ARS | Publication request: Emergence of new virulent races of Plasmopara halstedii inciting downy mildew on sunflower in the United States

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: May 11, 2011
Publication Date: June 15, 2011
Citation: Gulya Jr, T.J., Markell, S., Mcmullen, M., Harveson, R.M., Osborne, L.E. 2011. Emergence of new virulent races of Plasmopara halstedii inciting downy mildew on sunflower in the United States [abstract]. In: North Central Division Meeting Abstracts, June 15-17, 2011, Omaha, NE. S2.3-4.

Technical Abstract: Prior to 2009, no isolate of Plasmopara halstedii, the cause of sunflower downy mildew (DM), recovered from sunflower in the United States caused disease on resistance genes Pl6, Pl7 or Pl8. These genes, originally released by the USDA-ARS Sunflower Research Unit in 1985, have been used extensively to produce downy mildew-resistant (DMR) hybrids. In 2009, downy mildew was observed in a field of DMR sunflowers located in Bottineau county (north central) North Dakota. The sample was identified on a standard set of nine differential lines and categorized as race 734, and conferred virulence on the USDA line HA-335 which contains the Pl6 gene. In response, 49 isolates of P. halstedii were collected in a subsequent survey and virulence-phenotyped. Fourteen of 49 DM samples infected HA-335. In 2010, 17 of 160 isolates collected during surveys infected HA-335. These 31 isolates conferring virulence on Pl6 were identified as five races, namely 314, 704, 714, 734 and 774. Isolates virulent on Pl6 were predominantly from North Dakota, but also found in Minnesota and Nebraska; none were recovered from South Dakota. Race 714 comprised one-half of the 31 samples. To determine resistance in commercial hybrids, eighty DMR hybrids were tested in greenhouse trials with the five races. Only 17 hybrids were immune to all races. Thus, nearly 80% of commercial hybrids marketed in the United States as "DMR" are susceptible to the new races, presumably since these hybrids rely on either the Pl6 or Pl7 genes. None of the five newly identified races overcome the Pl8 gene in USDA line RHA-340, nor the Plarg gene in USDA lines RHA-419 and RHA-420. The identification of races conferring virulence on one of the most utilized resistance genes in commercial sunflower may facilitate the re-emergence of DM as a significant yield threat.