectively.