First Report on the Occurrence of a New Pathotype, 714, of Plasmopara halstedii (Sunflower Downy Mildew) in Hungary

Abstract

Downy mildew of sunflower, caused by Plasmopara halstedii (Farlow) Berlese et de Toni, is an economically important disease in Hungary and much of Europe. The known pathotypes (races) of the pathogen influence the resistance genes (Pl genes) incorporated into new sunflower hybrids to manage the disease. There are at least 36 pathotypes of P. halstedii worldwide (3), but the number of races is increasing rapidly. In 2010, race 704 was identified in Hungary for the first time (2). Race 704 has been reported to confer virulence on Pl6, a broad spectrum resistance gene that is widely used in sunflower hybrids. This has coincided with a significant increase in disease severity since 2010 in the country. Our objectives are to continuously monitor this pathogen and identify pathotypes of P. halstedii. Because of the unfavorable weather conditions for downy mildew in 2013, samples were collected at a single site (Kunszentmárton, South Hungary) in the beginning of July from NK Neoma sunflower hybrids. Disease incidence (early and late primary infection) was as high as 40%. Systemically mildewed plants showed severe stunting and leaf chlorosis, signs and symptoms consistent with downy mildew. P. halstedii was identified microscopically. Examination of isolates was carried out using a set of sunflower differential lines based on the internationally standardized method for race identification of P. halstedii (1). Inoculum of the isolates was increased on a susceptible cultivar (cv. Iregi szürke csíkos) and tested by inoculating 3-day-old seedlings of sunflower differential lines. Inoculated seedlings were planted in trays in glasshouse. After 8 to 9 days, seedlings were sprayed with distilled water, covered with black plastic bags, and left overnight to induce sporulation. Disease incidence was determined by examining cotyledons at 9 days after inoculation for sporulation and true leaves on 12 to 13 days after inoculation for secondary symptoms, such as leaf chlorosis and stunting (1). While several differential lines showed no typical susceptible/resistant reactions, i.e., the infection was much lower than 100%, it was concluded that the isolates were mixtures of different P. halstedii pathotypes. To obtain single isolates, we collected zoosporangia from the differential lines in question separately, and then inoculated the seedlings of the same genotype and a uniformly susceptible line. A single isolate caused as high as 100% infection on HA-335, containing resistance gene Pl6. Subsequent evaluation of this isolate with the entire differential set resulted in an aggregate virulence phenotype of 714. As resistance gene Pl6 is incorporated to the majority of sunflower hybrids grown in Hungary, pathotypes virulent on this gene, such as 704 and 714, are likely to spread. This underscores the need to prove the resistance to these races in the newly registered hybrids and for further research to identify P. halstedii pathotypes. It is also important to establish the identity of this new pathotype by already discovered 714 pathotypes in other countries like France and Italy and to discover the real conditions of local evolving of new pathogens. To our knowledge, this is the first report of pathotype 714 of P. halstedii in both Hungary and Central Europe. References: (1) T. J. Gulya et al. Helia 14:11, 1991. (2) K. Rudolf et al. Növényvédelem 47:279, 2011. (3) F. Virányi and O. Spring. Eur. J. Plant Pathol. 129:207, 2011.