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Leaf diseases of oats

Farmnote 33/95 [Reviewed, content current July 2005]

By Robert Loughman and Simon McKirdy, Plant Pathologists, Plant Pathology, South Perth

Summary

This Farmnote illustrates the leaf diseases of oats and discusses their symptoms and control measures. The diseases include septoria avenae blotch, stem and leaf rusts, barley yellow dwarf virus, ring spot and bacterial blights.

Septoria avenae blotch

Septoria avenae blotch is the most common oat disease in Western Australia. It occurs throughout the cereal growing areas, and is most severe in the high rainfall areas.

The disease is caused by the fungus Phaeosphaeria avenaria f.sp. avenaria (asexual stage: Stagonospora (formerly Septoria) avenae f.sp. avenaria). It is not one of the septoria diseases of wheat, which are caused by different organisms.

The fungus infects leaves, (Figure 1) sheaths and stems (Figure 2)and may also infect heads. Symptoms begin as mottled light and dark brown blotches, with dark brown centres. They are restricted and distinct at first but may enlarge to cover most of the leaf.

  
Figure 1. Leaves affected by septoria avenae blotch [with close up]

Figure 2. Stems affected by septoria avenae blotch

Lesions in the leaf sheath extend into the stem causing death and blackening (Figure 2) which may lead to lodging. The fungus sometimes causes a dark discolouration of the grain (Figure 3) when unseasonably late rain occurs.

Figure 3. Grain showing discolouration from septorial avenae blotch

Septoria avenae blotch may cause up to 50 per cent yield loss and crop lodging in extreme cases but losses of around 10 per cent are more common in high rainfall areas. Tall or slow maturing oats are less likely to be affected by the disease than short (dwarf) or fast maturing varieties(Figure 4).

Figure 4. Tall, late maturing oat varieties (right) are less prone to septoria avenae blotch than short, early maturing varieties (left)

Infected stubble is the main source of carryover infection from one season to another. The sexual stage of the fungus, Phaeosphaeria avenaria, occurs on infested stubble and produces ascospores which are spread moderate distances by wind. Oat stubbles in paddocks rotating from oat probably contribute most of the inoculum to nearby paddocks. In multiple cropped oats where stubble is not destroyed, ascospores land on the new crop in much larger quantities, resulting in the development of earlier and more severe outbreaks.

During the season, the fungus on diseased plants produces splash-borne pycnidio-spores which spread the disease onto new foliage during rain. These spores do not move between paddocks but may also be produced by infested stubble residues and contribute to the development of new disease in multiple cropped oats.

Control

Use resistant or moderately resistant varieties in disease-prone areas if suitable agronomic types are available. Consult the current issue of the Crop Variety Sowing Guide for Western Australia.

Sow at the time appropriate for the maturity of the variety. Septoria avenae blotch can be minimised by not growing continuous oat crops.

In continuous oat cropping, stubble from diseased plants should be destroyed by burning or ploughing. Burning is not advised on light soils subject to wind or water erosion. Heavy soils, that is, soils which are 50 to 60 per cent covered with clods of 2 to 3 cm diameter, may be considered for burning. Following crops should be sown at low speeds into moist soil.

Stem rust

Stem rust is caused by the fungus Puccinia graminis var. avenae. The disease appears as elongated reddish-brown pustules mainly on stems (Figure 5) but also on leaves and heads. The powdery spore masses in the pustules can dislodge readily.

  
Figure 5. Stem rust (Puccinia graminis var. arvenae) and closeup

Rusts grow and reproduce only on living plants and must continually infect new hosts. They survive over summer by infecting volunteer or wild oats and infect crops in the next season.

Seasons are at greater risk of a rust epidemic if

• rust was present in the previous season;
• summer and autumn rains allow wild or volunteer oats to grow over summer, harbouring and building up the rust; and
• spring conditions are suitably wet.

Each factor depends on locality, so it is possible to assess rust risk in your locality.

Oat stem rust will not attack wheat and wheat stem rust does not attack oats. Under favourable conditions, stem rust is very damaging and can destroy a crop (Figure 6) Epidemics are more frequent in the rust-prone, very high rainfall districts in northern and southern agricultural areas.

Figure 6. The red discolouration throughout this crop indicates a severe stem rust infection. Early infection and a susceptible variety, combined with seasonal conditions which favoured the disease, led to crop destruction.

Control

Avoid highly susceptible varieties in rust-prone areas. In seasons with increased risk of rust, resistant oat varieties are the main practical control measure (see Crop Variety Sowing Guide 1998). Grazing, spraying or tilling may help to control volunteer and wild oats over summer but the effect could be limited because of the abundance of wild oats.

