Wheat & Cereal
Fungal Diseases
Identification · Mycotoxins · Non-Chemical, Organic & Chemical Controls · Environmental Impacts
Fungal diseases are among the most damaging threats to wheat and cereal crops worldwide, reducing yields by 10–50% in severe cases and in some instances rendering grain unsafe for food or feed due to mycotoxin contamination. This complete guide covers disease identification, the mycotoxins DON and ZON and how to reduce them, non-chemical cultural controls, organic and biological treatments with their environmental impacts, and conventional chemical fungicides.
Contents
Disease Profiles
Most widespread rust in temperate climates. Favoured by cool, moist conditions (7–13°C). New aggressive races (e.g. Warrior, Triticale-aggressive) have overcome previously resistant varieties with alarming speed.
Key Symptoms
- Bright yellow-orange spore pustules in distinct stripes along leaf veins
- Pustules surrounded by pale yellow chlorotic tissue
- Powdery yellow dust on fingers when touched
- Severe infections cause leaf shrivelling and premature senescence
The single most economically damaging disease of wheat in the UK and Northern Europe. Spreads via rain splash. A latent period of 2–3 weeks makes early detection difficult and timing of control critical.
Key Symptoms
- Pale brown/tan irregular blotches with yellow margins on leaves
- Distinctive black pycnidia (tiny dots) visible within lesions
- Lesions progress upward from lower to upper leaves
- Flag leaf and ear infections cause the greatest yield losses
Thrives in warmer conditions than yellow rust (15–22°C). Epidemics develop rapidly in late spring and summer. New races regularly overcome resistant varieties, requiring annual resistance monitoring.
Key Symptoms
- Scattered, circular orange-brown pustules on upper leaf surfaces
- Pustules randomly distributed (not striped — key difference from yellow rust)
- Dark powdery spore masses; yellow halo around pustules
- Severe infection causes premature leaf death and reduced grain fill
Unlike most fungi, it spreads in warm, relatively dry conditions. High nitrogen and dense canopies increase risk significantly. Spreads rapidly by wind-dispersed spores between plants.
Key Symptoms
- Fluffy white-grey powdery colonies on upper leaf surfaces
- Initially small pustules expanding into large patches
- Yellow chlorosis and browning beneath colonies
- Black cleistothecia (overwintering bodies) appear in older lesions
Also called ear blight or scab. Critically important due to mycotoxin contamination (DON, ZON) which renders grain unsafe for food or feed. Worst at flowering in warm, wet conditions (>20°C, humid).
Key Symptoms
- Premature bleaching of individual spikelets on the ear
- Salmon-pink/orange mycelium visible between glumes in humid conditions
- Shrivelled, lightweight grain (Fusarium-damaged grains, FDGs)
- Visible pink-orange sporodochia in wet weather
A stem base disease causing 'lodging' — collapse of crop stems before harvest. Favoured by mild, wet autumns and winters. Builds up under frequent cereal cropping.
Key Symptoms
- Eye-shaped lesion with a brown border and pale centre at stem base
- Dark brown mycelium visible within lesion centre
- Weakened stems leading to lodging (crop collapse)
- Whiteheads (sterile, prematurely ripened ears) in severe cases
Mycotoxins: DON, ZON & T-2
What are mycotoxins and why do they matter?
Mycotoxins are toxic secondary metabolites produced by fungi — primarily Fusarium species — during infection of cereal crops. Unlike the fungi themselves, mycotoxins cannot be destroyed by cooking, milling, or most food processing. Contaminated grain must be rejected or downgraded. EU and UK regulations set strict maximum limits for mycotoxins in food and feed, and exceeding them can result in entire harvests being condemned. Fusarium head blight is the primary source on wheat, but mycotoxins can also originate from maize, stored grain and other cereals in the rotation.
The most prevalent mycotoxin in UK and European wheat. Produced mainly by Fusarium graminearum and F. culmorum during ear infection at flowering. DON is a type B trichothecene that inhibits protein synthesis and causes gastro-intestinal illness in humans and animals.
A resorcylic acid lactone produced primarily by Fusarium graminearum, often co-occurring with DON in the same infected grain. ZON is a potent oestrogenic mycotoxin that mimics oestrogen in mammals, causing reproductive disorders particularly in pigs at very low concentrations.
Type A trichothecenes produced mainly by Fusarium langsethiae and F. sporotrichioides, particularly important in oats and occasionally wheat. Among the most acutely toxic mycotoxins affecting cereals.
