Necrotrophic Pathogens of Wheat

This article was originally published in the Encyclopedia of Food Grains published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author’s institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Oliver R.P., Tan K.-C. and Moffat C.S. (2016) Necrotrophic Pathogens of Wheat. In: Wrigley, C., Corke, H., and Seetharaman, K., Faubion, J., (eds.) Encyclopedia of Food Grains, 2nd Edition, pp. 273-278. Oxford: Academic Press. © 2016 Elsevier Ltd. All rights reserved. Author's personal copy Necrotrophic Pathogens of Wheat RPOliver, K-C Tan, and CS Moffat, Curtin University, Bentley, WA, Australia ã 2016 Elsevier Ltd. All rights reserved. Topic Highlights give information of trends over decades (Brennan and Murray, 1988, 1998). Table 2 lists estimates of current wheat disease • Diseases of wheat. losses in Australia. It can be seen that across all regions, TS is • Genetic analysis of resistance to tan spot and Septoria the major disease, currently costing more than all rusts com- nodorum blotch (SNB) necrotrophic effectors. bined. SNB is restricted to Western Australia where it ranks as Tan spot effectors. the second most costly wheat disease. STB has declined signif- • • SNB effectors. icantly, from an annual cost of $152 million in 1998 (Brennan • Lateral gene transfer, effector interactions, and new fungal and Murray, 1998) to the current figure of $1 million per pathogens. annum. Table 2 also lists estimates of potential losses if no disease control measures were carried out. The difference between the actual and potential is then ascribed differentially Learning Objectives to breeding, fungicide, and cultural control methods. The distribution and trends in the diseases are quite dis- • Appreciation of the relative importance of necrotrophic and tinct. SNB is almost exclusively located in Western Australia, other wheat diseases. with only sporadic reports from other regions. STB has sharply • An understanding of the term effector. declined in both the southern and western regions and has Consideration of the nature of quantitative disease always been absent from the northern regions. TS is steadily • interactions. increasing in all regions although absolute levels are lower in • An understanding of the impact on plant breeding of the the southern region (Figure 1). effector model. The high levels of disease reported from TS and SNB was greeted with much skepticism in many quarters. The methodol- ogy of the report was based on questionnaires sent to regional Diseases of Wheat plant pathologists. These data were combined with estimates of area and crop value to arrive at consolidated estimates. Valida- The diseases of wheat have received particular attention tion of the estimates has been provided by two recent reports throughout the ages because of their often devastating effect that used fungicide control to estimate disease losses (Oliver on this all-important crop. The full range of nematode, viral, et al., 2014; Salam et al., 2013). Salam et al. analyzed yield bacterial, oomycete, and fungal pathogens cause significant responses to fungicide under situations in which either or both diseases. Much international attention has been given to the TS and SNB were the only significant diseases. Not surprisingly, biotrophic rust diseases, but in some parts of the world, necro- three fungicide treatments gave higher yield gains than one À À trophic pathogens cause far higher losses and have proved (515 kg ha 1 versus 297 kg ha 1 averaged over the whole much harder to control by either genetic or chemical means. region). However, it is not clear if the three fungicide treatment This article is primarily concerned with two major patho- schedule even gave complete control. Averaged over the entire gens of wheat from the order Pleosporales: Parastagonospora western region and using a figure of $263/tonne, this implies nodorum and Pyrenophora tritici-repentis. Some aspects of that the combined disease cost to WA growers is $300–600 m P. nodorum and P. tritici-repentis research have recently been per annum. Oliver et al. (2014) used a smaller number of À reviewed, and these reviews contain detailed background that fungicide trials and estimated losses of 350 kg ha 1 to TS only. underpins our current knowledge (Faris et al., 2013; Lamari It therefore appears that the estimates in Brennan and Murray of and Strelkov, 2010; Oliver et al., 2012). $212 m per annum in Western Australia are of the right order Taxonomic instability of these and other related leaf blotch and may well be an underestimate. pathogens from the class Dothideomycetes has been an issue. The distribution of the pathogens in the rest of world is The current and past names of the pathogens and their diseases understudied. Lamari and Strelkov reviewed the literature