Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

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significant evolutionary force at present. However, we consider it more likely that some gene flow continues as a result of the global commerce in grain. The obvious mechanism for gene flow on a regional basis is air-dispersed ascospores. In the case of P. nodorum, the most likely mechanism for intercontinental dispersal is infected seed (King et al., 1983). Since it has been shown that M. graminicola can infect seed (Brokenshire, 1975) we consider it likely that this also is the mechanism for long distance gene flow in M. graminicola. Whatever the mechanism, the high degree of similarity in populations around the world for both fungi suggests that they have been transported around the world by humans. Evidence for selection We recently completed an experiment to measure competition among 10 genotypes of M. graminicola in a field setting. The 10 isolates were inoculated onto three host treatments consisting of a moderately resistant wheat variety (Madsen), a susceptible wheat variety (Stephens) and a 1:1 mixture of these cultivars. Our most important finding in this experiment was that intense competition appeared to occur among the different genotypes. Significant changes in the frequencies of specific pathogen genotypes occurred over the season (McDonald et al., 1999). Some isolates showed evidence for adaptation to particular hosts. The results from this experiment provided our first direct evidence Population Genetics of Mycosphaerella graminicola and Phaeosphaeria nodorum 81 that selection operates on specific M. graminicola pathogen genotypes in a field setting. We have not yet conducted similar replicated field experiments to measure selection in P. nodorum. But we have indirect evidence that selection does not result in widespread clones that are adapted to specific host genotypes. In an experiment conducted in Switzerland in collaboration with Martin Wolfe’s group, we sampled 50 isolates of P. nodorum from each of nine wheat fields near Zurich. Three different wheat varieties were represented three times each among the nine wheat fields. Though fields planted to the same variety used the same source of seed, no genotypes were shared among field populations. Only six pairs of clones were found among the 432 isolates that were assayed. Isolates with the same DNA fingerprints always came from the same site within a field (Keller et al., 1997b). Taken together, all of our experiments suggest that nuclear genotypes do not persist through time for either fungus. Instead, the genes are the units of selection that are carried forward across generations. Selection operates on the population instead of the individual. In order to gain a representative spectrum of the diversity for virulence in natural populations, plant breeders should include the widest possible diversity of strains when screening germplasm for resistance to these fungi. Similarly, chemical companies should include at least several hundred strains in their screens for resistance to fungicides. Conclusions Given our present data, we have drawn the following conclusions regarding the evolutionary forces that affect the population genetics of M. graminicola and P. nodorum: • For both fungi, the mating system includes both sexual and asexual reproduction. Asexual reproduction may have an important impact over an area of a few square meters, but the sexual reproduction has much greater consequences for the evolutionary biology of both fungi. Genotypes are ephemeral but genes persist in populations through time. • Population sizes are large enough to make genetic drift negligible for both fungi. Large population sizes also ensure that ample mutations are present in every population to allow for a rapid response to selection, e.g. mutations from avirulence to virulence for major resistance genes. The population from Patzcuaro, Mexico, exhibits a genetic structure consistent with a founder effect. • Gene flow is sufficient to unite large geographical areas into a single genetic population. If gene flow is ongoing, then breeders should continue to test their resistant lines over the widest possible geographical area. If gene flow is episodic, continued vigilance is needed to limit the spread of new virulence genes and fungicide resistance genes. Quarantines in areas with low gene diversity, such as Australia, should be enforced to limit the evolutionary potential of these populations. If CIMMYT continues to use Patzcuaro as a field site to screen for resistance
