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Monitoring virulent strains within pathogen populations is crucial to improve host resistance deployment strategies. Such monitoring increasingly involves field pathogenomics studies of molecular polymorphisms in pathogen genomes based on high‐throughput screening technologies. However, it is not always straightforward to predict virulence phenotypes from these polymorphisms, and in planta phenotyping remains necessary. We developed a method for 'bulk phenotyping on checkerboard microcanopies of wheat near‐isogenic lines' (BPC) for estimating the frequency of virulence against a resistance gene in mixed populations of the fungal pathogen Zymoseptoria tritici, the causal agent of Septoria tritici blotch (STB) in wheat, without the need for strain‐by‐strain pathogen phenotyping. Our method involves the uniform inoculation of a microcanopy of two wheat lines—one with the target resistance gene and the other without it—with a multistrain mixture of isolates representative of the population to be characterized, followed by the differential quantification of infection points (lesions). Using Stb16q, a wheat resistance gene that has recently broken down in Europe, we found a robust correlation between the ratio of the mean number of lesions on each wheat line and the frequency of virulent strains in the inoculum. Using pairs of virulent and avirulent strains, as well as synthetic populations consisting of 10 virulent strains and 10 avirulent strains mixed in different proportions, we validated the principle of the method and established standard curves at virulence frequencies close to those observed in natural conditions. We discuss the potential of this method for virulence monitoring in combination with molecular methods. [ABSTRACT FROM AUTHOR]

Cultivar disease resistance ratings to nodorum blotch (caused by Parastagonospora nodorum) provide critical information to growers to assist with disease management. However, less has been done in terms of yield losses from diseases under different cultivar resistance scenarios. The purpose of this study was to characterize yield and quality loss responses to wheat nodorum blotch as influenced by variety partial resistance, epidemic intensity, and environment. The trial series includes four site/years in Western Australia. Five varieties, Magenta, Calingiri, Trojan, Janz, and Scout were used for all four trials. These varieties represented resistant/susceptible categories MRMS, MS, MSS, S, and SVS, respectively, and were selected on the basis that they were adapted to the western region, had similar maturities, and were resistant to diseases that were potential contaminants. Trials were sown in a strip‐plot design with three replicates and five disease levels, with intersecting main‐plots of varieties and disease levels, which facilitated comparisons across disease levels within varieties. Current categories of resistance were well reflected in yield and quality responses. A negative exponential relationship was observed between yield and disease, with a nonlinear yield loss function for both flag leaf and glume infection. This indicates that yield is most affected during early stages of disease on flag leaves and glumes. The functional form of yield loss response over four site years was similar among varieties, but varieties differed in their extent of disease expression. Partial resistance in commercial wheat varieties reduced yield loss by 40%–60% of losses observed in susceptible varieties. [ABSTRACT FROM AUTHOR]

Blast disease, caused by the Magnaporthe oryzae Triticum pathotype (MoT), is a major concern for wheat production in tropical and subtropical regions. The most destructive symptoms occur in wheat spikes. Infected spikes become bleached due to partial or total sterility, producing small and wrinkled grains. High disease pressure of the disease results in significant yield losses. This study aimed to identify wheat quantitative trait loci (QTLs) conferring resistance to blast disease at the heading stage. A doubled‐haploid population was developed from the cross between BRS 209 (susceptible) and CBFusarium ENT014 (resistant, carrying the 2NS translocation). A linkage map was constructed containing 5,381 molecular markers and the inclusive composite interval mapping method was employed for QTL detection. Four QTLs were mapped in response to two MoT isolates. The major QTL identified on the 2AS chromosome explained an average of 84.0% of the phenotypic variation for spike bleaching at 9 days postinoculation and reinforces the potency of the 2NS translocation. Recombination between the distal region of chromosome 2AS and the 2NS marker was found. These results could explain why some lines carrying the VENTRIUP/LN2 marker have a variable reaction to the disease. QTLs on 5B and 7B chromosomes were also identified. Two mechanisms of resistance were hypothesized: the hypersensitive response and resistance to colonization of host tissues. The KASP markers thus developed and simple sequence repeats (SSRs) allocated in QTL regions can be used in the future for the development of wheat blast‐resistant cultivars. [ABSTRACT FROM AUTHOR]

