Scientists uncover hidden susceptibility genes fueling wheat disease outbreaks

One of the most damaging fungal diseases of wheat is quietly exploiting the crop’s own genes, and a sweeping new study has now mapped exactly where that vulnerability lies. Researchers evaluating 619 elite hard winter wheat breeding lines and cultivars from the United States Great Plains have identified both well-characterized and previously unknown genes that make modern wheat susceptible to septoria nodorum blotch, a disease caused by the necrotrophic fungus Parastagonospora nodorum. The findings, published in Theoretical and Applied Genetics, offer wheat breeders a detailed genetic roadmap for building resistance into one of America’s most important market classes of wheat.

Septoria nodorum blotch has grown into an economically significant problem over recent decades across the eastern United States, northern Europe, Australia, and parts of North Asia. The fungus attacks both leaves, producing leaf blotch, and spikes, causing glume blotch, and can slash yields by as much as 50 percent on susceptible cultivars. In the Great Plains, where hard winter wheat dominates production, the disease has become increasingly prevalent in recent years. Farming practices such as minimum or no-till agriculture, which leave more infected crop residue on the soil surface, may have contributed to the surge. Compounding the problem, isolates of the fungus collected from hard winter wheat in Oklahoma show unusually high genetic and virulence diversity, suggesting local pathogen populations have adapted to the region’s cultivars.

The central puzzle of the wheat-P. nodorum system lies in its inverted genetics. Unlike classic disease resistance, where a plant gene recognizes a pathogen molecule and triggers defense, susceptibility here follows an inverse gene-for-gene model. The fungus produces proteinaceous molecules called necrotrophic effectors, and when a corresponding sensitivity gene in the wheat host recognizes one of these effectors, it triggers programmed cell death. That cell death, paradoxically, feeds the necrotrophic fungus, which thrives on dead tissue. To date, researchers have cloned seven wheat sensitivity genes, including Tsn1-B1, Snn1-B1, Snn2, Snn3-B1, Snn3-B2, Snn3-D1, and Snn5-B1, along with five fungal effector genes named SnToxA, SnTox1, SnTox3, SnTox5, and SnTox267. Thirteen distinct sensitivity gene-effector interactions have been documented in this pathosystem.

To determine how widespread these susceptibility genes are in contemporary hard winter wheat, the research team, led by scientists at Oklahoma State University in collaboration with the USDA Agricultural Research Service and Kansas State University, assembled a panel of 619 genotypes. The panel included 532 doubled-haploid breeding lines derived from 14 bi-parental crosses, 38 elite breeding lines from the Oklahoma State University program, and 49 cultivars from the Oklahoma State and Kansas State breeding programs. Each genotype was tested at the seedling stage against five P. nodorum isolates originally collected from the winter wheat cultivar Gallagher in Canadian County, Oklahoma in 2016, and against purified preparations of the five known necrotrophic effectors.

The results revealed a sobering picture of genetic vulnerability. When inoculated with the fungal isolates, between 40 and 67 percent of the genotypes proved susceptible, depending on the isolate, while resistant genotypes ranged from only 5 to 21 percent. Effector infiltration assays, in which purified effector proteins were injected directly into leaves using a needleless syringe, showed that 54 percent of genotypes were sensitive to SnToxA, 37 percent to SnTox3, 15 percent to SnTox5, 13 percent to SnTox267, and just 2 percent to SnTox1. These frequencies indicate that Tsn1-B1 and Snn3-B1/B2 are the most common sensitivity genes in this germplasm, while functional Snn1-B1 is rare. Notably, 75 genotypes showed resistant to intermediate reactions against all five isolates, including the Oklahoma cultivars Uncharted, Bentley, Big Country, and OK Corral.

The team genotyped the entire panel using genotyping-by-sequencing, which yielded 34,357 high-quality single nucleotide polymorphism markers after filtering, along with diagnostic kompetitive allele-specific PCR, or KASP, markers for the sensitivity genes Tsn1-B1, Snn1-B1, and Snn3-B1/B2. The diagnostic markers performed remarkably well for some genes: the Tsn1-B1 markers achieved 98 percent prediction accuracy against SnToxA sensitivity, and the Snn3-B1/B2 markers reached 92 percent accuracy against SnTox3. The Snn1-B1 marker, by contrast, showed only 75 percent accuracy, with a 25 percent false positive rate, suggesting that structural variations such as mutations, deletions, or inversions may have disabled the gene in many lines without being detectable by the current marker.

