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Structures of barley mildew populations in the Czech Republic and North Dakota in 1995 |
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Email: dreiseitl@vukrom.cz Accepted for publication: 08 June 2000 Citation: Cereal Rusts and Powdery Mildews Bulletin [www.crpmb.org/] 2000/0608dreiseitl Abstract Progenies of single barley powdery mildew colonies from the Czech Republic and North Dakota were analysed. There were considerable differences in the frequencies of studied virulences, virulence complexity and diversity between the populations. The virulence frequency of the Czech population can be explained by direct selection on commonly grown barley cultivars. This explanation does not seem to be completely valid for the population in North Dakota (particularly for the Va3 and Vp virulences).
Powdery mildew [Blumeria (Erysiphe) graminis f. sp. hordei] is the most common disease on barley in the Czech Republic (Dreiseitl & Jurecka, 1996), as in many other countries in Europe. An extensive programme aimed at resistance breeding has been initiated in order to limit the economic impact of this disease (Brückner, 1987). Consequently, all spring and some winter barley cultivars grown in the Czech Republic possess at least one specific resistance gene to powdery mildew (Dreiseitl & Jørgensen, 2000). These cultivars, in turn, can affect the population structure of the powdery mildew fungus (Dreiseitl & Schwarzbach, 1994; Dreiseitl & Pickering, 1999). In North Dakota, powdery mildew occurs only sporadically. Thus, breeding for resistance to this disease is not a priority. In contrast to the Czech Republic, there are only a few cultivars registered in the USA and Canada that carry specific powdery mildew resistance genes (Dreiseitl & Steffenson, 1996). The purpose of this study was to compare the population structure of powdery mildew populations in regions that differ markedly for the importance of the disease and the corresponding efforts aimed at resistance breeding in barley. Materials and Methods Progenies of single powdery mildew colonies on barley were analysed. Conidia of the Czech population were air-sampled using a mobile spore-trap (Schwarzbach, 1979) in the Czech Republic and cultivated on cv. Pallas for 10 days. The conidia were sampled when most spring barley fields were at GS 50 (Zadoks et al., 1974). In North Dakota, naturally occurring mildew colonies were sampled from the top three leaves of line B3213 at GS 73 in a plot within the research farm at NDSU, Fargo. Leaf segments of B3213 (which does not possess any known powdery mildew resistance gene) (Dreiseitl & Steffenson, 1996) were used for the increase of the young fungal colonies. The leaf segments were placed in Petri dishes on water agar with 35 ppm benzimidazole and used for inoculation four days later. Colonies of conidia were collected with a 'varipipette' and distributed over the leaf segments with a syringe in a micro-settling tower. Dishes with water agar and approximately 25-mm-long primary leaf segments of the differential near-isogenic Pallas lines (Kølster et al., 1986) were placed in the tower one by one for inoculation (Table 1). After inoculation, the dishes with leaf segments were kept at 22± 2oC in the laboratory. Seven days later, infection types were read according to the scale of Torp et al. (1978). The pathotypes were designated by coded triplets (Limpert et al., 1994; Limpert & Müller, 1994) which express their virulences on the first 12 differential lines (Table 1) in the Czech population and all the mentioned lines, except for cv. Borwina, in the North Dakota population. Results The lowest virulence frequency in the Czech powdery mildew population was found for genes Mla3 (13%) and Mlat (41%). From 53 to 82% of the isolates exhibited virulence for genes Mla13, Mla9, Mla1, Mla12, Mlk, Mla6, and MlLa. Over 90% of the isolates exhibited virulence for Mla7, Mlg, and Ml(Bw) (Table 1). No isolate with virulence for Mla1, Mla6, Mla9, Mla13 or Mlg was found in the North Dakota powdery mildew population. The virulence frequency for genes Mlk and Mlat was very low (4 and 5%, respectively). Virulence for genes Mla7, Mla12, and Mlp was detected in 27 to 65% of isolates. A high frequency of isolates (83 to 98%) were virulent for genes MlLa, Mla3, Mla8, and Mlh. The virulence complexity of isolates (assessed on the first 11 common differential lines in Table 1 only) was 7.00 and 2.45 in the Czech and North Dakota population, respectively. The powdery mildew population in North Dakota displayed considerably less diversity than the Czech population. Table 2 shows the most frequent pathotypes (the virulences given in front of the dash were investigated in both populations) identified in the two populations. The three most frequent pathotypes in the Czech population included 9% of the isolates studied. They all carried virulence for genes Mla6, Mla7, Mla12, Mlk, MlLa, Mlg, and Ml(Bw). The three most frequent pathotypes in the North Dakota population included 43% of the isolates studied. All of these isolates were virulent for genes Mla13 and MlLa (and