Phoma in Oilseed Rape: Beyond Fungicides

We are at that stage when agronomists across the country will be recommending the autumn phoma sprays. Almost a routine exercise and is usually met without any questions. The issue is that there is evidence that the fungicides are becoming less effective. So what else can we do? Here, I explore how we can improve phoma control beyond spending more on fungicides.

Phoma leaf spot, caused by Leptosphaeria maculans and L. biglobosa, has long been a familiar challenge in oilseed rape. The early signs are often just small leaf spots in autumn. But the real danger comes later. This is not simply a leaf disease; it is a systemic invader that spends months hidden inside the crop before reappearing as damaging stem cankers. Understanding what’s happening inside the plant helps explain why fungicides alone are not enough, and why biology and nutrition must be part of the solution.

How infection develops

The first infections usually occur in the autumn when spores, carried by rain splash, land on cotyledons or young leaves. Germination produces small, pale lesions, but the pathogen does not stay put. The hyphae grow down the leaf stalk (petiole) into the crown of the plant. Once in the crown, the fungus quietly colonises vascular tissue through the winter.

Credit: AHDB

By spring, this systemic colonisation shows up as crown lesions and stem cankers. These cankers infiltrate the plant’s vascular system, blocking water and nutrient flow, and often lead to premature ripening or lodging. By the time we see the damage, the fungus has been inside the plant for months. This delayed symptom expression is one reason phoma remains so difficult to manage.

Why fungicides alone struggle

Conventional advice has centred on azole fungicides applied at the 4–6 leaf stage. While this can protect leaves from new infections, fungicides cannot stop the fungus once it has already moved into the petiole and crown. In other words, chemistry only protects the visible part of the problem.

The challenge is compounded by recent evidence from the UK and Europe showing a slow decline in sensitivity to azoles. These fungicides are still effective, but not as much as they were ten or twenty years ago. This is the same pattern we’ve seen with septoria in wheat: gradual erosion of efficacy, followed eventually by reports of resistance. Relying on fungicides alone leaves farmers exposed to both biological and economic risks — more sprays, higher costs, shrinking margins.

How biology helps

Beneficial bacteria such as Bacillus amyloliquefaciens (BAA) provide a complementary line of defence. These bacteria support the crop in several ways:

• Competitive colonisation: Bacillus quickly occupies the same niches on leaf and root surfaces that phoma would exploit, physically reducing the space available for the pathogen.

• Antimicrobial metabolites: They produce lipopeptides (iturins, fengycins, surfactins) that directly suppress fungal growth.

• Iron competition: Both Bacillus and phoma need iron (Fe) for metabolism. Bacillus secretes siderophores that bind Fe tightly, depriving phoma of this critical resource and slowing its growth.

• Induced Systemic Resistance (ISR): Colonisation by Bacillus “primes” the plant’s immune system, triggering stronger and faster defence responses. This includes the thickening of cell walls, the production of defence enzymes, and the lignification of tissues.

By combining these effects, BAA doesn’t just reduce leaf infection pressure; it also helps limit the fungus’s ability to move systemically into the plant. 

Nutrition: the armour biology depends on

Biological protection is only as strong as the plant’s nutritional foundation. The way nutrients interact has a direct bearing on disease outcomes:

• Calcium (Ca) is essential for the formation of strong cell walls and maintaining membrane stability. Adequate Ca creates physical resistance to fungal ingress.

• Boron (B) works closely with Ca, cross-linking pectins in the cell wall and reinforcing structural integrity.

• Silicon (Si) forms a defensive layer beneath the cuticle, making it harder for pathogens to penetrate.

• Manganese (Mn) contributes to lignification and defence enzyme activity.

The balance between these nutrients is critical. Excess potassium (K) or ammonium (NH₄⁺) can suppress Ca uptake. That, in turn, limits boron movement because B transport in the plant is linked to Ca in the xylem. Too much K or NH₄ can therefore tip the crop into a state where tissues are thin-walled and poorly defended — effectively inviting Leptosphaeria maculans to invade. This explains why crops pushed hard with nitrogen and potash can sometimes suffer more severe phoma despite good fungicide programmes.

An integrated perspective

Phoma control should not be seen as a matter of “spray or not spray.” Fungicides have their place, but they are not enough on their own — and with declining sensitivity, their role will become more limited. The real resilience comes from an integrated system:

• Use fungicides strategically, at correct timing, and in mixtures to manage inoculum.

