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Published in Issue No. 122, page 32 to 35 - (12829) characters
Identification of powdery mildew and leaf rust resistance genes in common wheat (Triticum aestivum L. em. Thell.) cultivars grown in Bulgaria and RussiaNedialka Petrova Sai L.K. Hsam Penko Spetsov Friedrich J. Zeller Introduction
Wheat is the most widely grown and consumed food crop in many parts of the world. However, powdery mildew, caused by the pathogen Erysiphe graminis f.sp. tritici, is a destructive foliar disease in regions with a maritime climate. Likewise leaf rust, caused by the fungus Puccinia recondita tritici, threatens common wheat not only in hot and dry regions but also in places with mild climatic conditions (Dubin and Rajaram 1996). Wheat-yield losses may be prevented by timely applications of fungicides. However, for economic and environmental reasons, breeding for genetic resistance is the preferable and sustainable method of control.
To date, 25 genes resistant to powdery mildew (for a review see Szunics and Szunics 1999) and 47 genes resistant to leaf rust (Kolmer 1996; Dubcovsky et al. 1998) have been detected and assigned to specific chromosomes. The use in many European countries of genes resistant to powdery mildew has been reported by Zeller and Hsam (1998); however, only a few studies have been carried out in Bulgaria and Russia (Lebedeva 1994; Iliev 1999). Information on the occurrence and distribution of plants containing genes resistant to these diseases would help in exploiting and characterizing these plants in order to control the diseases. An attempt is made in the following investigation to determine the powdery mildew and leaf rust resistance genes present in the wheat cultivars and lines grown in Bulgaria and Russia.
Material and methods
Thirty-seven wheat cultivars and lines from Bulgaria, and 44 from Russia were screened for disease resistance. Russian cultivars and lines were provided by Tatjana Danilova, Moscow, Russia. The near-isogenic line of ‘Chancellor’ with known powdery mildew resistance genes and ‘TP114/2*Starke’ possessing gene Pm6 (Table 1) were supplied by R.A. McIntosh, Sydney, Australia. Eleven Erysiphe graminis tritici (Egt) isolates used to differentiate the known major resistance genes were collected from different parts of Europe and selected from single spore progenies. The Egt isolates were classified under Weihenstephan accession numbers (Table 1) and maintained at the Institut für Pflanzenbau und Pflanzenzüchtung, Weihenstephan. Tests for mildew resistance were conducted on primary leaf segments cultured on 0.6% w/v agar and 35mg/l benzimidazole in plastic boxes. The methods of inoculation, conditions of incubation and disease assessment followed were those of Hsam and Zeller (1997). Three major classes of host reactions were distinguished: r = resistant, I = intermediate, s = susceptible. Resistance genes found in the tested cultivars and lines were checked by comparing them with the disease-response patterns exhibited by the standard differential cultivars.
Leaf rust resistance tests were also performed on primary leaf segments in petri dishes on 6 g/l agar and a final benzimidazole concentration of 35 mg/l following the method of Felsenstein et al. (1998). Uredospores of race specific Puccinia recondita tritici (Prt) isolates were applied to leaf segments of the cultivars and assessed for disease response 11 days after inoculation using the 0 to 4 scale of Stakman et al. (1962). Infection type 1 was regarded as resistant (R), type 2 as moderately resistant (MR), type 3 as moderately susceptible (MS), and type 4 as susceptible (S). Most of the Prt isolates used (Moro, 365, F6/2, CH2/1, ASL351, B1/4, 672) were collected from Europe and Mediterranean regions and selected from single spore progenies. The isolate Prt8 was provided by Z.A. Pretorius, Bloemfontein, South Africa and Race 9 by P.L Dyck, Winnipeg, Canada.
Results and discussion
To compare response patterns 11 differential isolates of Egt were used to distinguish between host lines/cultivars with documented genes for resistance (Table 1). The differential responses of the resistance genes to specific Egt isolates enabled the identification of individual gene loci and genes in combination (Kowalczyk et al. 1998). Cultivars and lines that showed susceptible and intermediate response to 11 differential isolates of Egt are listed in Table 2. Seven Bulgarian cultivars showed susceptible responses and 8 cultivars from Bulgaria and 17 cultivars grown in Russia exhibited intermediate response to the specific Egt isolates used.
Several cultivars were characterized by identical disease-resistance patterns as the standard differential with documented resistance genes to powdery mildew. Bulgarian cultivars ‘Aglika’, ‘Dobrotitsa’, ‘Galatea’, ‘Kitten’, ‘Murgavets’, ‘Priaspa’, ‘Sadovska Belia’, ‘Slavianka’, ‘Todora’, ‘Yantar’, ‘Zora’ and ‘line 64/5’, as well as the Russian cultivars ‘Altaiskaya’, ‘ANK-1-E’, ‘Dneprenka’, ‘Kutulukskaya’, ‘Kurskaya’, ‘Lutescens 764h1’, ‘Lutescens 851h4’, ‘Lutescens 1889’, ‘Rosianka’ and ‘Saratovska’ carried resistance gene Pm5 (Table 3), as their responses to the Egt isolates corresponded well with the pattern displayed by the cultivar ‘Hope’ (Table 1). The cultivar ‘Enola’ showed the same response pattern as the differential cultivar ‘Disponent’ carrying resistance gene Pm8 from cultivated rye, Secale cereale. The Russian cultivar ‘Irina’ revealed a response pattern indicative of the presence of a combination of resistance genes Pm1+3a and line 2561 of the combined resistance genes Pm1+5, while the Bulgarian cultivars ‘Shabla’ and ‘Karat’ showed resistance genes Pm1+Pm8 and Pm5+Pm8, respectively. Somatic chromosomes in mitotic root-tip cells of cultivars ‘Enola’, ‘Shabla’ and ‘Karat’ that possess the Pm8 gene are characterized cytologically by the presence of only two satellited chromosomes (6B), indicating the deficiency of the short arm of chromosome pair 1B, which has been replaced by rye chromosome arm 1RS.
