Identification of Physiological Races of Plasmopara halstedii, the Causal Agent of Sunflower Downy Mildew, in Urmia
Sunflower downy mildew, caused by Plasmopara halstedii, is one of the most important diseases of this crop in West-Azarbaijan, Urmia. It causes economic yield losses during favorable environmental conditions. The pathogen is a polycyclic one, and it is likely that physiological changes occur in its population and subsequent appearance of new physiological races. Hence, identification of the existing races among the population in infected areas would play a significant role in breeding and the use of resistant cultivars to control the disease. Infected sunflower plants showing systemic symptoms were collected from different areas of Urmia in 2009. The isolates were inoculated and mass-produced on susceptible cultivar record, using Whole Seedling Immersion (WSI) inoculation method, and finally 15 isolates were obtained. The isolates were inoculated on nine differential lines (RHA-265, HA-304, PM-13, PM-17, 803-1, HA-R4, QHP-1, HA-335, RHA-274) by the same method. Three days old seedlings, after inoculation, were sown in pasteurized mixture of sand and perlite (3:2 v/v), and incubated in greenhouse for two weeks. To induce the sporulation, favorable conditions (16˚C, 90-100% R.H and darkness) were provided for 48 h. The results showed that differential lines had a similar reaction pattern to the isolates. Thus, one race was identified as dominant in the area. Following international system of nomenclature, the race was called number 100. This is the first report of identification of physiological races of sunflower downy mildew based on the international system for nomenclature in Iran.