Septoria tritici in winter wheat

Septoria tritici is commonly called septoria. It is also known as septoria leaf blotch.

Pathogen

Zymoseptoria tritici (teleomorph Mycosphaerella graminicola) – formerly known as Septoria tritici

Hosts

Mainly wheat but also occasionally on rye, triticale and some grass species.

Symptoms

Septoria often results in elongated, oval septoria lesions that are restricted by leaf veins, giving a rectangular appearance. Water-soaked patches often form, surrounded by leaf yellowing or death. Lesions may coalesce to form large areas of necrotic brown tissue. Mature lesions contain characteristic, small (but visible) black fruiting bodies (pycnidia). However, pycnidia are not always visible in immature lesions.

In most years, symptoms develop very early. On young autumn-sown wheat, symptoms may be evident by early December, and throughout the winter on the lowest leaves. During leaf production phases of growth in the winter and early spring, it is common for new leaves to appear green and healthy and for lower leaves to die back with typical symptoms. Stressed crops can show greater levels of infection.

Life cycle

Septoria survives the winter as dormant mycelium, pycnidia, and pseudothecia on crop debris, autumn sown crops and volunteers. The pathogen is not seedborne. Pseudothecia release windborne, sexually produced, ascospores. Pycnidia release asexually produced pycnidiospores. Ascospores released from previous wheat stubbles initiate epidemics in the winter and early spring. Pycnidiospores are rain-splashed from infected lower leaves. Pycnidiospores are usually responsible for the spread of the epidemic throughout the spring and summer. Physical spread of these pycnidiospores can also occur without rainfall, particularly when leaves 3 and 4 overlap the upper leaves as they emerge.

Once a spore has landed on a new leaf, it can take 12 hours for the spore to germinate. Infection of the new leaf usually takes place within 24 hours of the spore landing, providing conditions are damp. Symptoms appear after a 14–28 day latent period. The optimum temperature for Z. tritici is 15–20°C. Latent period reduces as temperatures get closer to this range.

Importance

Septoria tritici is the most important and damaging foliar disease on winter wheat in the UK. The pathogen reduces green leaf area for photosynthesis. It causes significant yield loss every year. It also affects grain quality. Losses of 50% may occur in severely affected crops. Unusually dry weather throughout May and June may reduce losses, but heavy dews can still allow infection. Higher rainfall areas, in the South and West, are most at risk.

High-risk factors

  • Susceptible varieties
  • Early drilling
  • Wet weather, especially during May and June
  • Windy weather, which can increase the physical spread of spores
  • Region (dry easterly regions are at less risk)
  • Mild winter

Management

Varietal resistance is key to the management of septoria tritici. Most varieties on the AHDB Recommended List have multi-gene resistance to this disease. This means it is difficult to breed varieties with complete resistance (RL septoria tritici rating of 9), but it does mean that the resistance is durable and less likely to breakdown if the pathogen population changes (as happens with yellow rust). A high RL septoria tritici rating can significantly reduce yield losses caused by the disease in an untreated crop and will give more flexibility in fungicide timing.

Early drilled crops are exposed to incoming ascospores for longer periods and, hence, tend to have higher levels of disease throughout the winter and early spring. Although the final level of disease is determined largely by weather conditions during stem extension, delaying drilling from mid-September to mid-October can reduce final disease pressure, irrespective of the variety. An AHDB-funded project showed that a moderately susceptible variety (RL septoria tritici rating of 5) sown in mid-September had comparable disease levels to a susceptible variety (RL septoria tritici rating of 4) sown in mid-October.

Fungicides

Fungicides are required for effective septoria tritici control in most crops. However, to reduce reliance on fungicides and the risk of fungicide resistance developing, all other cultural control methods should first be adopted to reduce the level of input required.

A typical programme for control of the disease starts at T1 to limit the disease on the lower leaves. However, the T2 timing is the most important to keep upper leaves free of disease and so optimise yield and quality. If the fungus is well into its latent phase, no fungicide will provide effective eradicant activity. There is very rarely a yield benefit associated with controlling septoria at the T0 timing, even in the highest disease pressure situations. However using septoria tritici active products at the T0 timing will reduce disease levels on the lower leaves, reducing the risk where T1 is delayed.