Cultivar mixtures can stabilize yield and reduce pathogen spread in plant populations. A field experiment was performed to determine (a) whether a large difference between the cultivars in the mixture (e.g., plant height or earliness) would have an impact on mixture performance, and (b) whether such differences would modify the classical rules for mixture design. Mixtures were constituted from wheat cultivars with diversity for many traits, including plant height, flowering date, disease resistance, and yield potential. The field experiment was conducted over 3 years, testing each year 72–90 mixtures of two, four, or eight cultivars, and their corresponding pure stands. Disease severity and yield of cultivar mixtures were strongly related to the mean values of the component cultivars in pure stands. Despite the considerable diversity of the mixtures tested, the classic rules (e.g., proportion of susceptible cultivars) already tested in mixtures with similar height and earliness were effective for decreasing disease severity. Agronomic heterogeneity for traits such as plant height, yield potential, or earliness of the cultivars in mixtures did not have a negative impact on disease severity and yield relative to pure stands. Increasing the number of cultivars in the mixture from two to eight had no impact on the mean disease severity and yield of the mixtures, but reduced the variability of disease severity and yield in the mixture relative to pure stands. These results suggest that it may be possible to increase within‐field wheat diversity by combining more contrasting cultivars in mixtures than was previously thought. [ABSTRACT FROM AUTHOR]

Known and unknown genes conferring seedling and adult plant resistance (APR) to leaf rust, stem rust and stripe rust were detected either singly or in combination in a set of 136 African wheat genotypes using multi‐pathotype tests with characterized Australian Puccinia triticina (Pt), P. graminis f. sp. tritici (Pgt) and P. striiformis f. sp. tritici (Pst) pathotypes. Lines Beladi 132, IYN 68/9.44, Kenya Kifaru and Kenya Mbweha were postulated to carry resistance against multiple pathotypes of Pt, Pgt and Pst, whereas IAR/W/163‐3, Grano Di Moggio Tipo 44 and Trigo 48 had resistance against all pathotypes tested in the current study. Field evaluation with the three rust pathogens detected low to high APR in more than 50% of lines, and while most tested positive with markers linked to known APR genes (csLV34, csLV46G22, TM10KASPAR, csGS, Cfb5006 and csSr2), many carried unidentified and useful resistance to all three rusts. Genetic analysis of F3 mapping populations based on seven genotypes showed either monogenic or digenic inheritance of APR to leaf rust, stem rust and stripe rust. The lines postulated to carry effective uncharacterized seedling genes and APR genes are of great potential value in diversifying resistance to help achieve durable control of all three rust diseases of wheat. [ABSTRACT FROM AUTHOR]

Partial resistance genes often need to be 'pyramided' into a crop cultivar to obtain commercially acceptable levels of disease resistance. Analysis of data from four different wheat mapping populations, segregating for partial resistance to four contrasting foliar pathogens, showed a diminishing benefit to disease control from increasing the numbers of resistance loci in wheat lines. To test whether a general function could describe the efficacy benefit from pyramiding, a simple multiplicative survival model ( MSM) was used to predict disease severities on mapping population lines carrying various combinations of two, three or four resistance loci. The effectiveness of each resistance locus was expressed as the disease severity in lines carrying resistance alleles at one locus, as a proportion of the severity in lines carrying no detectable resistance alleles. The predicted severity from any given combination of multiple resistance loci was calculated as the product of the proportional severities for the relevant single loci. A regression line fitted to transformed observed against predicted values explained 93% of the variation, with a slope and intercept not significantly different from 1 and 0, respectively. MSM may therefore provide a simple method to test contrasting types of partial resistance and search for synergistic combinations. The analysis suggests that diminishing returns are a general feature of partial resistance to foliar pathogens in wheat, a finding that is likely to apply to other crop pathosystems. Identifying and combining ever more QTL is likely to provide limited gains. The consequences of these findings for QTL analysis are described. [ABSTRACT FROM AUTHOR]

