Detection of Isolates of Puccinia graminis f. sp. Tritici Virulent to Sr31 Resistance Gene in Western Province of Iran

Stem or black rust of wheat, caused by Puccinia graminis Pers. f. sp. tritici Eriks. and Henn (Pgt), was the most feared and devastating disease of wheat at one time worldwide. The fear from stem rust was understandable because an apparently healthy looking crop about 3 weeks prior to harvest could reduce to a black tangle of broken stems and shriveled grain by harvest. The disease has been controlled by deploying resistance genes in commercial cultivars. Successful control of disease over forty years has significantly reduced the risk of crop losses and research activities as well. Almost 50 stem rust resistance (Sr) genes are now catalogued, several of which have been used in commercial wheat cultivars worldwide. With the exception of Sr2 that confers slow rusting race non-specific resistance at adult-plant stage, all other Sr- genes are race specific, and are expressed at both seedling and adult-plant stages. Resistance gene Sr2 in addition to other unknown minor genes known as 'Sr2-complex' provided the foundation for durable resistance to stem rust in germplasms developed in CIMMYT. In addition to Sr2 some other stem rust resistance genes derived from alien resistance genes sources such as Sr24, Sr26, and Sr31 and more recently Sr38 have been used widely. Deployment of stem rust resistance genes declined the importance of stem rust worldwide. All translocation except with Sr26, are also carrying additional resistance genes for other wheat diseases. Sr31 known to be located on 1B.1R translocation is completely linked with Yr9 and Lr26, genes conferring resistance to stripe and leaf rust. The 1B.1R translocation has been used widely in spring, facultative and winter wheat breeding programs in Europe, China, Russia, and USA and in several cultivars in developing countries derived from CIMMYT germplasms during mid 1980s. It was not until 1999 that the first virulent pathotype to Sr31 was detected in Uganda. This race is commonly known as Ug99 and also has been designated as TTKSK using North American nomenclature system. Following its first detection in Uganda, investigation in neighboring countries revealed that same race migrated to Kenya in 1999/2002, to Ethiopia in 2003, and finally it was reported in Yemen in 2006. The migration route of Ug99 matches with the migration route of Yr9 virulent race of Puccinia striiformis f. sp. tritici (Pst) during 1986/1991. The Yr9-viruelnt Pst pathotype migrated from Africa to Iran in 1992 and caused 1 and 1.5 million tones crop losses in 1992 and 1993 respectively. The pattern of airflows and wheat growing distribution in these areas supports the migration routes similar to that taken by Yr9-virulent Pst pathotype in 1990s. Similar genetic background of wheat cultivars growing in the possible migration route of Ug99 that are highly susceptible to this pathotype will also enhanced the potential risk of epidemics of stem rust in this epidemiological zone. Based on the historical events and predictions, Iran will be one of the frontline countries in the predicated migration route of Ug99. It tooks five years for Yr9-virulent Pst pathotype to reach this country. Conducive environmental conditions exist in Iran and if an epidemic occurs from Ug99 or any variant of Pgt carrying virulence for stem rust resistance gene Sr31 that is present in most of the Iranian wheat cultivars derived from CIMMYT germplasms, serious catastrophic crop losses would be expected.
Before use of CIMMYT wheat germplasms started from 1977/78, stem rust was one of the serious diseases of wheat in Iran. Although Pgt have been observed locally on local wheat cultivars (Scharif et al., 1971) but there was no significant outbreaks of the disease since the last outbreaks in 1975-76 that occurred in Caspian Sea and southern parts of the country (Bamdadian and Torabi, 1978). In 2007, several reports of stem rust in Lorestan and Hamedan provinces (upper Karkheh basin) received that considered unusual, particularly for that in Hamedan which winter wheat habit cultivars are grown and there is no record of such stem rust infections in this area. Further investigation in these areas showed very late occurrence of stem rust on experimental plots and occasionally on farmer fields. Some of the farmer fields showed moderate to high levels of infections (60%). Susceptible field responses of some of the known sources for 1B.1R translocation, such as Federation *4/Kavkaz and Falat # Seri 82, were suspected for possible presence of Ug99. Stem rust infections of both genotypes were higher in experimental plots at Hamedan Agricultural Research Center. These infections were considered as potential risk of presence of Ug99 as Iran has been predicted to be the next targeted country. Samples were collected from both sites and increased on susceptible cultivars Bolani and Morocco in greenhouse condition at Cereal Pathology Research Unit, Seed and Plant Improvement Institute, Karaj.
For race analysis, two isolates collected in 2007 each from Boroojerd and Hamedan, one isolate from previous race analysis conducted in 1996 that was collected from Boroojerd, and two isolates collected in 2007 each from Poldokhtar (Southwest) and Kelardasht (North) areas, were included in seedling test of Sr-Near Isogenic Lines. In addition to Sr-NILs, wheat cultivars known to carry Sr31, Falat (Seri 82), Shiroudi, and the universal susceptible cultivar Morocco and Local Red were also tested against the five isolates. Urediniospores mixed with talcum powder were inoculated onto 8-day-old seedlings. Inoculated plants were placed in a dew chamber for 24 h at 24-26°C under the natural light conditions and then placed in a greenhouse at 26-28°C with a photoperiod of 16 h. Seedling infection types, described by McIntosh et al. (1995) were recorded 14 days after inoculation. Preliminary results showed that the two isolates collected from Kelardasht and Poldokhtar (in 2007) and an isolate from Boroojerd collected from 1996, exhibited low infection type (LIT 1+ to 2=) on Line E/ Kavkaz and Federation *4/ Kavkaz, both known to carry Sr31, and also Falat and Shiroudi. Both isolates collected from Boroojerd and Hamedan in 2007, exhibited high infection type (3+4) on Line E/Kavkaz and Federation×4/Kavkaz and also on cvs. Falat and Shiroudi. The susceptible cultivars Local Red and Morocco were susceptible to all five isolates.
The results from the present study indicate presence of virulence for stem rust resistance gene Sr31in two samples collected from Boroojerd and Hamedan in 2007. Whether these isolates are Ug99 or developed from local population of Pgt needs further. Although Nasrollahi et al., (2001) previously detected an Pgt isolate virulent to Sr31 in BorooJerd, but lack of stem rust in iran during 1997-2006 due to unfavorable climatical conditions hindered more investigation on virulence factors of Pgt, therefore statement on virulence for Sr31 in the region during last ten years is rather difficult. However, virulence for Sr31 should be considered as serious threat to wheat production in Iran. Field evaluation of 100 Iranian wheat cultivars and advanced lines to the Sr31 virulent Pgt pathotype (Ug99) in Kenya in 2006 indicated that 98% of entries were completely susceptible and this indicates the potential risk of any Sr31 virulent race on wheat production. Here detection of virulence for Sr31 in Iran is reported. This finding alarms the potential serious impact of stem rust outbreaks on national wheat production in Iran.