Determination of virulence phenotypes of Plasmopara halstedii in the United States

Authors: GILLEY, MICHELLE - North Dakota State University; GULYA, THOMAS - Retired ARS Employee; Seiler, Gerald; Underwood, William; Hulke, Brent; Misar, Christopher; MARKELL, SAMUEL - North Dakota State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/26/2020
Publication Date: 9/21/2020
Citation: Gilley, M.A., Gulya, T.J., Seiler, G.J., Underwood, W., Hulke, B.S., Misar, C.G., Markell, S.G. 2020. Determination of virulence phenotypes of Plasmopara halstedii in the United States. Plant Disease. 104:2823-2831. https://doi/10.1094/PDIS-10-19-2063-RE.
DOI: https://doi.org/10.1094/PDIS-10-19-2063-RE

Interpretive Summary: Downy mildew caused by the oomycete pathogen Plasmopara halstedii is a persistent and economically significant disease on cultivated sunflowers. The pathogen typically attacks sunflowers at the seedling stage, resulting in seedling death or severe plant stunting and loss of yield. Several effective control measures are available for growers to manage this disease including fungicide seed treatment and genetic resistance conferred by single, dominant disease resistance genes. Genetic resistance is highly effective in mitigating losses to this disease. However, resistance conferred by single genes can be overcome by the emergence of new virulent forms of the pathogen, referred to as races. Consequently, it is essential to monitor the race structure of the pathogen population and evaluate the effectiveness of current resistance genes. In this study, a survey of the P. halstedii population in North Dakota and South Dakota was undertaken. Virulence of pathogen isolates was evaluated on a set of sunflower differential lines carrying distinct resistance genes to determine races and assess the effectiveness of known resistance genes. The results of this study provide important information on the pathogen population and resistance gene utility. Additionally, new sunflower differential lines are proposed to allow for more comprehensive assessment and monitoring of this important disease.

Technical Abstract: Downy mildew, caused by Plasmopara halstedii (Farl.) Berl. and de Toni, is an economically important disease in cultivated sunflowers, Helianthus annuus L. Resistance genes incorporated into commercial hybrids are used as an effective disease management tool, but the duration of effectiveness is limited as virulence evolves in the pathogen population. A comprehensive assessment of pathogen virulence was conducted in 2014 and 2015 in the US Great Plains States of North Dakota and South Dakota, where approximately 75% of the US sunflower is produced annually. The virulence phenotypes (and races) of 185 isolates were determined using the US Standard Set of nine differentials. Additionally, the virulence phenotypes of 61 to 185 isolates were determined on 13 additional lines that have been used to evaluate pathogen virulence in North America and/or internationally. While widespread virulence was identified on several genes, new virulence was identified on the Pl8 resistance gene, and no virulence was observed on the PlArg, Pl15, Pl17 and Pl18 genes. Results of this study suggest that three additional lines should be used as differentials and agree with previous studies that six lines proposed as differentials should be used in two internationally accepted differential sets. For effective disease management using genetic resistance, it is critical that virulence data be relevant and timely. This is best accomplished when determination of pathogen virulence is determined frequently and by using genetic lines containing resistance genes actively incorporated into commercial cultivars.