First report of the detection of the sunflower downy mildew pathogen pathotype 737 in the Russian Federation

Downy mildew, caused by the oomycete Plasmopara halstedii (Farl.) Berl. & de Toni, is one of the most harmful diseases affecting sunflower (Helianthus annuus L.). Effective disease control is based on the genetic resistance of cultivated sunflower, but the high variability of the pathogen leads to the emergence of new virulent pathotypes. The purpose of this publication is to report on pathotype 737, which is new to the Russian Federation. The research was conducted from 2021 to 2025 in the south of Russia (the Krasnodar, Stavropol, and Rostov regions). The virulence of P. halstedii was determined using the standard international set of nine sunflower differential lines. Pathotype 737 was first detected in the Russian Federation in the Krasnodar and Rostov regions in 2021. Its virulence was confirmed using monozoosporangial isolates. The new pathotype was found in foreign sunflower hybrids carrying the Pl6 resistance gene, indicating that it may have been introduced through imported seed material. In 2022, one isolate of this pathotype was found in the Krasnodar region, and subsequently (2023–2025) it was not detected in the regional population. Pathotype 737 has a broad spectrum of virulence, overcoming the resistance of eight out of nine lines of the standard set (except for line 8031); it does not affect lines RHA340 and RHA419. All isolates of pathotype 737 showed sensitivity to the fungicide mefenoxam. A distinctive feature of this pathotype is its high aggressiveness, which, by causing necrosis and maceration of host plant tissues, limits the formation of asexual sporulation, ensuring rapid spread and accumulation of infection. Despite overcoming the action of many sunflower resistance genes, it is assumed that the epidemiological potential of pathotype 737 will be low in local conditions due to disruption of the reproductive cycle. However, its oospores can remain in the soil, causing periodic outbreaks of the disease it causes.