Septoria and Stagonospora Diseases of Cereals - CIMMYT ...

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Durability of Resistance Durability of S. nodorum resistance within a crop cycle decreased with leaf age, and the older lower leaves proved more susceptible than the younger upper leaves (Jonsson, 1991). Due to this effect, late maturing germplasm is often wrongly considered more resistant than early maturing lines. Thus it is crucial to measure resistance at the same adult growth stage. More debated than durability within a crop cycle is the issue of durability of resistance across years and locations. Johnson in his 1992 general review of breeding for disease resistance highlights the difficulty of establishing the presence of differential hostpathogen interaction in the septoria foliar blights. He compares data gathered by the Eyal group (Eyal et al., 1973) with those of van Ginkel (van Ginkel and Scharen, 1985) and concludes that even the interpretation of quite similar data may differ. In practice, he concludes, no unequivocal demise of varieties due to truly differential races of the septoria foliar blights has been reported. The increase of disease severity over time on the same variety has been noted in the Netherlands, Australia, Israel (Jorgensen and Smedegaard-Petersen, 1999), and the USA (Mundt et al., 1999). However, it was not unequivocally shown whether this was due to an evolution of aggressiveness in the pathogen population or due to differential host-pathogen interaction based on the evolution of new virulence patterns. Mundt et al. (1999) conclude that the cultivation of susceptible hosts will result in selection for aggressiveness. In Germany several varieties were shown to retain their resistance to isolates of S. nodorum collected annually from all over the former East Germany for more than 10 years, except for small variations over years due to specific weather conditions (Walther, 1993). Clear proof of differential variety by isolate interaction at the seedling stage was presented by Kema et al. (1996). However, as in previous studies by this group and others, the size of the interaction component was about a magnitude smaller than that of the main effects due to varieties and isolates. In a later monocyclic field experiment, Kema and van Silfhout (1997) applied isolates specifically selected for their virulence differences at the seedling level, to adult plant plots. Again, small but significant interactions between isolates and varieties were apparent in this monocyclic situation. The significant interaction effect was largely due to two of the three isolates interacting differentially with two of the 22 varieties tested. Recent work on Kenyan isolates indicated that variety by isolate interaction in the field, while present, was small, with the interaction component accounting for only about 5% of the main effects (Arama, 1996). Breeding for Resistance to the Septoria/Stagonospora Blights of Wheat 121 Recent experiments on the genetic structure of S. tritici populations in response to host differences found a total lack of evidence for adaptation to the host genotype (McDonald et al., 1996). Different hosts had no differential effects on the dynamics of the pathogen population. On the other hand, Ahmed et al. (1996) found that susceptible varieties selected for more aggressive pathogen populations. However, the virulence levels of the pathogen isolates were associated with the variety of origin. Varietal mixtures of resistant and susceptible varieties have met with mixed success. Recent comparisons by Loughman et al. (1994b) of pure lines and mixtures did not find that mixtures have any advantage when it comes to durability of resistance. Interaction among isolates on the leaf, which can enhance or reduce expected disease severities (Eyal, 1992; Gilchrist and Velazquez, 1994), adds another component to the mix of virulence and aggressiveness. The implications for resistance breeding of the above findings on variety by isolate interactions remain to be tested further in real agricultural settings. However, it does appear that the response of the septoria/stagonospora foliar blights to their hosts cannot compare to the highly volatile population dynamics we see in the wheat rusts.
122 Session 6C — M. van Ginkel and S. Rajaram It is essential to determine the practical relevance of the presence of races of the septoria foliar blights when applying a resistance breeding strategy. Indeed, the importance of this topic cannot be overstated. New insights into the genetic structure of the pathogen population and its dynamics, especially as supported by biotechnological tools, may increase our understanding significantly in the near term. Breeding for Resistance at CIMMYT The methodology applied at CIMMYT for breeding for disease resistance in wheat has been outlined in previous publications, including the proceedings of several international septoria workshops (Mann et al. 1985; van Ginkel and Rajaram, 1989; van Ginkel and Rajaram, 1993; Gilchrist et al., 1995; van Ginkel and Rajaram, 1995). The methodology consists of three key components: utilizing proven resistance sources as parents, selecting for increased resistance in a shuttle breeding program, and confirming resistance by multilocation testing. The best identified lines are fed back into the crossing program as parents to further accumulate resistance, bringing the process full circle. The materials that provide the best resistance are made available by CIMMYT to all collaborators around the world who request them. Lists of such materials are published periodically (Gilchrist, 1994). Gilchrist et al. (this volume) have provided a new overview of lines found to be consistently resistant by CIMMYT researchers. Every year CIMMYT distributes to collaborators the newest wheat lines that combine S. tritici resistance with a full “agronomic package,” including adaptation to high rainfall environments, superior yield, a certain level of resistance