Wheat head blast (WHB), caused by the fungus Magnaporthe oryzae pathotype triticum, is a devastating disease affecting South America and South Asia. Despite 30 years of intensive effort, the 2NVS translocation from Aegilops ventricosa contains the only useful source of resistance to WHB effective against M. oryzae triticum isolates. The objective of this study was to identify non-2NVS sources of resistance to WHB among elite cultivars, breeding lines, landraces, and wild-relative accessions. Over 780 accessions were evaluated under field and greenhouse conditions in Bolivia, greenhouse conditions in Brazil, and at two biosafety level-3 laboratories in the United States. The M. oryzae triticum isolates B-71 (2012), 008 (2015), and 16MoT001 (2016) were used for controlled experiments, while isolate 008 was used for field experiments. Resistant and susceptible checks were included in all experiments. Under field conditions, susceptible spreaders were inoculated at the tillering stage to guarantee sufficient inoculum. Disease incidence and severity were evaluated as the average rating for each 1-m-row plot. Under controlled conditions, heads were inoculated after full emergence and individually rated for percentage of diseased spikelets. The diagnostic marker Ventriup-LN2 was used to test for the presence of the 2NVS translocation. Four non-2NVS spring wheat International Maize and Wheat Improvement Center breeding lines (CM22, CM49, CM52, and CM61) and four wheat wild-relatives (A. tauschii TA10142, TA1624, TA1667, and TA10140) were identified as resistant (<5% of severity) or moderately resistant (5 to <25% severity) to WHB. Experiments conducted at the seedling stage showed little correlation with disease severity at the head stage. M. oryzae triticum isolate 16MoT001 was significantly more aggressive against 2NVS-based varieties. The low frequency of WHB resistance and the increase in aggressiveness of newer M. oryzae triticum isolates highlight the threat that the disease poses to wheat production worldwide and the urgent need to identify and characterize new resistance genes that can be used in breeding for durably resistant varieties.

Wheat blast is a devastating disease caused by the Triticum pathotype of Magnaporthe oryzae. M. oryzae Triticum is capable of infecting leaves and spikes of wheat. Although symptoms of wheat spike blast (WSB) are quite distinct in the field, symptoms on leaves (WLB) are rarely reported because they are usually inconspicuos. Two field experiments were conducted in Bolivia to characterize the change in WLB and WSB intensity over time and determine whether multispectral imagery can be used to accurately assess WSB. Disease progress curves (DPCs) were plotted from WLB and WSB data, and regression models were fitted to describe the nature of WSB epidemics. WLB incidence and severity changed over time; however, the mean WLB severity was inconspicuous before wheat began spike emergence. Overall, both Gompertz and logistic models helped to describe WSB intensity DPCs fitting classic sigmoidal shape curves. Lin’s concordance correlation coefficients were estimated to measure agreement between visual estimates and digital measurements of WSB intensity and to estimate accuracy and precision. Our findings suggest that the change of wheat blast intensity in a susceptible host population over time does not follow a pattern of a monocyclic epidemic. We have also demonstrated that WSB severity can be quantified using a digital approach based on nongreen pixels. Quantification was precise (0.96 < r> 0.83) and accurate (0.92 < ρ > 0.69) at moderately low to high visual WSB severity levels. Additional sensor-based methods must be explored to determine their potential for detection of WLB and WSB at earlier stages.

Core Ideas Wheat grain yield response to foliar fungicide is highly dependent on environment. Late‐season N can offset protein dilution from fungicide‐driven yield increase. Foliar N rates necessary to impact grain protein have potential for leaf burn. Controlled‐release N and urea ammonium nitrate both impacted wheat responses similarly. In‐season canopy reflectance can aid in fungicide decision. Winter wheat (Triticum aestivum L) response to foliar fungicide and late‐season N is inconsistent. We evaluated various late‐season N fertilizer‐by‐foliar fungicide combinations to test whether wheat anthesis normalized difference vegetative index (NDVI) could aid fungicide decisions. Six site‐years were conducted with N fertility >120 kg N ha−1 during 2012–2013 and 2013–2014 in Oklahoma, using a three‐way factorial arrangement of a randomized complete block design with four replications. Fungicide treatment (with or without), foliar N source (urea ammonium nitrate [UAN] or controlled‐release N), and N rate (0, 2.8, 5.6, or 28 kg N ha−1) were applied during anthesis. Statistical analyses were performed by site‐year, by yielding environment (low [LY] <3.2 Mg ha−1 and high [HY] >3.2 Mg ha−1), and pooled across site‐years. Grain yield was 0.21 Mg ha−1 greater due to fungicide in the pooled analysis, mostly led by the 0.59 Mg ha−1 yield advantage in HY as opposed to non‐significance in LY. Fungicide increased test weight in 6 to 7 kg m−3 across all analyses, and 28 kg N ha−1 increased grain protein by ∼10 g kg−1 but caused leaf burn. Anthesis NDVI <0.6 occurred when preanthesis abiotic stresses restricted yield potential, not warranting fungicide application. Fungicide can benefit grain yield and test weight and could be considered when anthesis NDVI >0.6, but yield response will depend on grain‐filling conditions. Foliar N rates needed to increase grain protein in a well‐fertilized crop have potential for leaf burn.

