Brown rust in cereals

Also known as leaf rust.

Pathogen

Puccinia triticina (formerly known as Puccinia recondita f. sp. tritici) – wheat

Puccinia hordei – barley

Puccinia recondita – rye, triticale

Hosts

Puccinia triticina is specific to wheat. Other Puccinia spp. and pathotypes affect barley, rye and triticale but do not cross-infect.

Symptoms

Orange-to-brown colour pustules (about 0.5–1.0 mm in diameter) often develop in the autumn on early-sown crops. In early infections, pustules are frequently confused with yellow rust. There is greater differentiation of colour later in the season. Additionally, brown rust pustules tend to be scattered at random, compared with the more striped symptoms of yellow rust. Often seen on the leaves, symptoms can occur on the stem, leaf sheaths, and ears when infection is severe. When leaves begin to senesce, a 'green island' develops around individual pustules. Dark teliospores may be produced towards the end of the season.

Life cycle

The fungus can only grow and survive on live leaf tissue. It overwinters mainly on volunteers and early drilled crops. It spreads by airborne spores. Dark teliospores may develop on diseased plant tissue, indicating a second developmental stage of the fungus.

Temperatures between 15°C and 22°C are optimal for sporulation and germination. However, the disease is active between 7°C and 25°C (a wider temperature range than for yellow rust). Surface moisture on leaves (i.e. 100% relative humidity) is essential for spore germination. Consequently, brown rust epidemics in UK wheat crops normally occur during mid-to-late summer. At this time, windy days disperse spores and cool nights with dew favour the build-up of the disease. Symptoms can occur 5–6 days after infection at optimum temperatures. Cold weather slows disease development but does not kill the pathogen (unless the leaf dies).

Barley tends to suffer from brown rust infection earlier in the spring than wheat. In both crops, rapid spread can occur in warm weather later in the season.

Alternative host species exists for P. triticina but their role is not significant in the UK.

Importance

There is large seasonal and geographic variation in brown rust severity. Cold winters may reduce its spread. Climate change is leading to milder winters and higher than normal spring temperatures. Consequently, brown rust levels are now higher earlier in the spring and found in new locations (e.g. further North).

Wheat

Brown rust in wheat is common in Southern and Eastern parts of the UK, which have higher summer temperatures than other regions. Brown rust tends to develop later in the summer than yellow rust and results in a significant loss of green leaf area and, hence, yield and specific weight.

Barley

Brown rust in barley can be widespread, if conditions are conducive and there is dense cropping of barley. Brown rust epidemics on barley tend to start earlier in the spring than for wheat. Brown rust is usually more of an issue in winter barley than in spring barley, especially in early sown crops when the winter is mild. A severe attack of brown rust early in the season impacts on final yield, through reduced green leaf area and tiller retention.

High-risk factors

  • Susceptible varieties
  • Early drilling
  • Mild winter
  • Hot, humid weather, especially in April to July
  • Southern regions of the UK
  • Volunteers that provide a green bridge. Note: wheat volunteers are not a risk to barley and vice versa

Management

Varietal resistance offers an effective method to manage brown rust in both wheat and barley. AHDB Recommended List brown rust ratings reflect the resistance of adult plants during the main part of the growing season (from about stem extension onwards). Resistance ratings do not provide any guarantee of durability of resistance, as the pathogen population can change and new rust races can develop to overcome these resistances. Therefore, monitor all varieties regularly for symptoms.

Destruction of volunteers helps to prevent disease carry-over of the disease, as they act as a green bridge from one season to another. Wheat volunteers do not pose a brown rust risk for barley (and vice versa), since the different pathotypes do not cross infect. Delaying drilling of susceptible varieties can reduce risk. Managing nitrogen to avoid excessive concentrations in plants (e.g. from late doses of nitrogen) can also help.

Fungicides

Systemic seed treatments may help delay epidemics developing where risk is high. However, seed treatments are likely to provide less control of brown rust than yellow rust.

Winter wheat

Using fungicides to protect against infection is far more successful than eradicating established disease. SDHIs, strobilurins and most azoles are effective protectants for brown rust. Mixing a morpholine with a protectant fungicide can help eradicate established disease. 

T0 – two to four weeks earlier than T1

  • If brown rust is seen in susceptible varieties, and conditions are conducive, treatment will slow early brown rust epidemics and reduce disease pressure at T1 and T2. If brown rust is seen but conditions are cool, then treatment can be delayed until T1

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

  • If brown rust is seen in susceptible varieties, treatment will slow early brown rust epidemics and reduce disease pressure at T2
  • Sprays applied for septoria tritici control will normally also control rusts

T2 – fully emerged flag leaf (GS39)

  • A T2 spray should include a product with activity against brown rust, if it is present in the crop and conditions are conductive for its spread (warm), particularly on susceptible varieties 
  • Sprays applied for septoria tritici control will normally also control rusts

T3 – ear spray

  • Control brown rust at this timing, if it is present or if crops are in a high-risk situation, as it can be damaging if ears are affected
  • Consider adding a strobilurin, where brown rust risk is high
  • Azoles targeting head blight have brown rust activity. Consider the relative importance in brown rust versus head blight activity when choosing between active ingredients

Winter barley

In barley, control brown rust early. However, the azole/SDHI mixes used on other barley diseases will usually provide good control, so the disease rarely warrants specific treatment. Strobilurins have good activity on brown rust and can be used as an alternative. Late infections are less of an issue than in wheat, as the flag leaf is less important in barley than in wheat.

Autumn

  • A spray is only necessary if extensive brown rust affects overwintering capability

T0 – Early spring, GS 23–29

  • Should only be applied if overwintering brown rust levels are high on susceptible varieties

T1 – GS 30–32

  • Eradicate established disease and provide protection until the next timing
  • Azole/SDHI mixes used for other barley diseases will usually provide good protection of developing leaves. Use a strobilurin as an alternative

T2 – GS 39–59

  • Good timing to protect upper leaves against brown rust
  • Higher yield responses to fungicides at this timing can occur where brown rust epidemics and harvest is late (e.g. in the North)
  • Most azole/SDHI mixes used for other barley diseases usually provide good protection. Use a strobilurin as an alternative

Spring barley

T0 – GS 12–22

  • Treatment for brown rust not required

T1 – GS 25–31

  • Good timing to protect against brown rust
  • Azole/SDHI mixes used for other barley diseases will usually provide good control. Use a strobilurin as an alternative

T2 – GS 39–59

  • Best time to protect upper leaves from brown rust and maintain grain quality
  • Azole/SDHI mixes used for other barley diseases will usually provide good control. Use a strobilurin as an alternative
  • See fungicide performance for the latest dose-response curves of different actives

Summary

  • Grow a variety with a high resistance rating, but monitor disease levels throughout the season
  • Eradicate volunteers to remove green bridge
  • Delay drilling of susceptible varieties
  • Manage nitrogen applications to avoid excessive concentrations in plants
  • In wheat, apply a fungicide with activity against brown rust when the disease is seen and conditions are conducive or at T2 and T3 in high-risk situations
  • In barley, use fungicides early to control the disease. However, the azole/SDHI mixes used on other barley diseases will usually provide sufficient control
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