Genome-wide association studies using three statistical models, the mixed linear model, FarmCPU, and BLINK, confirmed the presence of the known sensitivity genes Tsn1-B1 on chromosome 5BL, Snn1-B1 on 1BS, Snn3-B1/B2 on 5BS, Snn2 on 2DS, and Snn5-B1 on 4BL, each associated with responses to its corresponding effector. But the analysis went further, uncovering numerous novel loci scattered across the wheat genome that had never before been linked to septoria nodorum blotch responses. In total, the BLINK model identified 23 significant loci associated with responses to the fungal isolates and 49 loci associated with effector responses, positioned on chromosome arms including 1AS, 1AL, 1BL, 1DS, 2AL, 2BL, 3AS, 3AL, 3BS, 3BL, 4AL, 4BL, 5AS, 5AL, 6AL, 6BS, 6BL, 7BS, 7BL, and 7DL.

The most striking discovery was a locus on chromosome arm 2AS, designated Qsnb.osu-2AS, which was associated with responses to all five P. nodorum isolates tested. For three of the isolates, the effect of this locus on disease response actually exceeded that of the Tsn1 gene. The significant markers in this region spanned a physical interval of roughly 17.69 megabases containing 268 high-confidence genes, several of which are annotated as involved in disease resistance and defense-related pathways. Encouragingly, the resistant allele of Qsnb.osu-2AS was already present at frequencies of roughly 45 to 60 percent in the germplasm, and the region shows some overlap with a previously reported resistance quantitative trait locus, Qsnb.cur-2AS2, mapped in an earlier study.

Translating these discoveries into practical breeding tools, the researchers developed new KASP markers linked to Snn5-B1, Snn2, and Qsnb.osu-2AS, sensitivity and resistance loci that previously lacked diagnostic markers. The marker KASP_S4B_643615365, targeting the Snn5-B1 region on chromosome 4BL, achieved 89 percent accuracy in predicting SnTox5 sensitivity across the full panel, while KASP_S2D_16184991, linked to Snn2 on chromosome 2DS, reached 80 percent accuracy for SnTox267 sensitivity. Two additional markers, KASP_S2A_9833162 and KASP_S2A_14367498, were validated for selecting the resistant allele at Qsnb.osu-2AS. Together with the existing diagnostic markers for Tsn1-B1 and Snn3-B1/B2, these tools give breeders the ability to deliberately eliminate susceptibility genes and stack resistance alleles through marker-assisted selection.

The study also revealed evidence of fungal weapons not yet characterized. Several cultivars, including Green Hammer, currently the second most grown wheat variety in Oklahoma, were insensitive to all five known effectors and lacked the corresponding sensitivity genes, yet remained susceptible to particular isolates. This pattern strongly suggests that P. nodorum harbors additional, undiscovered effectors and that wheat carries uncharacterized sensitivity genes targeted by them. The researchers also noted intriguing epistatic effects among effectors, consistent with earlier findings that SnTox1 expression can be suppressed in the presence of SnToxA, which may explain why the Snn1 locus showed no association with responses to the whole isolates even though it responds to purified SnTox1. Because Tsn1 also confers susceptibility to tan spot and spot blotch, two other major fungal diseases, the authors argue that eliminating Tsn1-B1 and Snn3-B1/B2, which are present at high frequencies in contemporary hard winter wheat, should be the top priority for breeding programs. The study focused on seedling-stage responses, and the team emphasizes that future work will need to identify loci governing adult plant resistance under field conditions, where genotype-by-environment interactions and quantitatively inherited resistance play a larger role.

Subject of Research: Identification of known and novel septoria nodorum blotch susceptibility and resistance genes in US hard winter wheat using effector assays, diagnostic markers, and genome-wide association studies.

Article Title: Identification of septoria nodorum blotch susceptibility genes in hard winter wheat

Article References: Ara, A. M., Holmes, D. J., Friesen, T. L., Carver, B. F., Bai, G., Amand, P. S., Bernado, A., Sharma, R., & Aoun, M. (2026). Identification of septoria nodorum blotch susceptibility genes in hard winter wheat. Theoretical and Applied Genetics, 139(10), Article 268. https://doi.org/10.1007/s00122-026-05359-7

Image Credits: AI Generated

DOI: 10.1007/s00122-026-05359-7

Keywords: septoria nodorum blotch, hard winter wheat, Parastagonospora nodorum, necrotrophic effectors, Tsn1, Snn3, genome-wide association study, KASP markers, marker-assisted selection, wheat breeding, disease resistance, Great Plains