Prompt timing is most important on varieties susceptible to septoria tritici (rated 5 or less on the AHDB Recommended List), so these should be the priority for treatment.

Septoria tritici has a high propensity to develop resistance to fungicide modes of action. Resistance to strobilurin products is widespread and they do not provide adequate control. Although resistance to azoles and SDHIs is also widespread in UK septoria populations, they still provide good levels of control, although azoles need to be applied at or close to full label rate to reach acceptable efficacy levels. As new modes of action and newer, more active, members of currently available modes of action are introduced, they should result in improved levels of septoria control, but will need to be protected from the development of resistance.

Winter wheat

Seed treatment

  • Some systemic azole seed treatments may give limited early control. However, none has a label claim and a yield benefit is unlikely

T0 – two to four weeks earlier than T1

  • There is very rarely a yield benefit associated with controlling septoria at the T0 timing, even in the highest disease pressure situations. It is sometimes used as an insurance against a delayed T1 in susceptible varieties
  • A protectant multi-site fungicide is most appropriate, where a fungicide is necessary

T1 – as soon as leaf 3 is fully emerged (GS31-33)

  • This is the first main timing for septoria tritici control. It aims to protect the fully emerged leaf 3. However, it will also give some protection from disease on leaves 2 and 4
  • Use an azole plus multi-site. The addition of an SDHI is unlikely to be economical in varieties with a high septoria tritici rating, especially if it is late-sown (from late October). However, omitting an SDHI on susceptible varieties (rating of 5 and below) that are September sown is a higher risk strategy
  • Apply products as soon as leaf 3 has fully emerged. Applying later or before the leaf has fully emerged will not give such effective control
  • For fungicide resistance management, use a balanced mixture of an azole + multi-site and only add an SDHI where disease risk merits it. Use the lowest dose possible to get the required control, but ensure that each component of the mix gives comparable control levels – this will require a robust does of azole in relation to that of any SDHI included.
  • Varieties susceptible to septoria tritici should be the priority for treatment

T2 – fully emerged flag leaf (GS39)

  • This is the most important spray timing for septoria tritici control, as it controls disease on the top two leaves, which contribute approximately 65% to yield
  • The aim is to eradicate disease already developing in healthy-looking leaves lower down the canopy and to protect the flag leaf, which has just emerged, for as long as possible
  • An azole plus SDHI should give good control of septoria tritici and prolong green leaf area of the top two leaves. The addition of a multi-site protectant will reduce the risk of azole and SDHI resistance as well as helping to protect the leaves from secondary infection. Check the label since their addition may reduce eradicant properties of some fungicides
  • Apply products when most flag leaves on main tillers are fully emerged
  • Varieties susceptible to septoria tritici should be the priority for treatment

T3 – ear spray

  • In the North and West, the septoria tritici risk continues throughout the season and treatment at T3 may be required to prolong the protection of the upper leaves
  • In the drier East, varieties with a high septoria tritici resistance rating should not require a spray for septoria tritici at this timing
  • On septoria tritici susceptible varieties, ensure azole applied for ear diseases at this timing is also active against septoria tritici
  • In cool and wet years, a multi-site protectant applied at this stage can help minimise damage associated with reinfection. However, check the number of applications permitted and the latest time of application

Summary

  • Grow a variety with a high septoria tritici resistance rating
  • Avoid early drilling, especially of susceptible varieties
  • The T2 fungicide timing is the most crucial, with T1 also targeted at septoria tritici control. However, the T0 spray rarely gives a yield benefit
  • Use a balanced mixture of an azole with a multisite (where possible). Only add an SDHI when disease risk merits it
  • Use the lowest dose possible to get the required control, but ensure that each component of the mix gives comparable levels of control
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Disease assessments

With a focus on yellow rust and septoria tritici in wheat, this video shows how disease severity is assessed in RL trials using a standardised scale (0 to 100 per cent)


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