The use of cultivar mixtures to control foliar fungal diseases is well documented for windborne diseases, but remains controversial for splash-dispersed diseases. To try to improve this strategy, a cultivar mixture was designed consisting of two wheat cultivars with contrasted resistance to Mycosphaerella graminicola, responsible for the rainborne disease septoria tritici blotch ( STB), in a 1:3 susceptible:resistant ratio rather than the 1:1 ratio commonly used in previous studies. The impact of natural STB epidemics in this cultivar mixture was studied in field experiments over 4 years. Weekly assessments of the number of sporulating lesions, pycnidial leaf area and green leaf area were carried out on the susceptible cultivar. In years with sufficient STB pressure, disease impacts on the susceptible cultivar in the mixture were always significantly lower than in the pure stand (e.g. 42% reduction of pycnidial leaf area for the three upper leaves in 2008 and 41% in 2009). In years with low STB pressure (2010 and 2011), a reduction of disease impacts was also shown but was not always significant. After major rainfall events, the number of sporulating lesions observed on the susceptible cultivar after one latent period was reduced on average by 45% in the mixture compared to the pure stand. All the measurements showed that a susceptible cultivar was consistently protected, at least moderately, in a mixture under low to moderate STB pressure. Therefore, the results prove that the design of an efficient cultivar mixture can include the control of STB, among other foliar diseases. [ABSTRACT FROM AUTHOR]

By carefully separating type I and type II resistances, the possible effects of plant height on fusarium head blight (FHB) resistance in wheat were assessed using near-isogenic lines (NILs) for several different reduced-height ( Rht) genes. Tall isolines all gave better type I resistance than their respective dwarf counterparts when assessed at their natural heights. These differences largely disappeared when the dwarf isolines were physically raised so that their spikes were positioned at the same height as those of their respective tall counterparts. The effects of plant height on type II resistance was less clear. For those NIL pairs which showed significant differences, it was the dwarf isolines which gave better resistance. As the Rht genes involved in these NILs locate at different genomic regions, the differences in FHB between the dwarf and tall isolines are unlikely to be the result of linkages between each of the different Rht loci with a beneficial or a deleterious gene affecting type I or type II resistance. Rather, the different FHB resistances are probably caused by direct or indirect effects of height difference per se, and microclimate may have contributed to the better type I resistance of the tall plants. Thus, caution should be exercised when attempting to exploit any of the FHB resistant loci co-located with Rht genes. [ABSTRACT FROM AUTHOR]

This study reports the discovery of a gene for resistance to septoria tritici blotch (STB) in two spring wheat cultivars, Courtot and Tonic. The gene, named Stb9, confers resistance to Mycosphaerella graminicola isolate IPO89011. It was mapped by quantitative trait loci (QTL) analysis using an existing map of Courtot × Chinese Spring and was located between markers Xfbb226 (3·6 cM) and XksuF1b (9 cM) on the long arm of chromosome 2B. Markers linked to Stb9 in Courtot were then shown to be linked to resistance to IPO89011 in F3 families of Tonic × Longbow. Allelism tests in which Tonic was crossed with Courtot confirmed that Tonic has a gene for resistance to IPO89011 at or very close to the Stb9 locus. SSR markers flanking Stb9 may be used in marker-assisted selection to introgress this gene into winter cultivars or in spring wheat breeding programmes outside Europe. [ABSTRACT FROM AUTHOR]

Wheat stem rust samples were collected in 2006 and 2007 in the Arsi, Bale, Shewa and northwest regions of Ethiopia to determine virulence diversity and race distribution in Puccinia graminis f.sp. tritici populations. Stem rust incidence was high in Arsi, Bale and east Shewa. In northwest Ethiopia, and north and west Shewa, stem rust was prevalent at low levels. A total of 152 isolates was analysed and 22 races were identified. Races TTKSR (Ug99), TTHSR and RRTTR were predominant, with frequencies of 26·6, 17·7 and 11·1%, respectively. These races were also detected in all regions. The highly virulent race designated Ug99 was present throughout the country and dominated in all regions except northwest Ethiopia. A variant of Ug99 virulent against the stem rust resistance gene Sr24 was not detected in this study. Four stem rust resistance genes ( Sr13, Sr30, Sr36 and SrTmp) were found to confer resistance to most of the races prevalent in Ethiopia. With the exception of Sr30, which is not effective against Ug99, these genes could be used in breeding for resistance to stem rust in Ethiopia. [ABSTRACT FROM AUTHOR]