82 Session 4 — B.A. McDonald, C.C. Mundt, and J. Zhan to M. graminicola and P. nodorum, it may want to consider introducing more diverse fungal populations from other parts of Mexico into this disease nursery. • Selection appears to operate on both nuclear and mitochondrial genomes in M. graminicola. Though selection may increase the frequency of particular genotypes over the course of a growing season, it appears that particular genotypes are unlikely to reach high frequencies within field populations because of the limited dispersal potential for conidia. But the genes in the most fit individuals will persist and be recombined to create novel genotypes in the next growing season. Over the course of many growing seasons, selection will change the frequency of genes that affect adaptation to the wheat host, but new genotypes will appear each season. It is too early to say if selection operates the same way in P. nodorum. In summary, the population genetics of P. nodorum and M. graminicola are very similar. This probably reflects the similarity in their life histories. Both fungi produce airborne sexual ascospores and splash-dispersed asexual spores. They both infect aboveground plant parts on the same host and they both infect seeds that can be transported globally as part of the world commerce in wheat. Use of multi-allelic, neutral genetic markers combined with hierarchical sampling has allowed us to achieve a much greater understanding of the population biology of both fungi. Acknowledgments BAM gratefully acknowledges the many collectors and collaborators around the world who responded to his request for infected leaf material. Funding for this project came from the USDA National Research Initiative Competitive Grants Program (Grant # 93-37303-9039), the National Science Foundation (Grant # DEB- 9306377), the Texas Agricultural Experiment Station (Hatch project #6928), and the Swiss National Fund (Grant # 5002-38966). References Boeger, J.M., Chen, R.S., and McDonald, B.A. 1993. Gene flow between geographic populations of Mycosphaerella graminicola (anamorph Septoria tritici) detected with RFLP markers. Phytopathology 83:1148-1154. Brokenshire, T. 1975. Wheat seed infection by Septoria tritici. Transactions of the British Mycological Society 64:331-335. Chen, R.S., and McDonald, B.A. 1996. Sexual reproduction plays a major role in the genetic structure of populations of the fungus Mycosphaerella graminicola. Genetics 142:1119-1127. Chen, R.S., Boeger, J.M., and McDonald, B.A. 1994. Genetic stability in a population of a plant pathogenic fungus over time. Molecular Ecology 3:209-218. Keller, S.M., McDermott, J.M., Pettway, R.E., Wolfe, M.S., and McDonald, B.A. 1997a. Gene flow and sexual reproduction in the wheat glume blotch pathogen Phaeosphaeria nodorum (anamorph Stagonospora nodorum). Phytopathology 87:353-358. Keller, S.M., Wolfe, M.S., McDermott, J.M., and McDonald, B.A. 1997b. High genetic similarity among populations of Phaeosphaeria nodorum across wheat cultivars and regions in Switzerland. Phytopathology 87:1134-1139. King, J.E., Cook, R.J., and Melville, S.C. 1983. A review of Septoria diseases of wheat and barley. Annals Applied Biology 103:345-373. Kohli, Y., Brunner, L.J., Yoell, H., Milgroom, M.G., Anderson, J.B., Morrall, R.A.A., and Kohn, L.M. 1995. Clonal dispersal and spatial mixing in populations of the plant pathogenic fungus, Sclerotinia sclerotiorum. Molecular Ecology 4:69-77. McDonald, B.A. 1997. The population genetics of fungi: tools and techniques. Phytopathology 87:448- 453. McDonald, B.A., and Martinez, J.P. 1990a. DNA restriction fragment length polymorphisms among Mycosphaerella graminicola (anamorph Septoria tritici) isolates collected from a single wheat field. Phytopathology 80:1368-1373. McDonald, B.A., and Martinez, J.P. 1990b. Restriction fragment length polymorphisms in Septoria tritici occur at a high frequency. Current Genetics 17:133-138. McDonald, B.A., and Martinez, J.P. 1991. DNA fingerprinting of the plant pathogenic fungus Mycosphaerella graminicola (anamorph Septoria tritici). Experimental Mycology 15:146-158. McDonald, B.A., Miles, J., Nelson, L.R., and Pettway, R.E. 1994. Genetic variability in nuclear DNA in field populations of Stagonospora nodorum. Phytopathology 84:250- 255. McDonald, B.A., Pettway, R.E., Chen, R.S., Boeger, J.M., and Martinez, J.P. 1995. The population genetics of Septoria tritici (teleomorph Mycosphaerella graminicola). Canadian Journal of Botany 73 (supplement), S292-S301. McDonald, B.A., Zhan J., Yarden O., Hogan K., Garton J., and Pettway R.E. 1999. The population genetics of Mycosphaerella graminicola and Phaeosphaeria nodorum. pp. 44-69 In: Septoria in Cereals: a Study of Pathosystems. Lucas, J.A., Bowyer, P., Anderson, H.M., eds. CABI Publishing, Wallingford, UK. Zhan, J., Mundt, C.C., and McDonald, B.A. 1998. Measuring immigration and sexual reproduction in field populations of Mycosphaerella graminicola. Phytopathology 88:1330-1337.