Produced primarily during storage by Aspergillus ochraceus and Penicillium verrucosum when grain moisture content is too high. Not a field-infection mycotoxin but a storage management failure indicator.
Reducing Mycotoxin Levels
⚠️ Critical principle: Mycotoxins cannot be eliminated once present in harvested grain. All strategies must focus on prevention during crop growth and harvest, plus rapid and effective grain storage management. Testing (ELISA or HPLC) before sale is essential in high-risk years.
Mycotoxin Reduction — Integrated Strategy by Stage
Cultural & Non-Chemical Management
Varietal & Genetic Resistance
The most effective long-term strategy. Consult the AHDB Recommended List each season — ratings of 7–9 for septoria, rust and mildew resistance significantly reduce disease pressure without inputs.
Sowing a mix of varieties (variety blending or strip trials) across a farm slows the spread of airborne spores and reduces risk of new races overcoming all crop on the farm simultaneously.
Subscribe to Rothamsted's Rust Tracker and AHDB disease alerts. Yellow rust Warrior race and new septoria lineages regularly overcome existing resistance — check resistance ratings are still valid each year.
Crop Rotation & Timing
Including oilseed rape, legumes or root crops in rotation reduces soil-borne pathogens (eyespot, fusarium) and disrupts the 'green bridge' — volunteer cereals that carry rust and septoria between seasons.
Sowing winter wheat after mid-October reduces early-season septoria and yellow rust infection. Cooler conditions at drilling slow spore germination and allow crop establishment before peak disease pressure.
Killing volunteer cereals and grass weeds around field margins 2–3 weeks before drilling removes the carry-over host for yellow rust and barley yellow dwarf virus before the new crop is established.
Nutrition & Canopy Management
Excessive nitrogen creates a dense, humid canopy highly susceptible to septoria and mildew. Split N applications and avoid over-application, particularly for disease-susceptible varieties in high-rainfall areas.
Open, well-ventilated canopies dry rapidly after rain, reducing the leaf wetness periods required for septoria, rust and mildew infection. Dense crops maintain humidity within the canopy, especially lower down.
Adequate K and silicon strengthen cell walls and improve intrinsic resistance to fungal penetration, particularly for powdery mildew and rust. Soil testing and targeted nutrition are key.
Monitoring & Decision Support
Use AHDB's Disease Risk Calculator and UK Met Office forecasts to make spray decisions based on actual risk — not calendar dates. Economic thresholds exist for each disease and crop growth stage.
Walk crops weekly from tillering onwards. Check lower leaves for early septoria, flag leaf emergence for rust, and ear emergence for fusarium. Photograph and compare to reference guides.
Burying or removing infected stubble post-harvest removes primary inoculum for septoria, fusarium and eyespot. Deep ploughing is more effective than minimal tillage for soil-borne pathogens.
Organic & Biological Treatments
Bacillus subtilis
A naturally occurring soil bacterium that produces lipopeptides (iturin, fengycin, surfactin) which disrupt fungal cell membranes and compete for space on leaf and root surfaces. Available as seed treatments and foliar sprays (e.g. Serenade, Taegro).
Efficacy: 20–40% reduction in fusarium and septoria lesion development. Most effective as part of an integrated programme rather than as a standalone treatment.
Trichoderma spp.
Biocontrol fungi that parasitise plant-pathogenic fungi and stimulate plant systemic defence responses. T. harzianum and T. asperellum are most used commercially. Applied as seed treatments or soil drenches.
Efficacy: Most effective against soil-borne pathogens including fusarium root/stem base diseases. Less consistent against foliar diseases. Sensitive to temperature and soil conditions.
Copper-Based Fungicides
Copper sulphate, copper hydroxide and Bordeaux mixture have broad-spectrum fungicidal activity by denaturing fungal enzymes and disrupting membrane function. Among the oldest fungicides used in agriculture.
Efficacy: Moderate activity against mildew and some protection against rusts. Less effective against septoria and fusarium under high disease pressure compared to synthetic fungicides.
Sulphur (Elemental & Wettable)
Elemental sulphur and wettable sulphur formulations have good activity against powdery mildew by disrupting fungal respiration. Has been used since ancient times and remains an important tool in organic cereal systems.
Efficacy: Good against powdery mildew, moderate against some rust strains. Not effective against septoria or fusarium. Works best preventatively — must be applied before or at very early stages of infection.