A lesion-mimic mutant was obtained from a mutagenic treatment performed with ethyl methanesulfonate on the Argentine bread wheat ( Triticum aestivum) cultivar Sinvalocho M.A. The HLP (hypersensitive-like phenotype) mutant exhibited tiny, discrete, white lesions in the absence of any pathogen, resembling the typical hypersensitive response (HR). The lesions only became evident once the fifth or sixth leaf emerged, and spread at random along the leaf blades and leaf sheaths of the developing plant, including tissues of the spike. Because the lesion-mimic mutant showed no lesions at the seedling stage, the phenotypes of both the mutant and its mother line were identical at this point. Histochemical studies showed that spontaneous hypersensitive-like lesions in the HLP mutant corresponded to cell death. In leaf-rust ( Puccinia triticina) infection experiments performed at seedling and adult-plant stages, adult HLP plants showed enhanced resistance to leaf-rust attack compared with plants of Sinvalocho M.A. of comparable developmental stage, suggesting that the HLP mutation may confer increased resistance to the fungus. Because enhanced resistance coincided with the presence of spontaneous HR lesions, activation of HLP plant defence responses appeared to be tightly linked to this phenomenon. Final plant height and yield components in the lesion-mimic mutant did not differ from those of the mother line, indicating that the HLP mutation caused no detrimental pleiotropic effects that significantly affected agronomic performance. These data support the direct use of mutations in disease-resistance breeding. [ABSTRACT FROM AUTHOR]

Twenty-four wheat cultivars and breeding lines were screened for isolate-specific resistance to septoria tritici blotch (STB) caused by 12 isolates of Mycosphaerella graminicola. New isolate-specific resistances that could be used in wheat breeding were identified. Major sources of resistance to STB used in world breeding programmes for decades, such as Kavkaz-K4500, Veranopolis, Catbird and TE9111, have several isolate-specific resistances. This suggests that ‘pyramiding’ several resistance genes in one cultivar may be an effective and durable strategy for breeding for resistance to STB in wheat. Several cultivars, including Arina, Milan and Senat, had high levels of partial resistance to most isolates tested as well as isolate-specific resistances. Resistance to isolate IPO323 was common, present in all but one of the major sources of resistance tested. This suggests that resistance to IPO323 may be an indicator of varietal resistance to STB in the field. [ABSTRACT FROM AUTHOR]

The onset of adult plant resistance (APR) to Puccinin striiformis was examined in seven Australian wheat cultivars under held conditions and in 14 cultivars under controlled environmental conditions. In most cultivars under field conditions the percentage leaf area affected by stripe rust at mid-tillering (GS 22-26; third leaf) was significantly less (P< 0·05) than on the more susceptible cultivar Teal. The expression of APR was more conspicuous during tillering to node formation; at these growth stages chlorosis and/or necrosis developed in association with rust colonies on the most resistant cultivars. Under controlled conditions, the primary leaves of all cultivars except Olympic and Flinders displayed some resistance when compared with Teal (lower infection types, longer latent periods and/or less percentage leaf area affected). Studies on the spread of stripe rust from infection loci established in selected cultivars in the field indicated that the resistance detected at early growth stages in Suneca in controlled environmental studies also appeared to be effective in the field. [ABSTRACT FROM AUTHOR]

In seven field experiments conducted over 6 years with a wide range of disease severities, triticale was intermediate in resistance to Gaeumannomyces graminis between wheat (susceptible) and rye (resistant). Use of triticale is suggested as an immediately available means of introducing take-all resistance into cereal cultivation. Octoploid triticale was slightly more susceptible than hexaploid triticale. There was little evidence of consistent variation in resistance among wheat or rye cultivars but a few hexaploid triticale cultivars varied in resistance. The resistance of triticale was not reliably expressed in the glasshouse tests used, so selection for resistance to take-all in a breeding programme would need to be conducted in the field. Individual pairs of rye chromosomes added to wheat did not significantly reduce its susceptibility. The feasibility of transferring the resistance of rye to wheat is considered. [ABSTRACT FROM AUTHOR]