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Sources of powdery mildew resistance in barley landraces collected from Algeria and Tunisia |
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Citation: Cereal Rusts and Powdery Mildews Bulletin [www.crpmb.org/] 2000/0607czembor Abstract Introduction Barley (Hordeum vulgare L.) is one of the most important cereal crops in the North Africa. It is grown as landraces in marginal, low-input, drought-stressed environments both for grain and straw (Czembor, 2000; Ceccarelli et al. 1987, 1995). Powdery mildew, caused by the fungus Erysiphe graminis DC. f. sp. hordei Em Marchal (synamorph Blumeria graminis (DC.) Golovin ex Speer f. sp. hordei), is a serious foliar disease that affects the crop in many major production regions around the world and it is of great economic importance. The primary loss from powdery mildew is reduced yield, which can reach up to 20% for Europe and up to 30% for North Africa, although average losses are smaller and about 10% (Atzema, 1998; Ceccarelli et al., 1995; Jørgensen, 1994, Rasmusson, 1985; Zine Elabidine, 1992). Powdery mildew on barley is considered as one of the most clearly characterized system of host-pathogen genetic interactions. Since 1907, when Biffen started genetic studies of barley resistance to powdery mildew, more than 100 mildew resistance genes have been identified in barley. In Europe, the use of specific resistance genes to control barley powdery mildew began in the 1930s with the work of Honecker. This work was stimulated by an extraordinarily heavy attack of this pathogen in Germany in 1929. Since that period, barley breeders commonly used such resistance genes as Mla6, Mla7, Mla9, Mla12 and Mla13 belonging to the Mla locus and the resistance alleles Mlk, Mlg, MlLa, Mlh and Mlra (Biffen, 1907; Brown & Jørgensen, 1991; Jørgensen, 1994). However, virtually all of these genes were gradually overcome by virulent races within 4-5 years when cultivars containing them were used on a large acreage (Atzema, 1998; Czembor & Czembor, 1998, 1999b). Today, landraces of major crops including barley are available only in Gene Banks and many examples of their successful use to solve breeding problems exist including lack of sufficient resistance to diseases (Hintum, 1996; Hodgkin, 1998). The total number of barley accessions worldwide is estimated to be about 280 000 (Knüpffer & Hintum, 1995). Barley breeders are constantly using these genetic resources including sources of resistance to powdery mildew. Most of powdery mildew resistance genes used in modern cultivars of barley originated from landraces. Many of these landraces originated from West Asia, Ethiopia and North Africa including Morocco (gene Mlat - resistance Atlas) (Czembor, 1976, 2000; Jørgensen, 1994; Jørgensen & Jensen, 1997). In the most accepted theory, postulated in 1885 by Körnicke and Werner, barley was derived from its wild ancestor Hordeum spontaneum C. Koch. when Neolithic men selected spikes with tough rachis (Körnicke & Werner, 1885; Zohary & Hopf, 1988; Ladizinsky, 1998). Many studies showed that the original area of cultivation of H. vulgare L. was the area of the Fertile Crescent (Nesbitt, 1995; Willcox, 1995; Zohary & Hopf, 1988). Recently, the discovery of wild barley in Morocco has been reported (Molina-Cano & Conde, 1980; Molina-Cano et al., 1982). This finding suggest that the area of North Africa may be possible center of origin for cultivated barley and that barley may be a multicentric crop, domesticated along the Mediterranean basin (Molina-Cano et al., 1987, 1999; Moralejo et al., 1994). Taking this into account, barley landraces collected from Algeria and Tunisia may be rich source of new genes for resistance to powdery mildew due to their high degree of diversification resulting from the long coevolution with populations of pathogen (Czembor 1999, 2000). The aim of the present investigation was to identify new sources of powdery mildew resistance in selections from barley landraces collected from Algeria and Tunisia. Materials and Methods Plant materials Seed samples of 168 H. vulgare L. landraces were provided kindly by Dr. J. Valkoun and Prof. S. Ceccarelli (International Center for Agricultural Research in the Dry Areas - ICARDA, Aleppo, Syria). Among them 64 were collected in Algeria during three missions. Two of these missions (ICARDA collection codes DZA89A and DZA89B) were conducted in 1989 in provinces: Alger, Batna, Bone, Medea, Tiaret, Constantine and the third one (ICARDA collection code DZA90) in 1990 in provinces: Oran, Mostaganem, Saida, Saoura and Tlemcen.. Another 104 landraces were collected in Tunisia in 1990 during 2 missions (ICARDA collection codes TUN90 and TUN90-2) in 15 provinces: Kairouan, Siliana, Beja, Nabeul, Sousse, Sidi Bouzid, Gafsa, Kasserine, Le Kef, Bizerte, Ariana, Kebili, Medenine, Gabes and Sfax. All investigated landraces were of a spring growth type, had six row heads and covered kernels. Generally in Polish conditions they had low resistance for lodging and were intermediate in heading date. Pathogen Twenty