Plant Extract Elicitors
Products such as acibenzolar-S-methyl (ASM/Bion), laminarin (Vacciplant) and harpin proteins trigger the plant's own Systemic Acquired Resistance (SAR) — essentially 'training' the plant immune system before infection occurs.
Efficacy: Typically 15–30% disease reduction. Works best as a complement to other programmes rather than a standalone. Effect is cumulative with multiple applications. Most useful for rust management.
Bicarbonate & Potassium Silicate
Potassium bicarbonate and sodium bicarbonate create an alkaline leaf surface environment hostile to mildew spore germination. Potassium silicate strengthens cell walls and reduces penetration by fungal hyphae.
Efficacy: Moderate control of powdery mildew only. Requires thorough coverage and repeated applications (5–7 day intervals). Limited practicality at field scale in arable systems.
Coniothyrium minitans
A fungal hyperparasite that attacks the sclerotia (resting structures) of Sclerotinia sclerotiorum in soil. Primarily relevant for oilseed rape in rotations with wheat, where it reduces the sclerotinia burden that can affect following cereal crops.
Efficacy: Very effective at reducing sclerotinia sclerotia in soil over 1–2 seasons. Applied as a soil drench. Less relevant for wheat specifically, but important in integrated rotation management.
Reynoutria sachalinensis Extract
An extract from giant knotweed that activates plant defence pathways (jasmonic acid and salicylic acid signalling), enhancing resistance to multiple pathogens. Commercially available as Regalia in some markets.
Efficacy: 20–35% reduction in mildew and rust severity in trials. More consistent results when applied preventatively. Useful as part of an integrated, resistance-based programme.
🌱 The case for organic and biological treatments: While currently less effective than synthetic fungicides under high disease pressure, biological and organic approaches have significant advantages: no resistance risk, no harmful residues, beneficial effects on soil biology, and compatibility with agri-environment schemes. Their efficacy is improving rapidly as understanding of plant immunity and beneficial microbial communities develops. In organic systems, combining resistant varieties + sulphur + copper (at restricted rates) + Bacillus subtilis provides the best currently available protection.
Conventional Chemical Fungicides
| Active Substance | Group / MoA | Target Diseases | Environmental Impact | Key Concerns |
|---|---|---|---|---|
|
Epoxiconazole Triazole (DMI) |
Group 3 — Sterol demethylation inhibitor | Septoria, yellow & brown rust, powdery mildew, eyespot | High | Classified as a suspected endocrine disruptor; reprotoxic Category 2. Toxic to aquatic organisms. Restricted in several EU member states; under continued regulatory review in UK post-Brexit. |
|
Prothioconazole Triazole (DMI) |
Group 3 — Sterol demethylation inhibitor | Septoria, rusts, fusarium, eyespot | Medium | Moderate aquatic toxicity. Metabolite prothioconazole-desthio detected in water catchments and groundwater. Widespread use accelerating reduced sensitivity in septoria populations across Europe. |
|
Tebuconazole Triazole (DMI) |
Group 3 — Sterol demethylation inhibitor | Fusarium (DON reduction), rusts, mildew, septoria | Medium | The primary fungicide for reducing DON and ZON in fusarium-infected grain when applied at anthesis. Suspected endocrine-disrupting properties. Widely detected in surface water. Moderately toxic to aquatic invertebrates. |
|
Bixafen / Fluxapyroxad SDHI |
Group 7 — Succinate dehydrogenase inhibitor | Septoria, rusts, mildew | High | Highly toxic to earthworms at field-relevant concentrations. Toxic to bees (contact). Persistent in soil. Resistance in septoria developing rapidly following widespread adoption since 2011. Now often used at reduced rates in mixtures. |
|
Isopyrazam / Benzovindiflupyr SDHI |
Group 7 — Succinate dehydrogenase inhibitor | Septoria, rusts, mildew, ramularia | High | As for bixafen/fluxapyroxad: earthworm toxicity, persistence, bee toxicity. Both are classed as very persistent, very bioaccumulative (vPvB) substances — a major regulatory concern. Cross-resistance between SDHIs limits substitution within this group. |
|
Azoxystrobin / Picoxystrobin Strobilurin (QoI) |
Group 11 — Quinone outside inhibitor | Rusts, mildew (septoria now fully resistant) | Medium | Complete resistance (G143A mutation) in septoria renders strobilurins largely ineffective against it in the UK. Very toxic to fish and aquatic invertebrates. Cannot be used alone — must always be mixed with a DMI. Limited to partner-only use for septoria management. |
|
Cyprodinil + Fludioxonil AP + PP seed treatment |
Groups 9+12 — Anilinopyrimidine + phenylpyrrole | Fusarium (seed-borne), septoria (seed-borne), Microdochium | Medium | Seed treatments apply much lower doses per hectare than foliar sprays, reducing environmental exposure. However, treated seed dust is highly toxic to pollinators and seed spillage during drilling is a significant unregulated exposure pathway for birds and mammals. |
|
Chlorothalonil Chloronitrile — WITHDRAWN |
Multi-site broad spectrum | Septoria (highly effective when available) | Very High | Approval withdrawn EU & UK (2019–2020). Included as a cautionary case study. Highly toxic to aquatic organisms; metabolites detected in rivers and groundwater across Europe. Classified possible carcinogen (Group 2B). Its withdrawal removed the most effective multi-site fungicide for septoria, accelerating resistance concerns in remaining actives. |
Environmental Considerations
Pollinators & Beneficial Insects
SDHIs and strobilurins are acutely toxic to bees on contact. Triazoles impair bee navigation and immune function at sub-lethal doses. Applying sprays during flowering or when bees are actively foraging is prohibited under product stewardship guidelines.
