First Report on the Occurrence of an Aggressive Pathotype, 734, of Plasmopara halstedii Causing Sunflower Downy Mildew in Hungary

Abstract

Downy mildew of sunflower (Helianthus annuus L.) is caused by Plasmopara halstedii (Farl.) Berl. et de Toni, leading to significant losses in crop production worldwide. The number of new and more aggressive pathotypes has increased rapidly over the last 10 years in Europe (Virányi et al. 2015, Bán et al. 2018), therefore, constantly monitoring the distribution of races is an important task. As part of regular surveys in June 2019, severe downy mildew was identified in some regions, appearing as chlorotic lesions along the veins of the adaxial side and white sporulation on the abaxial side of the leaves of severely stunted hybrids containing PI6 and PI7 resistance genes. The identification of the pathogen was performed microscopically based on morphological characteristics (average size of sporangia: 28x20 µm). Disease incidence (the ratio of diseased plants) ranged between 10 and 30% per field in three regions and resulted in moderate yield loss. Isolates (defined as a lesion per leaf) were collected from 4 to 8 infected leaves of each hybrid by region and stored at -70°C. Two, one and one isolates of P. halstedii were selected and characterized from the southeastern (Békés County), northern (Nógrád County) and northeastern (Borsod-Abaúj-Zemplén County) regions of Hungary, respectively. The pathotype of the four isolates was determined using the international standardized nomenclature method reviewed by Trojanová et al. (2017), including nine sunflower differential inbred lines (HA-304, RHA-265, RHA-274, PMI-3, PM-17, 803-1, HAR-4, QHP2 and HA-335). Zoosporangia from frozen sunflower leaves were washed off into bidistilled water and the concentration was adjusted to 3.5x104 sporangia/ml using a hemocytometer. Three-day-old seedlings with a radical of 1 to 1.5 cm long were immersed in the sporangial suspension and kept at 16°C overnight (Cohen and Sackston 1973). Inoculated seedlings were planted into trays containing clear moistened perlite (d = 4 mm) and grown in a growth chamber with a photoperiod of 12 h. The experiment was carried out twice with each isolate using 15 seeds/differential line with two replicates. Bidistilled water was sprayed on the plants 9 days after inoculation, and then trays were covered with a black polyethylene bag and maintained at 19°C overnight to induce sporulation. The first disease assessment was done based on cotyledons bearing white sporulation. Next, a second evaluation was performed 21 days after inoculation assessing stunting of the plants, chlorotic lesions on true leaves and damping-off. All 4 isolates examined caused disease on differential lines HA-304, RHA-265, RHA-274, PMI-3, PM-17 and HA-335, whereas the other lines showed no symptoms and signs of sunflower downy mildew. As a result, it was concluded that the presence of P. halstedii pathotype 734 was confirmed. This pathotype is likely widespread in Hungary as it could be detected from three different regions. Moreover, the possibility that pathotype 734 is present in Hungary has been raised before (Iwebor et al. 2018). This pathotype is already widespread in the USA and Russia and is considered to be highly aggressive, since it was able to infect hybrids with resistance genes PI6 and PI7 (Iwebor et al. 2018, Spring 2019). To our knowledge, this is the first report of pathotype 734 of P. halstedii in Hungary and Central Europe. Continuous monitoring and incorporation of new resistance genes into sunflower hybrids are essential steps in the future to control P. halstedii.

Updated Characterization of Races of Plasmopara halstedii and Entomopathogenic Fungi as Endophytes of Sunflower Plants in Axenic Culture

“…This shows that isolates having the same code (i.e., 715) can lead to different reactions in some genotypes that are widely used in sunflower breeding for resistance to downy mildew (i.e., RHA-340) and suggests the need of updating the nine differentials that were internationally agreed upon more than 20 years ago to be utilized for race identification of P. halstedii. In any case, our results agree with those of other authors from the Czech Republic and Hungary [32,35,36] and confirm that the previously suggested widening of distribution of highly virulent P. halstedii races [2] is a reality in Europe.…”

New and High Virulent Pathotypes of Sunflower Downy Mildew (Plasmopara halstedii) in Seven Countries in Europe

“…The first high virulent pathotype (704) was isolated in 2010 by Rudolf et al [ 22 ] from two sunflower fields in Hungary. In the following years, our research team confirmed the increased distribution of pathotype 704 in the country [ 27 ] and the emergence of three new P. halstedii pathotypes, 714, 724, and 734 [ 13 , 14 , 28 ] (New records are separated from previously published records by bold isolate numbers in Table 3 .). Previously, pathotype 724 was only identified from Hungary, but according to our present results, it was also detected in Romanian samples in 2019.…”

“…Gulya [ 11 ] has previously reported on 35 pathotypes. More recently, new global and highly aggressive pathotypes of P. halstedii have been identified in some areas of Europe, such as pathotype 354 in Germany [ 12 ], 724 and 734 in Hungary [ 13 , 14 ], 705 in Spain [ 15 ], and 705 and 715 in the Czech Republic [ 16 ]. Altogether, there are as many as 50 pathotypes worldwide [ 17 , 18 ].…”

Spatial Genetic Structure and Pathogenic Race Composition at the Field Scale in the Sunflower Downy Mildew Pathogen, Plasmopara halstedii

“…Only one race, 100, was present, and was widely distributed across France. In recent years, downy mildew has become a serious problem for the sunflower crop in France; until 2008, only 16 out of 50 worldwide races had been detected in various regions, suggesting that the pathogen population has evolved rapidly, resulting in a high diversity in virulence [ 13 , 14 , 20 , 21 , 22 , 23 ]. Furthermore, the wide use of fungicide to control this plant pathogen has led to the development of resistant strains that have decreased the fungicide’s efficacy.…”