three isolates of E. graminis f. sp. hordei Em Marschal were used (Table 1). They originated from the collections in Risø National Laboratory, Roskilde, Denmark; Danish Institute for Plant and Soil Science, Lyngby, Denmark, Edigenossische Technische Hochschule - ETH, Zurich, Switzerland provided kindly by Dr. H. J. Schaerer (ETH, Zurich, Switzerland) and IHAR Radzików, Poland. The isolates were chosen according to the virulence spectra observed on the 'Pallas' isolines differential set (Kølster et al., 1986), provided by Dr. L. Munk (Royal Agricultural and Veterinary University, Copenhagen, Denmark). They were purified by single pustule isolation, maintained and propagated on young seedlings of the cultivar 'Manchuria' (CI 2330). Frequent virulence checks assured their purity throughout the experiment. Disease Assessment After 8 - 10 days of incubation, the infection types were scored according to a 0 - 4 scale adopted from Mains & Dietz (1930) (Table 2). This scale was broaden by including score 0(4) describing infectiontype characteristic for gene mlo. The seedlings were classified into susceptible or resistant groups. Plants scored 0 - 2 were included in the resistant group and plants scored 3 and 4 were included in the susceptible group. Resistance tests This investigation was carried out during 1996 - 99 at IHAR Radzików, Poland. In winter 1996/97 about 30 plants per landrace were evaluated in the greenhouse with the R 303 isolate of E. graminis f. sp. hordei. R 303 represented the most avirulent isolate available allowing the expression of a maximum number of resistance genes. The cultivar 'Manchuria' was used as a susceptible control. Three (1.8%) landraces showed resistance reactions. Two of them (669 and 681) were collected in Algeria during mission DZA90 and another one (881) was collected in Tunisia during mission TUN90-2. Between one and three resistant plants for each landrace were grown in the greenhouse to obtain their seed. In this manner six single plant lines were created. These lines were tested with twenty three isolates during the 1998/99 winter in the IHAR Radzików greenhouse. The plants were grown with 16 h lights and 16-22 oC range of temperature. The inoculation was carried out when plants were 10 - 12 days old by shaking or brushing conidia from diseased plants. After 8-10 days of incubation the disease reaction types shown on the seedlings were scored. Postulation of resistance alleles Hypotheses about the specific resistance genes present were made from the comparison of the reaction spectra of the tested lines with those of differential lines. The lines giving the same reaction spectra with all isolates were classified in the same group. Identification of resistance genes was made by eliminating resistance genes not present in tested lines. Next step was determining the postulated and possible resistance genes. It was done on the basis of the gene for gene hypothesis. In the case when a compatible reaction (scores 3 and 4) was observed with one given isolate, it meant that the cultivar did not possess the resistance alleles for which the isolate was avirulent. Incompatible reactions (scores 0-2) with isolates possessing only one avirulence allele among the remaining possible resistance alleles made it possible to postulate that the matching resistance allele was present (Czembor & Czembor 1998, 1999b; Flor 1956). Results All 6 tested lines possessed resistance allele or alleles for powdery mildew of barley (Table 3). However only line 681-1-2 was identified with resistance to all prevalent in Europe powdery mildew virulence genes. This line was characterized by resistance reaction type 2 which was observed for inoculation with all isolates. Another 5 tested lines showed resistance to more than 73% of isolates. They were scored 2 for inoculation with most isolates used. The distribution of reaction type readings indicate the minimum number of genes involved because different genes for resistance may condition different reaction types. Based on this assumption it may be concluded that lines 681-1-3 and 881-1-1 may had many genes for resistance. The distribution of reaction type readings indicates that 84% of all reaction types observed were classified as powdery mildew resistance (score 0, 1 and 2). The majority (96%) of resistance reaction types observed in tested lines were intermediate resistance reaction type two. Rasistance allele Mlat was postulated to be present in two tested lines (669-2-2 and 881-1-1). In 4 tested lines it was impossible to determine which specific gene or genes for resistance were present. However in 2 lines (669-1-2 and 669-3-1) presence of allele Mlat was suggested as possible. All tested lines possesed unknown genes for resistance. Most probably these lines possessed alleles not present in the 'Pallas' near-isogenic line differential set. Discussion The results presented here demonstrate practical advantages of preserving the genetic diversity of barley in the form of landraces. Among 168 investigated landraces from Algeria and Tunisia three (1.8%) showed resistance