By Tim Ashley, Edaphos Agronomy
Introduction
The 2025 wheat season has revealed significant vulnerabilities in cereal production systems across the UK. Yellow rust (Puccinia striiformis f. sp. tritici) has emerged as the most problematic disease of the year, affecting a wide range of wheat varieties, including those previously considered resistant. The widespread reliance on fungicides, particularly tebuconazole, has once again highlighted the tension between short-term disease control and long-term system resilience.
This article outlines how yellow rust pressure should be viewed not just as a pathology to be controlled, but as a symptom of wider systemic weakness in the soil-plant-environment continuum.
Credit: AHDB
1. The Current Agronomic Response: Fungicide First
Tebuconazole, a triazole fungicide, remains a popular option due to its efficacy and low cost. A single application may cost around £5/ha, making it an attractive first-line response to yellow rust outbreaks. However, this reactive approach often leads to multiple applications throughout the season, compounding physiological and ecological side effects.
2. Physiological Effects of Tebuconazole
Tebuconazole is known to act as a mild plant growth regulator by inhibiting gibberellin biosynthesis. Under normal conditions, this can help reduce lodging by shortening stem internodes. However, under drought stress, it may suppress elongation and reduce canopy development, especially if applied at stem extension (GS30-32).
Studies have reported reduced root elongation and deformation of root apices in wheat following tebuconazole exposure (Shishatskaya et al., 2018). Such suppression is particularly problematic in dry conditions, where root depth and function are critical for moisture and nutrient uptake.
Additionally, triazoles can negatively affect urease enzyme activity in soil, potentially delaying the conversion of urea to ammonium (Kobyłecka et al., 2015). This interaction is especially concerning when urea-based fertilisers are applied early in the season and require microbial conversion during periods of water stress.
3. Soil Biological Disruption
Beyond plant physiology, tebuconazole can impact soil microbial communities. It has been shown to suppress populations of urease-producing microbes and reduce the activity of enzymes such as dehydrogenase and phosphatases (Zhang et al., 2018). This disruption may impair nitrogen cycling and nutrient mineralisation, particularly in biologically fragile soils.
Of particular concern is the documented inhibition of arbuscular mycorrhizal (AM) fungi, which are crucial for phosphorus uptake, water efficiency, and overall plant resilience (van der Heijden et al., 2008). Triazoles, while not specifically targeted at AM fungi, can reduce fungal colonisation and alter microbial balance.
Credit: Kobyłecka, E., et al. (2015). Effects of Fungicides on Urease Activity in Soil
4. Rethinking Disease as a Signal, Not Just a Threat
Yellow rust outbreaks should prompt a systems-level diagnosis. Repeated chemical control indicates an underlying failure in crop resilience.
Key contributing factors include:
- Genetic susceptibility or erosion of varietal resistance
- Nutritional imbalances, particularly low magnesium, manganese, or silicon
- Soil compaction, low organic matter, or disrupted microbiomes
- Inefficient nitrogen use due to delayed conversion or drought-stressed uptake
A systems agronomic approach asks: why is the pathogen succeeding? What plant or environmental weaknesses is it exploiting?
5. Predictive Tools and Proactive Strategies
Technologies such as the Cordulus Farm app now allow growers and advisers to predict yellow rust pressure based on real-time local weather data. Forecasting leaf wetness periods, temperature ranges, and humidity conditions enables more precise timing of interventions.
Rather than reacting to visible symptoms, growers can apply:
- Protectant fungicides (e.g. azoxystrobin) before infection establishes
- Biostimulants such as seaweed extracts, fulvic acids, or amino acid-based products to enhance stress tolerance and immune function
- Nutritional support (e.g. manganese, zinc, silicon) to strengthen the plant’s physiological defences
Credit: Cordulus.com
6. Toward Resilient Cropping Systems
The over-reliance on curative fungicides is indicative of a system under stress. A truly resilient cropping strategy integrates the following components:
- Improved soil structure and active carbon flow to support root development
- Balanced nutrition through mineral and microbial inputs
- Greater varietal diversity with both vertical and horizontal resistance
- Biological buffering through cover crops, composts, and reduced tillage
Yellow rust, when it appears, should be used as a diagnostic indicator rather than a seasonal inconvenience.
Conclusion
Tebuconazole remains a useful tool in the agronomic toolbox. But its repeated use under high disease pressure reflects a deeper systemic issue. Rather than focusing solely on chemistry, the goal should be to reduce the crop’s susceptibility in the first place.
Yellow rust offers a highly visible reminder that we need to design systems that are not only productive, but robust. Through integrated management of soil biology, plant health, and predictive technologies, it is possible to reduce dependency on reactive inputs and build cropping systems that can withstand the pressures of a changing climate and evolving disease landscape.
References
- Shishatskaya, E. I. et al. (2018). Influence of tebuconazole on wheat root tip development. Plant Biology International, 23(2), 145-153.
- Kobyłecka, E., et al. (2015). Effects of fungicides on urease activity in soil. Plant and Soil, 387, 147–159.
- Zhang, L. et al. (2018). Effects of triazole fungicides on soil enzyme activities and microbial communities. Ecotoxicology and Environmental Safety, 157, 216-224.
- van der Heijden, M. G. A. et al. (2008). The unseen majority: soil microbes as drivers of plant diversity and productivity in terrestrial ecosystems. Ecology Letters, 11(3), 296-310.