in are summarized in Table 1 (Quaedvlieg et al., 2013). The three 2010 for TS (Lamari and Strelkov, 2010). There are widespread diseases, Septoria nodorum blotch (SNB), tan spot (TS), and reports that the range of TS is increasing in northern and Septoria tritici blotch (STB), coexist in many places and have western Europe. In Germany, TS is consistently comparable likely been confused and misidentified at times. with STB and in some years is the most damaging disease (M. Disease loss assessments are rarely carried out and subject Weigand, personal communication). In Denmark, TS is fre- to many caveats and exceptions, despite their obvious impor- quently observed (Jørgensen and Olsen, 2007). In the United tance for farmers, breeders, agrochemical companies, Kingdom, TS is now frequent and is ranked third behind STB researchers, and funding agencies. More frequently, pathogens and powdery mildew (J. Thomas, personal communication). are ranked in terms of importance. One of the more reliable The consistent pattern is that TS is strongly associated with and recent assessments of wheat disease losses has been carried limited tillage methods and wheat-on-wheat rotations. SNB is out in Australia (Murray and Brennan, 2009). Previous studies only reported as an occasional disease in Germany, but it is Encyclopedia of Food Grains, Second Edition http://dx.doi.org/10.1016/B978-0-12-394437-5.00240-0 273 Encyclopedia of Food Grains, (2016), vol. 4, pp. 273-278 Author's personal copy 274 AGRONOMY OF GRAIN GROWING | Necrotrophic Pathogens of Wheat Table 1 Nomenclature of wheat leaf blotch pathogens and their diseases Preferred pathogen name Pathogen synonyms Order Preferred disease name Disease synonyms Parastagonospora Stagonospora nodorum Pleosporales Septoria nodorum blotch Stagonospora nodorum nodorum Phaeosphaeria nodorum (SNB) blotch Leptosphaeria nodorum Glume blotch Septoria nodorum Pyrenophora tritici- Drechslera tritici-repentis Pleosporales Tan spot (TS) Yellow spot repentis Helminthosporium tritici-repentis Yellow leaf spot Zymoseptoria tritici Septoria tritici Capnodiales Septoria tritici blotch (STB) Septoria leaf blotch Mycosphaerella graminicola Septoria tritici blotch Table 2 Potential and actual losses to selected wheat diseases in Australia Contribution to control (potential minus actual) from breeding, fungicides, Disease Rank Actual/AUD million Potential/AUD million and cultural methods (AUD million) Breeding Fungicide Cultural TS 1 212 676 200 108 155 SNB 3 108 230 36 35 51 STB 27 1 21 15 4 1 Stripe rust 2 127 994 431 359 78 Stem rust 8 8 478 438 8 24 Leaf rust 11 12 197 152 17 16 Data from Murray, G.M., Brennan, J.P., 2009. Estimating disease losses to the Australian wheat industry. Australasian Plant Pathology 38, 558–70. 9 8 7 6 5 TS 4 SNB 3 STB 2 1 0 1988 1998 2008 1988 1998 2008 1988 1998 2008 Northern Southern Western Figure 1 Average annual disease losses to SNB, TS, and STB in the northern (Queensland and northern NSW), southern (southern NSW, Victoria, Tasmania, and South Australia), and western (Western Australia) cropping regions. Figures are % losses to each disease. Data from Murray, G.M. and Brennan, J.P., 2009. The current and potential costs from diseases of wheat in Australia. common in parts of France (M. LeBrun, personal communica- (Oliver et al., 2014). This is not due to baseline levels of fungi- tion), western counties of England (S. Gurr, personal cide sensitivity, but rather to poor field performance (Beard communication), and Norway (M. Lillemo, personal commu- et al., 2009; Bhathal et al., 2003; Tormen et al., 2013). There nication). It seems that SNB is most apparent in regions with have been reports of evolved resistance to QoI fungicides (Blixt intense rain showers. Rain splash of spores is required for et al., 2009; Reimann and Deising, 2005). Overall, there is every progression of SNB throughout the canopy. It is clear that the reason to pursue the breeding of disease-resistant cultivars. incidence of these diseases and their economic impact need further study. Fortunately, molecular diagnostic tools are read- ily available (Oliver et al., 2008). Genetic Analysis of Resistance to TS and SNB A range of fungicides are available for SNB and TS. The Necrotrophic Effectors primary modes of action in use are demethylation inhibitors, quinone outside inhibitors (QoIs), and succinate dehydrogenase Our understanding of the interaction between wheat and the inhibitors. In general, control levels in the field are modest TS and SNB pathogens has been revolutionized by the Encyclopedia of Food Grains, (2016), vol. 4, pp. 273-278 Author's personal copy AGRONOMY OF GRAIN GROWING | Necrotrophic Pathogens of Wheat 275 recognition of the importance of necrotrophic effectors (NEs).

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