for E. graminis f. sp. hordei. However, only line 681-1-2 was resistant to all isolates used. This line had resistance to all powdery mildew virulence genes prevalent in Europe. This conclusion is based on the fact that isolates used in this experiment had virulences corresponding to all major resistance genes used in the past and currently in Europe. Another 5 lines showed resistance for inoculation with most isolates used. Taking this into account these lines should be used in breeding of barley as a new effective sources of resistance to powdery mildew. The frequency of powdery mildew resistant landraces (landrace 681) in the present study, 0.6 per cent, is similar or lower than assessed in other studies (Czembor, 1976, 1999, 2000; Czembor et al., 1979; Czembor & Czembor, 1999a, 2000a, 2000b; Czembor & Johnston, 1999; Brückner, 1964; Honecker, 1938; Jönsson & Lehmann, 1999; Jørgensen & Jensen, 1997; Leijerstam, 1996; Leur et al., 1989; Lehmann & von Bothmer, 1988; Lehmann et al., 1998; Negassa, 1985; Nover & Lehmann, 1973; Ralski & Mikoajewicz, 1958; Wiberg, 1974). Most probably this was caused by using the various methods and isolates of powdery mildew for screenings landraces for resistance. In last 30 years use of fungicide treatment against E. graminis f.sp. hordei become common practise to control barley powdery mildew. However, future strategies for the control of powdery mildew will have to focus increasingly on more ecologically acceptable methods because any usage of chemicals (pesticides, fungicides, herbicides, and mineral fertilizers) in agriculture is increasingly criticized in societies of many countries. Perhaps the most effective method is breeding for resistance. Application of resistance is considered also as relatively inexpensive and convenient for the farmer because use of fungicides require investments in machinery, chemicals, labour and special training (Brown, 1996; Czembor & Gacek, 1990, 1995; Gullino & Kuijpers, 1994; Jacobsen, 1997). However breeding for resistance to powdery mildew of barley is faced with a highly mobile pathogen, whose gene-pool forms an almost infinite source of genetic variation (Hovmøller et al., 2000; Müller et al., 1996; O'Hara & Brown, 1997). About 36 genes for specific resistance have been used in more than 700 european barley varieties since the first gene, Mlg, was introduced on a large scale in the 1930s in Germany (Jørgensen, 1994; Wolfe & Schwarzbach, 1978; Wolfe & MacDermott, 1994). However, 28 of these alleles are closely linked or allelic, which limits the possible number of gene combinations in breeding of new varieties (Brown & Jørgensen, 1991; Czembor & Gacek, 1990; Jørgensen, 1994; Wolfe & McDermott, 1994). Almost all of these genes were successively overcome by the appearance of pathotypes with matching virulence. These varieties had to be discarded because they were far too susceptible to be of any further value. This susceptibility was due to loss of partial resistance during breeding for race-specific resistance (Vertifolia effect) (Czembor & Gacek, 1990; Vanderplank, 1982). The opposite situation occurs when farmers grow barley landraces. In this case powdery mildew rarely develops to levels that significantly damage the yield. This has been attributed both to the stabilizing effect of the genetic heterogeneity within the landraces and to the presence of resistance sufficient to control the limited disease development (Andrivon & Vallavielle-Pope, 1992; Leur et al., 1989). This was confirmed in this study. The most frequent observed resistance reaction type in tested lines was 2 (96%) and all six tested lines showed this reaction for inoculation with more than 50% isolates used. This is different from resistance reaction conferred by most of powdery mildew resistance genes used in Europe which confer mostly reaction type 0 and 1 (Brown & Jørgensen, 1991; Czembor & Czembor, 1998, 1999b; Jensen et al., 1992, Jørgensen, 1992a, 1994). In all the selected lines presence of unknown genes alone or in combinations with specific ones were detected. Most probably these lines possessed alleles not present in the 'Pallas' near-isogenic line differential set. Therefore these lines represent new sources of resistance to powdery mildew. Presence of high number of unknown genes in barley landraces is in agreement with findings in other studies (Czembor, 1976, 1999, 2000; Czembor et al., 1979; Czembor & Czembor, 1999a, 2000a, 2000b; Czembor & Johnston, 1999; Brückner 1964; Honecker, 1938; Jönsson & Lehmann, 1999; Jørgensen & Jensen, 1997; Leijerstam, 1996; Leur et al., 1989; Lehmann & von Bothmer, 1988; Lehmann et al., 1998; Negassa, 1985; Nover & Lehmann, 1973; Ralski & Mikoajewicz, 1958; Wiberg, 1974). However, per cent of lines in which unknown genes were identified is higher than those observed in other investigations (Czembor, 1999, 2000; Czembor & Czembor, 1999a). Rasistance allele Mlat was postulated to be present in two tested lines (669-2-2 and 881-1-1) and in another two lines 669-1-2 and 669-3-1 presence of allele Mlat was suggested as possible. This