Wheat yield has significantly increased over the past 50 years, partly due to advancements in breeding for higher yield potential and disease resistance. However, fungal pathogens such as leaf rust (*Puccinia triticina*), stripe rust (*Puccinia striiformis*), powdery mildew (*Blumeria graminis*), and Fusarium head blight (*Fusarium culmorum*) remain major constraints, causing substantial yield losses in susceptible cultivars. Plant resistance is the most environmentally friendly and cost-effective method of plant protection, making innovations in resistance a major goal of wheat breeding. This study aimed to quantify the breeding progress concerning resistance to these major fungal pathogens and its effects on grain yield in relation to nitrogen fertilization, fungicide treatment, and the year of cultivar release. Understanding this relationship is crucial for developing sustainable wheat production strategies. Previous research has shown yield progress in German winter wheat, but comprehensive quantification of pathogen resistance improvements and their impact on yield under different agricultural management practices is lacking. This research addresses that gap using a large panel of German winter wheat cultivars released between 1966 and 2013.

The literature extensively documents the impact of fungal pathogens on wheat yield. Studies have shown yield losses of up to 70% in susceptible cultivars. While fungicides have played a role in disease management, resistant cultivars are essential for sustainable and environmentally friendly plant protection. Previous research has highlighted the impact of nitrogen fertilization on disease severity, with higher N levels often increasing susceptibility to biotrophic and hemibiotrophic pathogens. Several studies have investigated long-term breeding progress in German winter wheat yield, reporting genetic gains. While the general improvement of wheat resistance to fungal pathogens over the last 50 years is acknowledged, studies quantifying this progress and its effect on grain yield remain limited. This research builds upon previous work by Voss-Fels et al. (2019), which analyzed breeding progress in economically successful European winter wheat, focusing specifically on the subset of economically important German cultivars.

This study used a panel of 178 German winter wheat cultivars released between 1965 and 2013, selected for their economic and agronomic importance. The cultivars were grown in field trials at the Julius Kühn Institute in Quedlinburg, Germany, over three consecutive growing seasons (2014/15, 2015/16, 2016/17). A full factorial design incorporated two nitrogen levels (110 kg N ha⁻¹ and 220 kg N ha⁻¹) and two fungicide treatments (with and without fungicides). Artificial inoculation with *P. striiformis*, *P. triticina*, and *F. culmorum* was conducted to ensure comparable pathogen pressure. Powdery mildew infection occurred naturally. Disease severity was scored regularly, and the area under the disease progress curve (AUDPC) was calculated. Agronomic traits, including grain yield (GRY), thousand kernel weight (TKW), kernels per spike (KPS), ears per square meter (ESM), biomass (BM), and harvest index (HI), were measured. Statistical analyses, including ANOVA, orthogonal contrasts, correlation analysis, and principal component analysis (PCA), were performed using JMP 14.0 and SAS 9.4 to analyze the data. Partial correlation analysis was employed to account for the competitive effects of stripe rust on leaf rust. The average ordinate of infestation (AO) was calculated for each disease using a specific formula. Bivariate analysis explored relationships between GRY and fungal diseases, and breeding progress was assessed using linear regression analysis considering the year of cultivar release.

Stripe rust was the most prevalent disease, followed by leaf rust, powdery mildew, and Fusarium head blight. Nitrogen fertilization significantly increased susceptibility to all pathogens. Genotype (G) and year (Y) effects were highly significant for all diseases, with significant G x Y interactions indicating varying disease impacts across years. Plant protection significantly increased biomass and grain yield. Elevated N fertilization increased biomass and ears per square meter but decreased thousand kernel weight, harvest index, and grain yield. Grain yield showed substantial genetic variation. Principal component analysis revealed that GRY, BM, HI, and TKW formed a cluster, while ESM showed weak correlations with other yield components. Strong negative correlations were observed between the sum of all fungal diseases and GRY, explaining approximately 50% of yield differences. Stripe rust showed the strongest negative correlation with GRY, followed by Fusarium head blight and powdery mildew. Leaf rust showed a weaker negative correlation with GRY primarily in cultivars with low stripe rust infestation, suggesting competitive suppression. Yield losses per percentage of tissue damage were highest for Fusarium head blight, followed by powdery mildew, and then the rust diseases. Breeding progress showed a significant negative correlation between all fungal diseases and year of cultivar release, indicating sustained improvement in disease resistance. Although absolute progress was higher at high N, relative progress was similar across N levels. The relative annual improvement in resistance was highest for powdery mildew and stripe rust, followed by leaf rust. Fusarium head blight showed the slowest improvement. Grain yield increased significantly over time in all treatments, with the most pronounced increase observed without plant protection, particularly at high N. Improved fungal resistance accounted for 11% of yield increase at low N and 28.5% at high N.

This study demonstrates that breeding progress in German winter wheat has resulted in substantial increases in grain yield, significantly driven by improved resistance to major fungal pathogens. The findings highlight the importance of breeding for durable resistance to minimize yield losses. The similar relative improvement in resistance at both nitrogen levels suggests that breeding strategies can effectively improve resistance across different agricultural management intensities. The competitive interaction between stripe rust and leaf rust underscores the complexity of disease management and the potential for one pathogen to mask the effects of another. While this study primarily focused on German cultivars, the findings may have broader implications for wheat breeding programs globally. The observed negative correlation between grain yield and pathogen severity emphasizes the economic importance of incorporating disease resistance into breeding strategies. Future research should focus on identifying the specific genes and QTLs contributing to the observed improvements in resistance, as well as investigating the interaction between different resistance mechanisms. This knowledge can inform more precise breeding strategies, focusing on pyramiding different resistance genes or deploying genes conferring quantitative resistance.

This study demonstrates that breeding for improved resistance to major fungal pathogens has been a significant contributor to the increase in grain yield in German winter wheat over the past five decades. The findings highlight the importance of continuing breeding efforts to develop cultivars with durable resistance to fungal diseases, particularly Fusarium head blight, and enhanced yield potential under varying agricultural management practices. Future research could focus on identifying the genes responsible for the observed resistance improvements and explore breeding strategies to further enhance resistance while maintaining other important agronomic traits. Additionally, investigating the combined effects of biotic and abiotic stresses, such as drought, on wheat yield and disease resistance would provide valuable insights for optimizing future breeding strategies.

The study was conducted at a single location, potentially limiting the generalizability of the results to other environments. Although artificial inoculation was employed to standardize pathogen pressure, the natural infection levels of powdery mildew might have influenced the overall disease pressure and the effects of fungicides. The use of a specific set of pathogen isolates might not represent the full spectrum of pathogen diversity encountered in agricultural settings, although it did include newer and more virulent races. The study’s time frame (1965-2013) limits the ability to predict long-term trends in disease resistance and yield. The analysis was based on a linear model, which may not fully capture the complexities of the interactions between genetics, environment, and disease.