Wheat blast is an explosive new fungal disease of wheat caused by an Magnaporthe oryzae (synonym of Pyricularia oryzae) host-adapted subpopulation, the M. oryzae Triticum pathotype (MoT). MoT has been found in South America, South Asia, and Africa, but not in the United States. Wheat blast caused by the MoT fungus was first reported in Brazil in 1985 and subsequently spread to Bolivia, Paraguay, and Argentina in the 1990s and 2000s. The disease first appeared in Bangladesh in 2016 and in Zambia in 2017. The MoT fungus is seedborne, and the most likely route for movement across oceans was though grain trade. Wheat head (spike) blast is the predominant form of the disease in the field, although foliar and stem blast also occurs. The disease has proven hard to control when weather conditions are conducive, often resulting in devastating yield and quality losses. The only currently effective resistance, contained in the 2NvS translocation from the wild wheat relative Aegilops ventricosa, confers partial resistance that is variable depending on the genetic background of the specific wheat variety. Fungicides are not fully effective in controlling wheat head blast if warm, humid weather occurs during the heading stage. A major disease management strategy in areas where the disease occurs involves timing the wheat planting date so that heading does not coincide with warm rainy weather. A climate suitability model for the United States indicates that all of U.S. soft red winter wheat and about half of the hard red winter wheat are at risk.

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Resistance to wheat spike blast (WSB), caused by the Magnaporthe oryzae triticum pathotype (MoT), has relied upon a single major source: the 2NvS translocation introgressed from the wild relative Aegilops ventricosa Tausch. However, this resistance is partial and recently partially overcome by newer MoT races. To characterize potential novel loci conferring resistance to WSB, we conducted a genome‐wide association study (GWAS) using a diverse panel of 384 wheat genotypes phenotyped under three controlled‐environment conditions using MoT isolates T‐25 (301 genotypes), B‐71 (87 genotypes), and 008 (49 genotypes). Genotyping‐by‐sequencing identified 13,175 single nucleotide polymorphisms (SNPs) after filtering. Principal components analysis (PCA) identified two clusters based on the presence or absence of the 2NvS translocation, and the first three PCAs explained 13% of the genetic variation. Three individual analyses were performed (full [all genotypes combined], 2NvS genotypes only, and non‐2NvS genotypes only) using a linear mixed model and a threshold of significance of false discovery rate at 5%. Association analysis detected 25 significant SNPs for the full GWAS with isolate T‐25, in which 21 were mapped on chromosome 2A in the same physical position as the 2NvS translocation. Highly significant linkage disequilibrium among these SNPs suggested they might tag the same quantitative trait locus (QTL). No significant associations were identified with isolates B‐71 and 008, likely due to the small sample size. A QTL pyramiding analysis showed that the combination of multiple QTL was not statistically different from the individual effect of the 2A QTL. Further validation of these genomic regions can aid breeding for broad spectrum and durable WSB resistance.

Global wheat production is seriously threatened by the filamentous fungal pathogen, Magnaporthe oryzae, causing wheat blast disease. The pathogen was first identified in South America and recently spread across continents to Bangladesh (South Asia) and Zambia (South-central Africa). M. oryzae strains closely related with a South American field isolate B71 was found to have caused the wheat blast outbreaks in South Asia and Africa. Here, we studied the genetic relationship among isolates found on the three continents. Using an improved reference genome for B71 and whole genome sequences of isolates from Bangladesh, Zambia, and South America, we found strong evidence to support that the outbreaks in Bangladesh and Zambia were caused by the introductions of genetically separated isolates. Structural variation analysis using whole genome short-read sequencing data indicate all isolates closely related to B71 maintained at least one supernumerary mini-chromosome and, interestingly, some Zambian isolates contain more than one mini-chromosome. Long-read sequencing and de novo genome assemblies of two Zambian isolates show that both contain a mini-chromosome similar to the B71 mini-chromosome, although pervasive structural variation exists among them. Genome assemblies also provide evidence that one Zambian isolate carries an additional mini-chromosome that is highly divergent from the B71 mini-chromosome. Our findings show that while the core genomes of the multiple introductions are highly similar, the mini-chromosomes have undergone marked diversification. The maintenance of the mini-chromosome during the multiple introductions, and the rapid sequence and structural variation suggests the mini-chromosomes may serve important virulence or niche adaptation roles under diverse environmental conditions.

Wheat blast, caused by the fungus Magnaporthe oryzae Triticum pathotype (MoT), is a devastating disease affecting South America, Bangladesh, and Zambia. Resistance to wheat blast has strongly relied on the 2NvS translocation; however, newer MoT isolates have increased aggressiveness, threatening the 2NvS translocation’s effectiveness and durability. To identify genomic regions associated with wheat blast resistance, we performed a quantitative trait loci (QTL) mapping study using 187 double-haploid (DH) lines from a cross between the Brazilian wheat cultivars ‘TBIO Alvorada’ and ‘TBIO Sossego’ , which are moderately resistant and susceptible to blast, respectively. The DH population was evaluated under greenhouse in Brazil and Bolivia, and field conditions in Bolivia. Contrasting models best explained the relationship between traits evaluated according to differences in disease levels and the presence of the 2NvS. A large effect-locus, derived from ‘TBIO Sossego’, was identified on chromosome 2AS which was confirmed to be 2NvS translocation and explained 33.5 to 82.4% of the phenotypic variance. Additional significant loci were identified on 5AL, 1DS, 4DS, 5DL, and 6DL chromosomes arms with phenotypic variance < 6%, but they were not consistent across trait – environment combinations . QTL pyramiding analyses showed that some specific loci had an additive effect when combined with the 2NvS, suggesting that stacking multiple loci might be an effective strategy for helping to manage wheat blast. The markers associated with the 2NvS can be used as dominant diagnostic markers for this alien translocation. Additional characterization of these loci using a broader set of MoT isolates is critical to validate their effectiveness against current MoT populations.