First report of virulence to resistance gene Sr25 by the stem rust pathogen (Puccinia graminis f. sp. tritici) in Ardabil, North West of Iran

 

 
Stem (black) rust, caused by Puccinia graminis Pers. f. sp. tritici Erikss. & Henning, is one of the most destructive diseases of wheat around the world. It could be controlled through deployment of race-specific resistance genes (Jain et al. 2009). However, such kind of resistance is mostly short lived due to emergence of new virulences. For example, due to emergence of Ug99 resistance genes Sr5,Sr6, Sr7a, Sr7b, Sr8a, Sr8b, Sr9a, Sr9b, Sr9d, Sr9e, Sr9f, Sr9g, Sr9h, Sr10, Sr11, Sr12, Sr16, Sr17, Sr18, Sr19, Sr20, Sr21, Sr23, Sr24, Sr30, Sr31, Sr34, Sr36, Sr38, Sr41, Sr49, Sr54, SrMcN, SrWld-1 are no longer effective (Singh et al. 2015) and cannot be used in breeding programs. Monitoring of new virulence factors has remained very important in the evaluation and identification of new sources of resistance. Considering to the importance of stem rust especially the race of Ug99 (also named TTKSK), during cropping season of 2015-2016, virulence of the wheat stem rust was investigated by planting 47 isogenic lines (in national trap nursery) under field conditions. This survey was conducted in Ardabi Agricultural Research Station (38.17°N, 48.39°E, 1350 m Height), North West of Iran. Each entry was planted in two 1 meter rows which were spaced 30cm apart. Plots were spaced at 65 cm. A susceptible spreader (Morocco) row was sown around the borders of the experiment and 10 entry intervals.All the required cultural practices were carried out during the experiment. Disease severity was estimated according to the modified Cobb,s scale; 0% = immune, and 100% = fully susceptible when disease was well-developed (Peterson et al. 1948). The infection type (IT) of disease was also recorded based on Roelfs et al. (1992). The presence of virulence factors was determined by susceptible infection type while monitoring the disease on differential sets. In other words, corresponding genes against virulence factors of pathogen in plants were considered as ineffective genes and corresponding genes against avirulence factors of pathogen were considered as effective resistance genes. Results showed that there is virulence to resistance gene Sr25, a gene from Thinopyrum elongatum, which is located on chromosome 7DL and linked with leaf rust resistance gene Lr19. The resistance Sr25 had been effective or partially effective against stem rust worldwide, including race Ug99 (Singh et al. 2011). The detection of Sr25 virulence is significant since Sr25 is an important gene to be targeted for breeding wheat cultivars resistant to Ug99. In this research, ineffectiveness of Sr25 was observed in first time in Ardabil (Northwest of Iran), and thus we should not use this resistance gene in breeding program (at least in Ardabil). Before of this report, viulence to Sr25 had also been reported in India (Jain et al. 2009). Considering to rapid spread of Ug99 and its threat in some parts of Iran (Nazari et al. 2009), and in order to obtain of durable resistance, adult plant resistance (APR) or slow rusting genes such as Sr2, Sr55, Sr56, Sr57 and Sr58 should be deployed in combination with each other or with effective seedling (all-stage) resistance genes; Sr22, Sr26, Sr33, Sr35, Sr45 and Sr50.