is in agreement with the fact that virulence to Mlat is very common in Moroccan mildew population and that Mlat resistance gene was originally described from western North Africa (Caddel, 1976; Jørgensen, 1994; Yahyaoui et al., 1997). New sources of resistance to powdery mildew originated from landraces should be a relatively easy to incorporate into a barley breeding programs in comparison to those originating from mutants or wild barley. A good example of this is introduction of Mlo resistance into modern European barley cultivars. All twenty five different mlo alleles with exception of mlo11 were obtained by mutagenesis. However, almost all barley cultivars with Mlo resistance have the same allele mlo11 which originated from the Ethiopian landrace L92 (Atzema, 1998; Jørgensen, 1992a, b, 1994; Pickering et al., 1995) Furthermore undesirable agronomic traits that are usually derived from wild relatives do not have to be bred out when using landraces as a source of powdery mildew resistance. Using barley landraces in breeding programmes also has other advantages including the incorporation of desirable agronomic traits such as good adaptation to dry land conditions (Ceccarelli et al., 1987, 1991, 1995; Lakew et al., 1997; Yahyaoui et al., 1996). Many germplasm collection missions to Algeria and Tunisia gathered very diverse plant material including barley. Most probably it was due to big contrasts in these countries, both in geographical conditions and in agricultural practices, between the Mediterranean area and the Sahara desert (Da'aloul, 1995; Echikh at al., 1997; Malki at al., 1995; Neffati & Pistrick, 1993; Pistrick et al., 1993). Collecting missions in North Africa are recommended by many investigators because barley landraces in these countries are subject to genetic erosion. This erosion is caused mostly by drought and desertification (Damania, 1988; Perrino et al., 1986; Tazi et al., 1989; Zine Elabidine et al., 1995). Very important for farmers and barley breeders is the durability of the resistance genes to powdery mildew. It may be increased by using many different strategies for deploying resistance genes in barley. Most common of these strategies are: multiline cultivars, combining different resistance genes into one cultivar and deploying many cultivars with different resistance genes in space (e.g. cultivar mixtures) or time (winter versus spring barley) (Czembor & Gacek, 1995, 1996; Jørgensen, 1994, Newton & Gacek, 1998). New sources of resistance, including sources described in this study, may be used by barley breeders and farmers in different strategies (Jørgensen, 1994; Jørgensen & Jensen, 1997; Wolfe 1984). The results presented here come from the tests performed on seedlings. This will not necessarily predict adult plant resistance and field performance of the selected resistant lines. However, determination of powdery mildew resistance genes based on tests performed on seedlings is very effective and sufficient for breeders and pathologist needs (Brown & Jørgensen, 1991; Czembor & Czembor, 1998, 1999b; Jensen et al., 1992; Jensen & Jørgensen, 1981; Jørgensen, 1992a). Generally, confirmation of resistance composition can only be established by a test for allelism through crosses and backcrosses among appropriate hosts (Czembor & Johnston, 1999; Heitefuss et al., 1997). Also different levels of partial resistance in tested lines may influence conclusions concerning postulation of the presence of specific resistance genes (Czembor & Johnston, 1999; Jørgensen, 1994). This study confirmed findings in other investigations that barley landraces possess mildew resistance genes different from genes present in cultivated varieties (Czembor 1976, 1999, 2000; Czembor et al. 1979; Czembor & Czembor 1999a, 2000a, b; Czembor & Johnston, 1999; Brückner 1964; Honecker 1938; Jönsson & Lehmann 1999; Jørgensen & Jensen 1997; Leijerstam 1996; Leur et al. 1989; Lehmann & von Bothmer, 1988; Lehmann et al. 1998; Negassa 1985; Nover & Lehmann 1973; Ralski & Mikoajewicz 1958; Wiberg 1974). The use of new effective sources of resistance described in this study, especially line 681-1-2 , may increase the diversity of the powdery mildew resistance genes present in barley cultivars in Europe. Acknowledgements The author thank Dr. J. Valkoun and Prof. S. Ceccarelli (International Center for Agricultural Research in the Dry Areas - ICARDA, Aleppo, Syria) for providing seed samples of barley landraces from Algeria and Tunisia, Dr. H. J. Schaerer (Edigenossische Technische Hochschule - ETH, Zurich, Switzerland) for the powdery mildew isolates and Dr. L. Munk (Royal Agricultural and Veterinary University, Copenhagen, Denmark) for the near-isogenic lines of Pallas. References Andrivon D, de Vallavieille-Pope C, 1992. Race-specific resistance genes against Erysiphe graminis f. sp. hordei in old and recent French barley accessions. Plant Breeding 108, 40-52. Atzema JL, 1998. Durability of mlo resistance in barley against powdery mildew caused by Erysiphe graminis f. sp. hordei, Ph. 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