Water Contamination
Triazoles and their metabolites are regularly detected in UK rivers, groundwater and drinking water sources. Buffer zones of 3–10m from watercourses, tramline management and avoiding pre-rain applications are all required under stewardship.
Soil Health & Earthworms
SDHIs (bixafen, fluxapyroxad, isopyrazam) are acutely toxic to earthworms at agronomic application rates. Repeated use suppresses soil microbial diversity and mycorrhizal fungi, impairing nutrient cycling, drainage and soil structural stability.
Birds & Mammals
Treated seed poses documented risks to seed-eating birds (e.g. grey partridge) and small mammals. Triazoles bioaccumulate in food chains. Seed spillage during drilling is a significant, historically under-regulated exposure route.
Endocrine Disruption
Multiple triazoles (epoxiconazole, tebuconazole, propiconazole) are suspected endocrine disruptors in vertebrates. Effects on amphibian and fish reproduction are documented in laboratory studies. Regulatory review is ongoing in the EU and UK.
Fungicide Resistance
Intensive fungicide use has driven multi-resistant septoria populations across Europe — an irreversible ecological change. Strobilurin resistance is complete. SDHI and triazole resistance is at commercially damaging levels. Resistance management through mode-of-action rotation is now essential to preserve remaining efficacy.
Treatment Comparison
| Approach | Septoria | Rusts | Mildew | Fusarium / DON | Organic Certified | Environmental Risk | Resistance Risk |
|---|---|---|---|---|---|---|---|
| Resistant varieties | ✔ Good | ✔ Good | ✔ Good | ~ Partial | ✔ | Very Low | Race evolution possible |
| Rotation / cultural | ✔ Good | ✔ Good | ✔ Good | ✔ Good | ✔ | Very Low | None |
| Bacillus subtilis | ~ Moderate | ✗ Limited | ~ Moderate | ~ Moderate | ✔ | Very Low | Very Low |
| Sulphur | ✗ Poor | ~ Limited | ✔ Good | ✗ Poor | ✔ | Low | Very Low |
| Copper fungicides | ~ Moderate | ~ Moderate | ~ Moderate | ✗ Poor | ✔ (restricted) | Medium (soil accumulation) | Low |
| SAR elicitors (laminarin) | ~ Limited | ~ Moderate | ~ Moderate | ✗ Limited | ✔ (some) | Very Low | None |
| Triazoles (DMI) | ✔✔ High | ✔✔ High | ✔ Good | ✔ Good (tebuconazole) | ✗ | Medium–High | Resistance developing |
| SDHIs | ✔✔ High | ✔✔ High | ✔ Good | ✗ Poor | ✗ | High | Rapid resistance development |
| Strobilurins (QoI) | ✗ Resistant | ✔✔ High | ✔ Good | ~ Limited | ✗ | Medium–High | Full resistance in septoria |
⚠️ Best practice recommendation: No single approach provides adequate protection. The most effective, sustainable and environmentally responsible strategy combines: high-rated resistant varieties + good rotation and canopy management (the non-negotiable foundation) + biological/organic treatments where certified systems require it + targeted synthetic fungicides applied at the correct timing based on disease risk thresholds, using mixture products to manage resistance. Monitoring variety resistance ratings and pathogen race changes annually is essential — what worked last season may not work this season.