The response pattern of a group of four Russian cultivars (‘Harkovskaya’, ‘Lutescens 640’, ‘PPG 38/1’ and ‘Sibirskaya 41’) corresponded to that of differential British wheat cultivar ‘Maris Huntsman’, known to have the combined resistance genes Pm2+Pm6 (Table 1). The Bulgarian cultivars ‘Vratsa’ and ‘Svilena’ appear to possess a combination of resistance genes Pm1+Pm2 +3a and Pm1+Pm5+Pm8, respectively.
A group of five Bulgarian cultivars and 11 Russian cultivars showed resistance to specific Egt isolates, but susceptible and/or intermediate response to other isolates. The resistance of these cultivars was tentatively characterized as unknown (u) resistance, because their response patterns did not correspond to any of the lines possessing major single resistance genes or gene combinations. The Bulgarian cultivars ‘Kristal’ and ‘Pliska’ expressed resistance response to all Egt isolates used. However, the origin of the resistance genes could not be determined, as no information on their ancestries was available.
Recently virulence variabilities of Egt populations in Bulgaria were conducted by Iliev (1999). Results showed the following virulence frequencies for various resistance genes to powdery mildew: Pm1 (97.6%), Pm2 (39.1%), Pm3a (60.5%), Pm5 (89.6%). This indicates that with the exception of cultivars ‘Kristal’ and ‘Pliska’, and possibly some other cultivars (Table 2), Bulgarian common wheat is not very resistant to powdery mildew pathotypes.
Nine isolates of Puccinia recondita tritici (Prt) were used to inoculate Bulgarian and Russian common wheat cultivars and the response patterns compared with a differential set of wheat lines possessing individual documented genes resistant to leaf rust (data not shown). The Bulgarian cultivar ‘Dobrotitsa’ was susceptible to all isolates. Eight further cultivars grown in Bulgaria and 27 Russian cultivars gave an intermediate response to the differential isolates (Table 4) possibly indicating the presence of quantitatively inherited resistance genes. Gene Lr3ka was the most commonly documented resistance gene in Bulgarian cultivars (‘Yantar’, ‘Todora’, ‘Priaspa’, ‘Milena’, ‘Slavianka-196’, ‘Laska’, ‘Rusalka’, ‘Trakya’, ‘Prosto’). The expression of gene Lr26 attributed to the 1BL.1RS wheat rye translocated chromosome was not clearly detected in those cultivars possessing 1BL.1RS and carrying Pm8 resistance, such as ‘Enola’, ‘Shabla’, ‘Karat’ and ‘Svilena’. The isogenic Lr26 line is resistant to all the Prt isolates used except ASL351 to which it is susceptible. Cultivars ‘Enola’ and ‘Slivenka’ showed very similar disease reactions. Similarly, the cultivars ‘Sadovo-1’, ‘Sadovo-772’, ‘Diamant’ and ‘Bononia’ revealed very similar disease response patterns. However, the source of their resistance could not be deduced from the available information of the pedigrees.
Among the cultivars tested, three cultivars, namely ‘Pobeda’, ‘Kristal’ and ‘Anglika’ from Bulgaria, as well as ‘Analog 4’ from Russia, possessed a wide spectrum of disease response to the nine Prt isolates employed. The cultivar ‘Pobeda’ showed cytologically a 1BL.1RS wheat-rye translocation and possessed a complex pedigree (T. sphaeroccum var. rotundatum//T. durum/Secale montanum/2/Bezostaya-1/3/Mexican-225). It is very likely that S. montanum, a wild species of rye, is involved in the wheat-rye translocated chromosome as ‘Probeda’ also expressed powdery mildew resistance that differed from that of Pm8 contributed by S. cereale (Table 3). Further information could not be obtained from cultivar ‘Kristal’ possessing a pedigree of (F-9-75/3837-5-5//7523-11). Nevertheless, this cultivar, together with the cultivar ‘Pobeda’, should be useful in combating powdery mildew and leaf rust diseases in Bulgaria and many other European countries. Further plant exploration in the region is needed to detect new genetic variation for resistance for use in future wheat breeding.
The authors thank Heidrun Glöckner for the excellent technical help provided and gratefully acknowledge the financial support provided for N. Petrova by the Deutsche Forschungs-Gemeinschaft (DFG), Bonn.
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