Identification of virulence/avirulence spectrum of Blumeria graminis f.sp hordei, the causal agent of barley powdery mildew in different regions of Iran

Introduction Powdery mildew of barley is one of the most important diseases of barley worldwide that occur also in most of the barley cultivation areas in Iran. The use of resistant cultivar is known as the most effective, the most economic and the most environmentally safe method of disease control. Due to the high level of genetic diversity, the fungal pathogen have a big capacity for producing new physiological races and breaking down of effective sources of resistance. This study was carried out to determine the virulence/avirulence spectrum of the pathogen in major powdery mildew conductive regions, to monitor their annual changes in receny years and to evaluate some commertial cultivars of barley to powdery mildew. Materials and methods Trap nurseries consist of 77 lines and cultivars including 19 Pallas near-isogenic lines, a supplementary set including 34 cultivars carrying known or unknown resistance gene (s) and 24 Iranian commercial barley cultivars were planted under the natural disease development at six powdery mildew hot spot locations in Mashhad, Gorgan, Ghaemshahr, Pars Abad (Moghan), Dezful and Zarghan during three consecutive cropping seasons (2016-2019). Evaluation of powdery mildew resistance was performed in 00-99 scroing scale based on the double digit system at the flowering stage. The virulence/avirulence spectrum of the pathogen in different regions was determined by considering the infection types 1-4 as incompatibile reaction (resistance) or avirulence and the infection types 5-9 as compatibile reaction (sucseptibility) or virulence. Research findings The results of this study showed that there is a considerable variation in the virulence spectrum of the pathogen in different locations, occurring changes and appearing new virulence factores for some resistance genes. Based on the results Mlk and Mlk(1) resistance genes were ineffective in all locations during the three year and Mlh, Mla22 and Mla23 resistance genes were also ineffective at least in two locations during the three year field experiments. The results also showed that the mlo resistance gene was effective in all locations and Mla3, Mla6, Mla9, Mla13, Ml(Ru3), Mla12, Mla14 genes were effective at least in three locations during the three years. Based on the results new virulence factors for the MlLa, Mlg and MlCP genes appeared in some regions. The results of evaluation of the supplementary set showed that all the mlo carrying lines/cultivars including Viskosa, Wren, Alexis, Brenda and Chalice were effective in all locations/years. The majority of commercial cultivars showed a range of reaction from moderately resistant to susceptible in different locations. ConclusionBased on the high level of variation in the virulence spectrum of the pathogen in different locations, appearing new virulence factores for some resistance genes and susceptible reaction of the commercial cultivars, continuous research about monitoring the virulence spectrum of the barley pathogen pathogen and idenfication of new sources of resistance is necessary. Sources of effective resistance genes including Mla3, Mla6, Mla9, Mla13, Ml(Ru3), Mla12 and Mla14 could be used in breeding programs to develop new resistant cultivars.