Leaf rust

Oat leaf rust is also known as crown rust. The word 'crown' refers to the shape of a type of spore produced by this fungus and is not related to the disease symptoms. The disease is caused by the fungus Puccinia coronata var. avenae. The characteristic symptom is the development of round to oblong, orange to yellow pustules, primarily on leaves (Figure 7) but also on stems and heads. The powdery spore masses in the pustules are readily dislodged. The pustule areas turn black with age.

  
Figure 7. Leaf rust (Puccinia coronata var. avaenae) may first appear in crops as 'hot spots' from an initial infection. Hot spots in early spring allow leaf rusts to build up to very severe levels by the end of the season.

Figure 7. Close up of leaf rust on leaves

Oat leaf rust is potentially a very damaging disease, reducing both grain and forage yields. It does not infect wheat and wheat leaf rust does not infect oats. The fungus is carried over on volunteer oats and wild oats from season to season.

Control

Control is similar for stem rust (above). When oats are grown for high quality or export hay, early cutting should be considered before the disease builds up and causes obvious damage to leaves(Figure 8).

Figure 8. Clouds of leaf rust spores were released when this infected hay crop was cut. The early harvest prevented further damage to leaves.

Barley yellow dwarf virus

Barley yellow dwarf virus (BYDV) is widespread in high rainfall areas of Western Australia. It infects cereals and grasses, but grasses (pasture and wild) are the main reservoirs. The virus survives between growing seasons in grasses which persist through the summer. Infection is spread from the grass reservoirs to oats and other cereals through the migration of cereal and grass aphids. Because of the role of aphids in establishing infection, BYDV is always more severe following wet summers when aphid survival and build-up has occurred.

Symptoms first appear three to five weeks after infection as yellow-brown or orange-brown, diffuse blotches near the leaf tip. The blotches enlarge and fuse until most of the leaf is affected and appears orange-brown. Later it changes to crimson-pink which is recognised easily in the field (Figure 9 and Figure 10).

Figure 9. Reddening of oat leaves from the tip down and striping of young leaves is caused by barley yellow dwarf virus

Figure 10. Barley yellow dwarf virus infection is often seen on plants at the edges of the crop

Oats affected as seedlings may show additional symptoms of severe stunting, increased tillering and floret abortion. Infection after tillering causes a characteristic 'reddening' of later emerging leaves and tip-reddening and death of older leaves. The distribution of infection within the paddock is normally patchy but occasionally the whole crop may show symptoms.

Control

Some varieties of oats are tolerant of the virus, while others are highly susceptible (see Crop Variety Sowing Guide 1998).

Recent trial results indicate that spraying the crop in its early stages with synthetic pyrethroid can provide good protection and is feasible in high risk areas.

Minor diseases

Ring spot

Ring spot is a common disease caused by the fungus Drechslera campanulata which is widespread throughout agricultural areas. The fungus infects the leaf and leaf sheath, causing small purplish-black spots which develop bleached centres (Figure 11). The disease often occurs with septoria avenae blotch and symptoms may easily be confused. It has a wide host range and produces similar symptoms on other cereals and grass weeds.

Figure 11. Leaf spots caused by Drechslera campanulata.

Fungus-infested stubble produces spores which spread to nearby plants.

Control

No direct control measures are available nor does control appear to be warranted. The disease may be more severe following grassy pasture or where grass weeds are not controlled.

Bacterial blights

Stripe blight (Pseudomonas syringae pv. striafaciens) forms long, brown stripes on leaves during winter, which join into blotches that cause leaf collapse (blight) (Figure 12).

Figure 12. A range of symptoms of stripe blight caused by Pseudomonas syringae pv. striafaciens.

Halo blight (Pseudomonas syringae pv. coronafaciens) causes buff-coloured, oval-shaped spots surrounded by a pale halo with a water-soaked appearance. Spots turn brown and fuse together into blotches.

These bacteria need moist conditions to spread. Normally, crops outgrow the infection in spring. Losses are not known to be significant in Western Australia.

Acknowledgment: The authors acknowledge the contribution of Tanveer Khan, Crop Industries, and Roger Jones, Plant Pathology, South Perth.

Further reading

• Bulletin 4341 'Crop Variety Sowing Guide 1998' (Agdex 102/30)
• Farmnote No. 27/87 'Cereal smuts - life cycles' (Agdex 110/633).
• Farmnote No. 19/93 'Barley yellow dwarf virus in cereals' (Agdex 110/633).
• Farmnote No. 45/93 'Wind erosion: monitoring the paddock status' (Agdex 571).
• 'Cereal leaf and stem diseases' H. Wallwork (Ed) (1992) Grains Research and Development Corporation. Available from Kondinin Group, 177 Great Eastern Highway, Belmont, 6104 ($18 posted).