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

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 recent years and to evaluate the resistance of some Iranian commercial cultivars irrigated barley to powdery mildew.

 Trap nurseries consisting 77 lines and cultivars including 19 Pallas near-isogenic lines, a supplementary set including 34 cultivars carrying known or unknown resistance gene (s) along with 24 Iranian commercial barley cultivars were planted under natural disease development conditions 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 the reaction of studied barley cultivars and lines to powdery mildew 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 (susceptibility) or virulence.

The results of this study showed that there is a significant variation in the virulence spectrum of the pathogen in different locations of Iran, occurrence of changes in their virulence/avirulence spectrum, and appearing new virulence factores for some plants carrying resistance genes. Based on the results, Mlk and Mlk(1) resistance genes were ineffective in all locations during all three years, and Mlh, Mla22 and Mla23 resistance genes were also ineffective in all three years at least in two locations. The results also showed that the mlo resistance gene was effective in all locations, and Mla3, Mla6, Mla9, Mla13, Ml(Ru3), Mla12 and Mla14  genes were effective at least in three locations during all three years. Based on the results, new virulence factors for MlLa, Mlg and MlCP genes appeared in some regions. Evaluation of the supplementary set in different regions of Iran showed that all cultivars/lines carrying one of the alleles of the mlo resistance gene including Viskosa, Wren, Alexis, Brenda and Chalice were effective in all locations/years. Also, the majority of commercial cultivars showed a range of reaction from moderately resistant to susceptible in different locations.

Based on the high level of variation in the virulence spectrum of the pathogen in different locations and appearing new virulence factores for some effective resistance genes, as well as the susceptible or semi-susceptible reaction of most commercial cultivars, continuous research to monitor the virulence spectrum of the pathogen population and identify new genetic sources of disease resistance is necessary. Also, genetic resources with the effective resistance genes including Mla3, Mla6, Mla9, Mla13, Ml(Ru3), Mla12 and Mla14 can be used as resistant parents to develop new resistant cultivars in breeding programs.