The samples of Plasmopara halstedii (the causal agent of sunflower downy mildew) were collected on sunflower fields in south of the Russian Federation: in the Krasnodar, Rostov and Stavropol regions, and in the Republic of Adygea in 2016 and 2017. Virulence code of 545 isolates was identified; i. e. 280 isolates originating from 24 fields in 2016, and 265 isolates from 17 fields in 2017. Races 330, 334, 710 and 730, widespread within the region in previous years, were found in all mentioned areas. In addition, races 713, 733 and 734 have been detected in several districts of the Krasnodar region since 2016. This is the first report of these races in Russia and above that the first record throughout Europe and Asia for the races 733 and 734. The identification of new P. halstedii races was confirmed by the method of cross inoculations from individual differential lines. The phenotype corresponding to virulence code 734 was in some cases disclosed as a mixture of race 334 with 710 or 730. The presence of several P. halstedii races on an individual plant was also confirmed for the first time in the territory of the Russian Federation.
The population of oomycete Plasmopara halstedii (Farl.) Berl. et de Toni (sunflower downy mildew pathogen) has been monitored in Krasnodar and Rostov regions and the Republic of Adygea for more than 15 years. Prior to the beginning of the 2000s there were races 100, 300, 310 and 330 in the regions. In the period from 2004 to 2007 races 100, 300, 310 and 700 were recorded sporadically. The race 330 was the most common; in a number of agrocoenoses it was 100 % of samples. In some fields races 710 and 730 prevailed. In 2008–2011 only races 330, 710 and 730 were found; the race 330 have been still prevailed and was also found on Ambrosia artemisiifolia L. Since 2012, in the majority of fields races 710 and 730 prevailed, and the race 330 wasn’t allocated in many of them; for the first time in Russia pathotype 334, that able to overcome Pl6, was found in Krasnodar region. In the period of 2013–2015 increased distribution of the race 334 in the Krasnodar region and the Republic of Adygea was observed. At the same time, in 2014 in one field in the Rostov region only races 310 and 330 (prevailed) were identified. The virulence of the pathogen population is closely connected with the cultivated assortment of sunflower. Further spread and accumulation of P. halstedii race 334 and the emergence of new pathogen pathotypes in the said regions are predicted.
Sunflower downy mildew caused by Plasmopara halstedii (Farl.) Berl. et de Toni is a destructive and widespread disease. More than 50 races of P. halstedii have been recorded worldwide. In 2020, in the Russian Federation (Zernogradsky district, Rostov region), a globally new race 337 was identified for the first time. The pathogen was identified on the plants of a foreign sunflower hybrid bearing the resistance gene Pl<sub>6</sub>. According to the five-digit racial nomenclature, its virulence profile was determined as 337 53. It is the first P. halstedii race recorded in the Russian Federation that simultaneously infects all differential lines of the 3<sup>rd</sup> triplet, i.e., HA-R4, HA-R5 and HA-335. The sunflower lines RHA-274, 803-1, PSC8, RHA-419 and RHA-340 were resistant to it. All the collected isolates of the new race were susceptible to the fungicide mefenoxam.
The accelerated morphofunctional variability of Orobanche cumana, a flowering parasitic plant of the family Orobanchaceae, can be traced to the influence of anthropogenic factors. This broomrape species is an obligate parasite of sunflower. Continuous breeding of the crop for immunity to broomrape has been conducted in the Russian Federation for more than 100 years. In the last 30 years, the accelerated return of sunflower in crop rotation to the previous field in 1-3 years (instead of the scientifically justified 8–10 years) resulted in the rapid emergence of new races of broomrape and contamination of fields with its seeds, which provided intense competition between individuals for the nutrition received from the host plant. Under these conditions, interrow cultivation in sunflower sowings infested by the broomrape contributed to permanent injury of developing stems of many individuals of the parasite, which led to the removal of apical dominance of one shoot in some species of the broomrape. There have appeared the bushy forms with equal development of many shoots from a tubercle, which were detected for the first time by us. Strengthening of the haustorial part of the parasitic plant, externally manifested as thickening of the host plant root under the tubercle, was required for feeding the multiple shoots. Such thickening is not a defensive reaction of the host plant but is caused by changes in the haustorial area of the parasite and is observed when sunflower roots are affected by species of the race G of O. cumana that became widely spread and dominant during the last decade. Thus, in the co-evolution of the parasitic plant and its host, we first discovered bushy forms of O. cumana and their characteristic change in the haustorial zone, which is externally manifested by the visible thickening of the sunflower root in this area.
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