Rhynchosporium in barley

Note: Also called leaf scald.

Pathogen

Rhynchosporium commune

Hosts

The disease affects barley, rye, triticale and a number of grasses, particularly ryegrasses. Specialised forms of the pathogen are, generally, restricted in their host range.

Symptoms

Initial symptoms often appear in random patches soon after sowing winter barley. Symptoms due to the seedborne phase of the disease do not appear until January/February, as the disease initially develops inside the leaves and roots without symptoms showing.

Typical, early symptoms are oval or irregular, pale green-grey, water-soaked lesions. Infection often occurs in the leaf axil. Symptoms often first appear at the base of the leaf close to the stem. Symptoms then spread to the rest of the leaf, leaf sheaths, ears and grain, particularly in wet conditions. As the lesions age, they acquire a dark-brown margin (the centre remains pale green or turns pale brown). Lesions often coalesce and form large areas, around which leaf yellowing is common. These can cause chlorosis and eventual death of the rest of the leaf.

In winter barley, symptom expression can be high during tillering in early spring. In spring barley, it is rare to see symptoms until after tillering, except in very early sown crops. In both crops, symptoms can build up rapidly after emergence of the flag leaf under favourable conditions.

Rhynchosporium can be confused with other abiotic spotting symptoms, but symptoms are always present on the lower leaves if they are present on the upper leaves, which can help diagnosis.

Life cycle

Rhynchosporium does not survive well in soil. The seedborne phase is not fully understood. The fungus survives for up to a year in crop debris. Rain-splashed spores, from infected trash, stubble and volunteers, infect seedlings. Rain-splash continues to spread the disease up the canopy, infecting later-emerging leaves, ears and grains. Long-distance spread of airborne spores by wind can also occur. This route of infection can be important in spring barley planted next to infected winter barley crops. The disease is polycyclic. This means it is capable of causing several infection cycles during the growing season. Cool, moist conditions favour the disease. The disease cycle can repeat every 14 days. The ideal temperature range is 18–20°C but temperatures over 20°C slow disease development. There is also potentially a sexual stage of this disease.

Importance

Rhynchosporium is the most damaging disease of UK barley. It can be very severe, particularly in the South-West, West and North of the UK, where conditions are generally wet. In winter barley, yield losses can exceed 1.5 t/ha and grain quality can be reduced. The most serious effect on yield, in both winter and spring barley, results from attacks that develop between first node detectable and boot-swollen growth stages (GS 31–45). Visible levels of disease of 1–2% at GS 31-32 cause economic loss, if left untreated.

High-risk factors

  • Very early sowing
  • Wet weather
  • Tight rotations
  • Infected trash, stubble, volunteers and seed
  • Nearby infected crops
  • Western and northern regions
  • Reduced tillage, where trash remains on the soil surface

Management

Grow varieties with a high resistance rating. However, symptomless-infection and sporulation can also occur on both resistant and susceptible varieties.

Minimise barley trash, stubble and volunteers, and extend rotations. Avoid saving seed from infected crops. A thick lush canopy creates the ideal microclimate for rhynchosporium, so manage nitrogen carefully. Early sown spring crops (December-February) and very early sown winter crops will be at more risk.

Fungicides

Most fungicides have good protectant activity. However, few treatments have good eradicant activity in high-pressure situations. An effective azole, in mixture with a strobilurin or SDHI fungicide, is a good foundation for disease management. For additional eradication, include a morpholine in the mixture.

Winter barley

Autumn/winter

Only necessary if extensive rhynchosporium affects overwintering capability

T0 – Early spring, GS 23-29

If extensive symptoms are seen on susceptible varieties, a low-dose fungicide application with eradicant activity can result in an average yield benefit of 0.2 t/ha

T1 – GS 30-32

This is the most important time to control rhynchosporium. In most crops, this is the first spray timing for rhynchosporium control, even though the disease may already be well established

T2 - GS 39-59

Rhynchosporium risk is lower. However, consider an application if wet weather between flag leaf and ear emergence has caused infection of upper leaves

Note: Consider later protection of upper leaves if weather is wet between GS 39–GS 59. Check the latest fungicide timing for malting crops.

Spring barley

T0/GS 12-22

No treatment required for rhynchosporium

T1/GS 25-31

In most crops, this is the first spray timing for rhynchosporium control. It is the most important timing and is likely to be required on susceptible varieties in high-disease pressure situations

T2/GS 39-59

Rhynchosporium risk is lower. However, consider an application if wet weather between flag leaf and ear emergence has caused infection of upper leaves

Note: Consider later protection of upper leaves if weather is wet between GS 39–GS 59. Check the latest fungicide timing for malting crops.

Summary

  • Grow a variety with a high rhynchosporium resistance rating, but continue to monitor disease on resistant varieties
  • Avoid using seed from infected crops
  • Minimise barley trash, stubble and volunteers
  • Avoid early sowing of spring crops (December–February) and very early sowing of winter crops
  • Manage nitrogen applications to avoid excessive concentration in plants
  • Early control with fungicides is important. Later applications may also be necessary, if the weather is wet
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Rhynchosporium symptoms on barley appear as grey, water-soaked irregular patches with dark brown margins


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