Mlh and Mla8 as well). Due to the large sample size difference in the populations, the other diversity parameters were not compared. Discussion The structure of the Czech powdery mildew population on barley is similar (with the exception of a higher portion of virulence for the gene Mla1) to that observed in neighbouring European countries (Limpert et al., 1991). This reflects an intensive utilization of corresponding resistances in cultivars grown in this part of the European continent (Brown & Jørgensen, 1991; Dreiseitl & Jørgensen, 2000). The absence of virulence for Mla1, Mla6, Mla9, Mla13 and Mlg in the North Dakota population was documented. The first four genes were highly effective against the European mildew population prior to their incorporation in commercial cultivars in Europe. The absence of virulence for gene Mlg is surprising from the present European point of view since the occurrence of corresponding virulence for this gene is often close to 100%. This result is also somewhat unexpected for North America because 11 barley cultivars are known to carry Mlg (Dreiseitl & Steffenson, 1996). However, Louter & Falk (1991) reported that the virulence for this gene ranged from 0 to 7% in southern Ontario from 1986 to 1989. By contrast, a high frequency of virulence for genes Mla3 and Mlp was detected in North Dakota. Neither of these two genes, however, was identified in 139 North American cultivars (Dreiseitl & Steffenson, 1996). Louter & Falk (1991) did not study the virulence for gene Mla3. However, they did report virulence for gene Mlp ranging from 33 to 43% (our result is 65%). Isolates with virulence for this gene have not been detected in Central Europe (Dreiseitl, unpublished; Jahoor, pers. commun.). The high frequency of these virulences in North Dakota cannot be explained by direct host selection. Virulences Va3 and Vp may be associated with other characteristics of the pathogen that facilitate its survival in the Northern Great Plains. The key difference between our results and those obtained by Louter & Falk (1991) are in virulences Va1 and Vk. These researchers observed virulence frequencies of 33 and 43% for Va1 and Vk, respectively, whereas in this study, the frequencies were 0 and 4%. Both these genes are present in a limited number of North American cultivars (Dreiseitl & Steffenson, 1996). Thus, the differences in virulence frequency could be explained by potential differences in the cultivar assortments of both regions. Acknowledgements This study was supported by the U.S.-Czech Science and Technology Program (grant No. 95-042) and the Grant Agency of the Czech Republic (grant No. 522/00/1062). References Brown JKM, Jørgensen JH, 1991. A catalogue of mildew resistance genes in European barley varieties. In: Jørgensen JH ed. Integrated Control of Cereal Mildews: Virulence Patterns and Their Change, 1991. Roskilde, Denmark, 263-286. Brückner F, 1987. Contribution of the world barley assortment to breeding for powdery mildew resistance (in Czech). In: Sbornik ke 100. vyroci narozeni N.I.Vavilova, 1987. Praha, 59-68. Dreiseitl A, Jørgensen JH, 2000. Powdery mildew resistance in Czech and Slovak barley cultivars. Plant Breeding 119, 203-209. Dreiseitl A, Jurecka D, 1996. Disease occurrence on spring barley in the Czech Republic in 1989-1995. Ochrana Rostlin 32, 221-229. Dreiseitl A, Pickering RA, 1999. Pathogenicity of Blumeria graminis f. sp. hordei in New Zealand in 1997. New Zealand Journal of Crop and Horticultural Science 27, 273-280. Dreiseitl A, Schwarzbach E, 1994. Composition of the powdery mildew population on barley in Central Moravia of the Czech Republic in 1992. Rostlinná Výroba. 40, 545-554. Dreiseitl A, Steffenson BJ, 1996. Postulation of powdery mildew resistance genes in North American barley cultivars. Barley Newsletter 40, 82-90. Kølster P, Munk L, Stølen O, Løhde J, 1986: Near-isogenic barley lines with genes for resistance to powdery mildew. Crop Science 26, 903-907. Limpert E, Andrivon D, Knittel R, Fischbeck G, 1991. Barley mildew in Europe: patterns of composition of the pathogen population during the period 1985-1988. ?In: Jørgensen JH ed. Integrated Control of Cereal Mildews: Virulence Patterns and Their Change, 1991. Roskilde, Denmark, 87-103. Limpert E, Clifford B, Dreiseitl A, Johnson R, Müller K, Roelfs A, Wellings C, 1994. Comparing systems of designation of pathotypes of plant pathogens. Journal of Phytopathology 140, 359-362. Limpert E, Müller K, 1994. Designation of pathotypes of plant pathogens. Journal of Phytopathology 140, 346-358. Louter JH, Falk DE, 1991. Virulence genotypes of Erysiphe graminis f. sp. in hordei southern Ontario from 1986 to 1989. Canadian Journal of Plant Pathology 13, 280 (Abstract). Schwarzbach E, 1979. A high throughput jet trap for collecting mildew spores on living leaves. Phytopathologische Zeitschrift 94, 165-171. Torp J, Jensen HP, Jørgensen JH, 1978. Powdery mildew resistance genes in 106 Northwest European spring barley varieties. Kgl. Vet.-og Landbohøjsk Årsskr.: 75-102. Zadoks JC, Chang TT, Konzak CF, 1974. A decimal code for the growth stages of cereals. Weed Research 14, 415-421. |