• Support soil and leaf surfaces with biological inoculants that include Bacillus AA, which compete with and suppress the pathogen.

• Ensure a balanced nutrient supply — especially Ca, B, Si, and Mn — while avoiding excesses of K and NH₄ that undermine defence.

• Focus on keeping carbon flowing into the soil to build a diverse microbial community that works alongside the crop.

The combination of biology and nutrition creates conditions where phoma struggles to establish, spreads more slowly, and causes less damage. Fungicides then become an insurance policy rather than the sole defence.What I would suggest:Not a recommendation but a suggestion to discuss with your agronomist:

Ingredient	Suggested Rate (starting point)	Role in Mix / Notes
Rampart* (Aiva)	~ 0.3 – 0.5 L/ha*Follow Citric acid mixing protocol	Silicon boost for structure / resilience
Pulsar (Aiva)	~ 3 – 6 L/ha (depending on intensity)	Multi-nutrient backbone Aiva Fertiliser
Calfite from Unium	~ 0.75 L/ha	Core P + Ca input
Boron	~ as per label	Micronutrient in demand at this stage
Armour (Aiva)	~ 0.25 – 0.4 L/ha	Defence / elicitor support
Nurture N	~ 1 L/ha	Carbon source with fulvic acid: Chelator / uptake enhancer Aiva Fertiliser
Citric Acid	Depends on water hardness	Essential in order to mix Rampart
Water volume	200 – 300 L/ha

Conclusion

Phoma is a classic example of how a disease exploits weak points in the crop’s defences. By understanding the infection pathway and the nutrient interactions that either strengthen or weaken resistance, we can design management systems that rely less on chemistry and more on the natural resilience of the crop–soil–microbe partnership.

The future of phoma control isn’t just about waiting for the next fungicide active ingredient. It’s about building crops that are structurally stronger, nutritionally balanced, and biologically supported. That shift doesn’t just reduce disease pressure; it protects farm margins and improves system resilience for the long term.

Scientific Footnote: Reduced fungicide sensitivity

Phoma has long been controlled by azole (DMI) fungicides, but recent research shows a slow decline in sensitivity, echoing the pattern seen with septoria in wheat.

• 2017 baseline: UK populations were still well controlled by azoles, although L. biglobosa was generally less sensitive than L. maculans. (Sewell et al., 2017, Pest Management Science).

• 2024 update: Work from Rothamsted and European collaborators found reduced sensitivity in western-European L. maculans populations, including to prothioconazole-desthio and mefentrifluconazole. The mechanism is linked to CYP51 promoter insertions, which up-regulate the enzyme targeted by DMIs. Fungicides still work, but the shift is statistically clear. (King et al., 2024, Plant Pathology; Rothamsted, 2024).

• European context: Broader surveys in Central/Eastern Europe also report variation and reduced sensitivity (Fajemisin et al., 2022, Agriculture).

• Stewardship guidance: FRAC (2024) stresses rotating/mixing MoAs and avoiding back-to-back solo azoles.

Key message: This isn’t widespread resistance, but a slow decline in sensitivity. If fungicides are made to do all the work, that decline accelerates. Biology and nutrition extend fungicide life and protect margins.

Scientific Footnote: Bacillus amyloliquefaciens (BAA) vs Phoma

There is growing evidence that BAA and related Bacillus species can suppress L. maculans under lab and controlled conditions:

• Cholerton (2015, Nottingham PhD): BAA significantly reduced L. maculans leaf lesions in planta at the leaf spot stage.

• Hanif (2021): Identified B. amyloliquefaciens strain B89 and B. velezensis B94 as strong antagonists, inhibiting fungal growth through volatile compounds and near-complete suppression in seedling assays.

• Fernando et al. (Saskatchewan projects): Field-oriented work showed BAA strains producing compounds like surfactin, fengycin, bacillomycin D, antagonising L. maculans in canola.

• Mechanisms: antibiosis, induced resistance, Fe competition via siderophores, and niche exclusion.

Caveat: Most evidence is lab- or seedling-stage; consistent field suppression is less well proven. Effectiveness is strain- and environment-dependent. BAA should be seen as part of an integrated resilience strategy alongside nutrition and chemistry, not a silver bullet.