A White Paper on Global Wheat Health Based on Scenario Development and Analysis.

Scenario analysis constitutes a useful approach to synthesize knowledge and derive hypotheses in the case of complex systems that are documented with mainly qualitative or very diverse information. In this article, a framework for scenario analysis is designed and then, applied to global wheat health within a timeframe from today to 2050. Scenario analysis entails the choice of settings, the definition of scenarios of change, and the analysis of outcomes of these scenarios in the chosen settings. Three idealized agrosystems, representing a large fraction of the global diversity of wheat-based agrosystems, are considered, which represent the settings of the analysis. Several components of global changes are considered in their consequences on global wheat health: climate change and climate variability, nitrogen fertilizer use, tillage, crop rotation, pesticide use, and the deployment of host plant resistances. Each idealized agrosystem is associated with a scenario of change that considers first, a production situation and its dynamics, and second, the impacts of the evolving production situation on the evolution of crop health. Crop health is represented by six functional groups of wheat pathogens: the pathogens associated with Fusarium head blight; biotrophic fungi, Septoria-like fungi, necrotrophic fungi, soilborne pathogens, and insect-transmitted viruses. The analysis of scenario outcomes is conducted along a risk-analytical pattern, which involves risk probabilities represented by categorized probability levels of disease epidemics, and risk magnitudes represented by categorized levels of crop losses resulting from these levels of epidemics within each production situation. The results from this scenario analysis suggest an overall increase of risk probabilities and magnitudes in the three idealized agrosystems. Changes in risk probability or magnitude however vary with the agrosystem and the functional groups of pathogens. We discuss the effects of global changes on the six functional groups, in terms of their epidemiology and of the crop losses they cause. Scenario analysis enables qualitative analysis of complex systems, such as plant pathosystems that are evolving in response to global changes, including climate change and technology shifts. It also provides a useful framework for quantitative simulation modeling analysis for plant disease epidemiology.

Molecular Mapping of a Leaf Rust Resistance Gene on the Short Arm of Chromosome 6B of Durum Wheat.

Leaf rust, caused by Puccinia triticina, is an important disease of durum wheat (Triticum turgidum subsp. durum) worldwide, and the most effective way to control it is through the use of resistant cultivars. A partially dominant leaf rust resistance gene present in the International Maize and Wheat Improvement Center-derived Chilean cv. Guayacan INIA and its sister line Guayacan 2 was mapped to chromosome arm 6BS by identifying linked amplified fragment length polymorphisms (AFLPs) and mapping two of the molecular markers in common wheat (T. aestivum) linkage maps of the International Triticeae Mapping Initiative and Oligoculm × Fukuho-komugi populations. Comparison of infection type responses of the two resistant durums with common wheat testers carrying the previously mapped resistance genes Lr36 and Lr53 on 6BS, and their chromosomal positions, indicated that the resistance gene in durum wheat Guayacan INIA is a new leaf rust resistance gene, which was designated as Lr61. Gene Lr61 is effective against the P. triticina race BBG/BN predominant in northwestern Mexico and other races infecting durum wheat in various countries.

Identification and Molecular Characterization of Leaf Rust Resistance Gene Lr14a in Durum Wheat.

Leaf rust, caused by Puccinia triticina, is an important disease of durum wheat (Triticum turgidum subsp. durum) and only a few designated resistance genes are known to occur in this crop. A dominant leaf rust resistance gene in the Chilean durum cv. Llareta INIA was mapped to chromosome arm 7BL through bulked segregant analysis using the amplified fragment length polymorphism (AFLP) technique, and by mapping three polymorphic markers in the common wheat (T. aestivum) International Triticeae Mapping Initiative population. Several simple sequence repeat (SSR) markers, including Xgwm344-7B and Xgwm146-7B, were associated with the leaf rust resistance gene. Resistance response and chromosomal position indicated that this gene is likely to be Lr14a. The SSR markers Xgwm344-7B and Xgwm146-7B and one AFLP marker also differentiated common wheat cv. Thatcher from the near-isogenic line with Lr14a, as well as durum 'Altar C84' from durum wheat with Lr14a. This is the first report of the presence of Lr14a in durum wheat, although the gene originally was transferred from emmer wheat 'Yaroslav' to common wheat. Lr14a is also present in CIMMYT-derived durum 'Somateria' and effective against Mexican and other P. triticina races of durum origin. Lr14a should be deployed in combination with other effective leaf rust resistance genes to prolong its effectiveness in durum wheat.

Effect of Leaf Rust on Grain Yield and Yield Traits of Durum Wheats with Race-Specific and Slow-Rusting Resistance to Leaf Rust.

Leaf rust, caused by Puccinia triticina, is an important disease of durum wheat (Triticum turgidum) in many countries. We compared the effectiveness of different types of resistance in International Maize and Wheat Improvement Center-derived durum wheat germ plasm for protecting grain yield and yield traits. In all, 10 durum wheat lines with race-specific resistance, 18 with slow-rusting resistance, and 2 susceptible were included in two yield loss trials sown on different planting dates in Mexico with and without fungicide protection under high disease pressure. Eight genotypes with race-specific resistance were immune to leaf rust. Durum wheat lines with slow-rusting resistance displayed a range of severity responses indicating phenotypic diversity. Mean yield losses for susceptible, race-specific, and slow-rusting genotypes were 51, 5, and 26%, respectively, in the normal sowing date trial and 71, 11, and 44% when sown late. Yield losses were associated mainly with a reduction in biomass, harvest index, and kernels per square meter. Slow-rusting durum wheat lines with low disease levels and low yield losses, as well as genotypes with low yield losses despite moderate disease levels, were identified. Such genotypes can be used for breeding durum wheat genotypes with higher levels of resistance and negligible yield losses by using strategies that previously have been shown to be successful in bread wheat.

New Genes for Leaf Rust Resistance in CIMMYT Durum Wheats.

Leaf rust, caused by Puccinia triticina, has become an important disease of durum wheat (Triticum turgidum) in Mexico since the detection in 2001 of BBG/BN, a new race virulent on all common cultivars and on more than 80% of CIMMYT's durum wheat collection. We investigated the genetic basis and diversity of resistance in nine durum genotypes that are highly resistant to the new race. These resistant durums were crossed with the susceptible cv. Atil C2000 and intercrossed in a half diallel arrangement. Five diverse sources of resistance were identified by evaluating parents, F1, F2, and F3 populations in greenhouse and/or field trials under artificial epidemics of race BBG/BN. The same pair of partially dominant complementary genes determined resistance in Jupare C2001, Hualita, and Pohowera. Somateria and Llareta INIA shared the same dominant resistance gene, whereas a partially dominant gene conferred resistance in two sister lines, Guayacan 2 and Guayacan INIA. A different partially dominant gene present in Storlom was linked in repulsion to another partially dominant gene in Camayo. These diverse resistance genes can be used effectively to control leaf rust, preferably by deploying them in combinations.