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First Report on the Occurrence of a Globally New Pathotype, 724, of Plasmopara halstedii Causing Sunflower Downy Mildew in Hungary

Bán, R., Kovács, A., Körösi, K., Perczel, M., Turóczi, G., Zalai, M., Pálinkás, Z., Égei, M.
Plant disease 2018 v.102 no.9 pp. 1861
European Union, Helianthus annuus, Plasmopara halstedii, chlorosis, cotyledons, disease incidence, downy mildew, evolution, genes, greenhouses, growth retardation, hybridization, hybrids, leaves, pathogens, pathotypes, plastic bags, seedlings, sporangia, sporulation, spraying, spring, surveys, trays, weather, Hungary
Plasmopara halstedii (Farlow) Berlese et de Toni is the causal agent of downy mildew of sunflower (Helianthus annuus L.). The widespread use of new commercial sunflower hybrids accelerates the evolution of new pathotypes of P. halstedii. In 2016, the global number of P. halstedii pathotypes was as many as 44 (Trojanová et al. 2017). In Hungary, until 2010, the presence of six pathotypes was proven. In 2014 the increased distribution of pathotype 704 and the occurrence of a new pathotype, 714, were reported (Bán et al. 2014). In addition, several cases of downy mildew infections have been reported and investigated since 2010. Our aim, therefore, was to monitor downy mildew and identify pathotypes of P. halstedii. Because of the favorable weather conditions for downy mildew during the spring of 2017, the pathogen caused heavy infections in several regions of Hungary. Disease incidence was especially high, as much as 23% at Mezőkovácsháza and 14% at Szeghalom (southeast Hungary). Isolates were collected at the beginning of June from 5 to 15 leaves of sunflower hybrids SY Diamantis (Mezőkovácsháza) and SY Neostar CLP (Szeghalom), respectively. Both of these commercial hybrids are known to be genetically susceptible to P. halstedii pathotypes 704 and 714, because neither of them contains advanced resistant genes. All diseased plants showed the typical symptoms of downy mildew, such as stunting and leaf chlorosis. The currently accepted differential sunflower set with nine differential lines (HA-304, RHA-265, RHA-274, PMI-3, PM-17, 803-1, HAR-4, QHP-2, and HA-335) and the international method for pathotype identification of P. halstedii were used to examine the isolates (Trojanová et al. 2017). Experiments were carried out twice, with each experiment containing three replicates. Two downy mildew isolates, one each from Mezőkovácsháza and Szeghalom, were tested by inoculating 3-day-old seedlings of the sunflower differentials (35,000 sporangium/ml) following the whole seedling immersion method. Inoculated seedlings were planted in trays in a glasshouse. After 8 to 9 days, sporulation was stimulated by spraying distilled water onto the seedlings and covering the trays with black plastic bags overnight. Disease assessment was first performed based on the appearance of white sporulation on cotyledons. A second evaluation of true leaves was made on 21-day-old plants. Each replicate for a differential line inoculated with the two isolates showed clear resistant or susceptible reactions to the pathogen. At the first evaluation, HA-304, RHA-265, RHA-274, PM-17, and HA-335 differential lines showed white sporulation on the cotyledons. At the second evaluation, the plants of these lines were stunted, and chlorotic lesions appeared on the true leaves, indicating systemic infection. No other lines showed disease symptoms. The two isolates were identified as pathotype 724. To our knowledge, this is the first report of P. halstedii pathotype 724 in Hungary and worldwide. Although P. halstedii is seedborne (Ioos et al. 2007), it is unlikely that this new pathotype was introduced into Hungary through seed, owing to strict European Union phytosanitary regulations. It is instead likely that pathotype 724 developed in situ owing to hybridization of coexisting pathotypes (Ahmed et al. 2012). This survey underscores the need for further research to identify P. halstedii pathotypes in Hungary and worldwide.