to other prevalent diseases such as the rusts and scab, and acceptable grain quality. The parental stocks emphasized in developing these lines have a relatively long history of proven resistance in many locations around the world. The first genepool exploited for resistance to S. tritici by CIMMYT breeders was that represented by such lines as IAS20 from Brazil and Klein Atlas from Argentina. The Tinamou line is an example of a widely adapted, resistant line that was developed from those resistance sources. Then followed the winter wheats, in particular those from the former USSR, such as Aurora. Examples of elite lines emanating from that work are the group of Bobwhite lines, such as the cultivar PROINTA Federal and the Milan lines, which have shown strong disease resistance throughout the world, including in the hotspots of eastern Africa and the Southern Cone of South America (Kohli, 1995; Dubin and Rajaram, 1996). Subsequently, Chinese materials, such as the Ning, Shanghai, and Suzhoe series, plus other varieties from China, were heavily crossed by CIMMYT breeders. A key resistant line that resulted from that effort was Catbird. This history is discussed in more detail by Gilchrist et al. (this volume). New sources of resistance, such as highly promising synthetic wheats, are currently being exploited. CIMMYT’s Wide Crosses Program produces synthetic wheats by crossing CIMMYT elite durum wheats to accessions of Aegilops squarrosa (syn. Triticum tauschii and Ae. tauschii). The resulting triploid seedlings are treated with colchicine to double the number of chromosomes, resulting in manmade (hence “synthetic”) hexaploid bread wheats. Certain of those wheats have shown remarkable levels of S. tritici resistance, and low severities bordering on immunity have also been observed. Since synthetic wheats are relatively easy to cross with common wheats, their resistance can be readily introgressed into agronomically acceptable plant types, and combined with other resistances. CIMMYT’s breeding methodology emphasizes regularly exposing segregating materials to disease epidemics. Since the breeding program also targets resistance to several other diseases besides S. tritici, straw is used as the source of primary inoculum. Straw residues may carry inoculum for tan spot, fusarium head scab, fusarium foliar blight, and different species of bacteria in addition to the septorias. In special cases and in specific studies, pure liquid S. tritici spores are applied. CIMMYT applies a shuttle breeding methodology in which materials are alternately grown in a high rainfall site in the Mexican highlands (Toluca) and an irrigated site in northwestern Mexico (Cd. Obregon). As a result, the material
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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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Provided that S. nodorum fungal gen
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;; yy ;; yy 6 28% 1 32% ;y; ; y y ;
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Table 1. Sources of isolates or DNA
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Septoria/Stagonospora Leaf Spot Dis
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Interrelations among Septoria triti
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42 Session 2 — B.M. Cunfer disper
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44 Session 2 — B.M. Cunfer beyond
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46 Aggressiveness of Phaeosphaeria
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48 Session 2 — P. Halama, F. Lala
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50 Growth of Stagonospora nodorum L
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52 Session 3A — G.H.J. Kema and E
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54 A Possible Gene-for-Gene Relatio
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56 Diallel Analysis of Septoria Tri
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58 Session 3A — X. Zhang, S.D. Ha
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60 Session 3A — L. Gilchrist, C.
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62 Session 3A — L. Gilchrist, C.
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64 Session 3B — E. Arseniuk and P
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66 Session 3B — E. Arseniuk and P
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68 Cultivar variances Cultivar vari
Page 77 and 78: 70 Session 3B — E. Arseniuk and P
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 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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Page 135 and 136: Session 6C — G. Shaner 128 An exa
Page 137 and 138: Session 6C — G. Shaner 130 Anothe
Page 139 and 140: Session 6C — M. Díaz de Ackerman
Page 141 and 142: 134 Septoria tritici Resistance Sou
Page 143 and 144: Session 6C — L. Gilchrist et al.
Page 145 and 146: Session 6C — L. Gilchrist et al.
Page 147 and 148: 140 Selecting Wheat for Resistance
Page 149 and 150: Session 6C — R.M. Gonzalez I., S.
Page 151 and 152: Session 6C — R.M. Gonzalez I., S.
Page 153 and 154: Session 6C — R. Loughman, R.E. Wi
Page 155 and 156: 148 Field Resistance of Wheat to Se
Page 157 and 158: 150 Using Precise Genetic Stocks to
Page 159 and 160: Session 6C — C.M. Ellerbrook, V.
Page 161 and 162: 154 Evaluating Triticum durum x Tri
Page 163 and 164: 156 Sources of Resistance to Septor
Page 165 and 166: Session 6C — H. Toubia-Rahme and
Page 167 and 168: 160 Partial Resistance to Stagonosp
Page 169 and 170: Session 6C — C.G. Du, L.R. Nelson
Page 171 and 172: Session 6C — D.E. Fraser, J.P. Mu
Page 173 and 174: Session 6C — C.G. Du, L.R. Nelson
Page 175 and 176: Session 6C — M. Mincu 168 Arcsin
Page 177 and 178: 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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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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