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 U.S. Great Plains states of North Dakota and South Dakota, where approximately 75% of the U.S. sunflower is produced annually. The virulence phenotypes (and races) of 185 isolates were determined using the U.S. 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. Although widespread virulence was identified on several genes, new virulence was identified on the Pl₈ resistance gene, and no virulence was observed on the Pl_Arg, 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 pathogen virulence is determined frequently and by using genetic lines containing resistance genes actively incorporated into commercial cultivars.

霜霉病，由Plasmopara halstedii（Farl。）Berl引起。和de Toni是一种在栽培向日葵中的重要经济疾病向日葵L.掺入商业杂种中的抗性基因被用作有效的疾病管理工具，但是随着病原体种群中毒力的发展，有效期受到限制。2014年和2015年，在美国大平原州北达科他州和南达科他州进行了病原体毒力的综合评估，这些地区每年约生产美国向日葵的75％。使用美国标准品（共9个差异）确定185个分离株的毒力表型（和种族）。此外，在另外13条品系上确定了61至185个分离株的毒力表型，这些品系已用于评估北美和/或国际上的病原体毒力。尽管在几个基因上发现了广泛的毒力，但在Pl上发现了新的毒力₈抗性基因，并且在Pl _Arg， Pl ₁₅， Pl ₁₇和Pl ₁₈基因上未观察到毒力。这项研究的结果表明，应使用另外三行作为差异，并与以前的研究一致，建议在两个国际公认的差异集中使用六行作为差异。对于使用遗传抗药性进行有效的疾病管理，至关重要的是毒力数据的相关性和及时性。当频繁确定病原体毒力并使用含有积极掺入商业品种中的抗性基因的遗传品系时，可实现最佳效果。