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Septoria and Stagonospora Diseases
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ii The Organizing Committee express
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iv 87 Genetic Variability in a Coll
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vi Foreword In the mid-1970s the id
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2 Opening Remarks — S. Rajaram ar
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4 Opening Remarks — S. Rajaram Ta
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6 Opening Remarks — S. Rajaram cu
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8 Opening Remarks — S. Rajaram br
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10 Opening Remarks — S. Rajaram T
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12 Opening Remarks — S. Rajaram t
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14 Opening Remarks — S. Rajaram C
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16 Opening Remarks — S. Rajaram r
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Session 1: Pathogen Biology Biology
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Table 2. The Septoria tritici/Stago
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Molecular Analysis of a DNA Fingerp
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histidine kinase in yeast, although
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According to the previous observati
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cultivars. The presence of pycnidia
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Page 42 and 43: Table 1. Sources of isolates or DNA
Page 44 and 45: Septoria/Stagonospora Leaf Spot Dis
Page 46: Interrelations among Septoria triti
Page 49 and 50: 42 Session 2 — B.M. Cunfer disper
Page 51 and 52: 44 Session 2 — B.M. Cunfer beyond
Page 53 and 54: 46 Aggressiveness of Phaeosphaeria
Page 55 and 56: 48 Session 2 — P. Halama, F. Lala
Page 57 and 58: 50 Growth of Stagonospora nodorum L
Page 59 and 60: 52 Session 3A — G.H.J. Kema and E
Page 61 and 62: 54 A Possible Gene-for-Gene Relatio
Page 63 and 64: 56 Diallel Analysis of Septoria Tri
Page 65 and 66: 58 Session 3A — X. Zhang, S.D. Ha
Page 67 and 68: 60 Session 3A — L. Gilchrist, C.
Page 69 and 70: 62 Session 3A — L. Gilchrist, C.
Page 71 and 72: 64 Session 3B — E. Arseniuk and P
Page 75 and 76: 68 Cultivar variances Cultivar vari
Page 79 and 80: 72 Session 3B — N.E.A. Murphy, R.
Page 81 and 82: 74 Inheritance of Septoria Nodorum
Page 83 and 84: 76 Session 3B — N.E.A. Murphy, R.
Page 85 and 86: 78 Session 4 — B.A. McDonald, C.C
Page 87: 80 Session 4 — B.A. McDonald, C.C
Page 91 and 92: 84 Session 4 — E. Arseniuk, H.S.
Page 93 and 94: 86 Session 4 — C. Cowger, C.C. Mu
Page 95 and 96: 88 each isolate. Two of the probes
Page 97 and 98: 90 Mating Type-Specific PCR Primers
Page 99 and 100: 92 Session 4 — Bennett, R.S., S.-
Page 101 and 102: 94 Session 5 — M.W. Shaw and the
Page 103 and 104: 96 Session 5 — M.W. Shaw The path
Page 105 and 106: 98 Spore Dispersal of Leaf Blotch P
Page 107 and 108: Session 5 — C.A. Cordo, M.R. Sim
Page 109 and 110: 102 Epidemiology of Seedborne Stago
Page 111 and 112: Session 5 — D.A. Shah and G.C. Be
Page 113 and 114: 106 Session 6A / Session 6B — J.M
Page 119 and 120: Session 6A / Session 6B — C.C. Mu
Page 125 and 126: 118 Session 6C — M. van Ginkel an
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Session 6C — M. Díaz de Ackerman
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134 Septoria tritici Resistance Sou
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Session 6C — L. Gilchrist et al.
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Session 6C — L. Gilchrist et al.
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140 Selecting Wheat for Resistance
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Session 6C — R.M. Gonzalez I., S.
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Session 6C — R.M. Gonzalez I., S.
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Session 6C — R. Loughman, R.E. Wi
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148 Field Resistance of Wheat to Se
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150 Using Precise Genetic Stocks to
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Session 6C — C.M. Ellerbrook, V.
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154 Evaluating Triticum durum x Tri
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156 Sources of Resistance to Septor
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Session 6C — H. Toubia-Rahme and
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160 Partial Resistance to Stagonosp
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Session 6C — C.G. Du, L.R. Nelson
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Session 6C — D.E. Fraser, J.P. Mu
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Session 6C — C.G. Du, L.R. Nelson
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Session 6C — M. Mincu 168 Arcsin
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170 Comparison of Greenhouse and Fi
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Session 6C — S.L. Walker, S. Leat
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Session 6D — L.N. Jørgensen, K.E
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176
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Concluding Remarks — Z. Eyal 178
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184 Dr. Patrice Halama Professor In
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186 Dr. Hala Toubia-Rahme Research
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