Septoria tritici blotch of wheat

Septoria tritici blotch (STB) is an important stubble-borne foliar disease of wheat in Victoria.

This disease has increased in importance in the high rainfall cropping regions during the last five years, even though it has been well controlled in Victoria for the last 30 years through the use of partially resistant wheat varieties.

The increase in STB in the high rainfall zone has been favoured by:

• stubble retention
• intensive wheat production
• susceptible cultivars
• favourable disease conditions.

STB is prone to developing resistance to fungicides. Resistance to some triazole (Group 3) fungicides was recently detected in Victoria by Dr Andrew Milgate, NSW Department of Primary Industries.

To minimise the chance of further resistance developing it will be important pay careful attention to fungicide strategies and use an integrated approach to management.

What to look for

The fungus causes pale grey to dark brown blotches on the leaves, and to a lesser extent stems and heads. The diagnostic feature of Septoria tritici blotch is the presence of black fruiting bodies (pycnidia) within the blotches.

These tiny black spots give the blotches a characteristic speckled appearance. When the disease is severe, entire leaves may be affected by disease lesions.

In the absence of the black fruiting bodies, which are visible to the naked eye, similar blotching symptoms may be caused by yellow leaf spot or nutritional disorders such as aluminium toxicity or zinc deficiency.

The only other disease that has black fruiting bodies within the blotches is Septoria nodorum blotch, but this disease is rare in Victoria.

Economic importance

When susceptible and very susceptible varieties are grown, Septoria tritici blotch is likely to cause annual average losses of up to 20 per cent, with much higher individual crop losses possible.

Disease cycle

Septoria tritici blotch, also called Septoria leaf spot or speckled leaf blotch of wheat is caused by the fungus Mycosphaerella graminicola (asexual stage Zymoseptoria tritici, synonym Septoria tritici).

Septoria tritici blotch survives from one season to the next on stubble. Following rain or heavy dew in late autumn and early winter, wind borne spores (ascospores) are released from fruiting bodies (perithecia) embedded in the stubble of previously infected plants. These spores can be spread over large distances.

Early ascospore infections cause blotches on the leaves. Within these blotches a second type of fruiting body, pycnidia, are produced. Asexual spores ooze from pycnidia when the leaf surface is wet and spores are dispersed by splash to other leaves where they cause new infections.

This phase of disease development depends on the rain splash of spores, therefore Septoria tritici blotch will be most severe in seasons with above average spring rainfall. A combination of wind and rain provides the most favourable conditions for spread of this disease within crops.

Management

An integrated approach that incorporates variety selection, cultural practices, crop rotation and fungicides is the most effective way to manage Septoria tritici blotch.

Variety selection

The majority of commercially grown varieties now have partial resistance (that is, they are moderately susceptible) to Septoria tritici blotch. This resistance has to date been durable and sufficient to effectively control this disease in Victoria.

It is important to very susceptible varieties as they will build up inoculum levels. This will cause yield loss in that variety, and in adjacent moderately susceptible wheat crops.

For information on the resistance status of varieties consult a current Victorian Cereal Disease Guide.

Cultural practices

Following an outbreak of Septoria tritic blotch do not sow wheat into infected stubble and avoid early sowing, as a high number of ascospores are released early in the season.

If this is not possible, destroying stubble by grazing or cultivation will reduce the number of spores available to infect the new season's crop. This practice is not practicable in light soil areas where stubble must be kept to prevent erosion.

Such practices will have more effect if undertaken on a district basis.

Crop rotations

Crop rotations are important to ensure wheat is not sown into paddocks with high levels of stubble-borne inoculum.

A 1-year rotation out of wheat is generally effective to provide disease break. However, the fungus may survive for over 18 months on stubble during very dry seasons.

Fungicides

Some seed applied fungicides can suppress early infection and should be used in areas where Septoria tritici blotch is known to occur.

Effective foliar fungicide sprays are available if necessary. However, it is important to correctly identify Septoria tritici blotch before spraying with a fungicide as nutritional disorders such as aluminium toxicity or zinc deficiency can be confused with Septoria tritici blotch.

In high risk areas, the timing of fungicides will be important to achieve adequate disease control. In early sown susceptible varieties, a fungicide application at growth stage 31-32 may be required to suppress the disease and protect emerging leaves. Once the flag leaf has fully emerged, at GS39, another fungicide application may be required to protect the upper canopy.

Since STB is prone to developing resistance to fungicides it is important that fungicide strategies to reduce the likelihood of resistance developing are adopted. This is extremely important as fungicide resistance to Septoria tritici blotch had been detected in Australia.

Fungicide resistance

Increasing resistance of Zymoseptoria tritici to some triazole (Group 3) fungicides was recently detected in Victoria by Dr Andrew Milgate, NSW Department of Primary Industries.

Two mutations of Septoria tritici blotch giving resistance to triazole fungicides were identified. These mutations reduce the effectiveness of fungicides, rather than making them completely ineffective. However, continued use of triazole fungicides will put further selection pressure on the pathogen and potentially new mutations will be selected.

Fungicides with reduced effectiveness to Septoria tritici blotch include:

• triadimefon
• triadimenol
• tebuconazole
• propiconazole
• epoxiconazole (not registered for control of Septoria tritici blotch in Australia).

Dr Milgate found that resistance may not be causing reduced spray efficacy at present, but a strategy to prolong effectiveness will prolong the life of this fungicide group.

Managing fungicide resistance

There are a number of methods thought to reduce the selection rate for further mutations.

The first method is to alternate different triazoles, as not all triazole fungicides are affected equally by mutations of the Septoria tritici blotch fungus. This means not using the same traizole fungicide more than once in a crop, if multiple sprays are required during the season.

The second is to used fungicides that combine triazoles, such as propiconazole and cyproconazole, or tebuconazole and flutriafol, which are registered for Septoria tritici blotch.

The third is to use fungicides with different modes of action. However, in Australia there is a limited choice of fungicides with different modes of action. Products that combine a strobilurin (Group 11) fungicide with a triazole (Group 3) fungicide may reduce the risk of resistance development.

Strobilurins on their own are considered to be at high risk of developing resistance due to their single site mode of action.

In the United Kingdom, resistance to strobilurins is so widespread in Septoria tritici blotch populations, they are no longer effective, even in mixtures. Resistance of Septoria tritici blotch to strobilurins has been recently detected in New Zealand.

While not yet registered in Australia, SDHI (Group 7) carboxamide fungicides mixed with triazole (Group 3) fungicides are being used in New Zealand and the United Kingdom to manage Septoria tritici blotch.

When using fungicides, it is important that growers always follow label guidelines and ensure maximum residue limits are adhered to.

Biosecurity

Resistant mutations of the Septoria tritici blotch fungus have been identified in other countries, including New Zealand, the United Kingdom and mainland Europe. It is important to not accidentally introduce these resistant mutations into Australia after travelling overseas.

The risk of introducing exotic diseases or new mutations of a pathogen into Australia can be minimised by having a biosecurity hygiene plan following overseas travel.

Basic biosecurity hygiene includes washing clothes and cleaning footwear before returning to Australia. If high risk areas have been visited, consider leaving clothing and footwear behind. Remind family members, employees or others travelling to also take these precautions.

Further information

• Victoria Cereal Disease Guide
• CropLife Fungicide Activity Groups
• Decimal Growth Scales of Cereals
• GRDC Managing Septoria Tritici Blotch Disease in Wheat
• Plant Heath Australia Fact Sheet: Is Your Farm at Risk?
• SARDI Cereal Seed Treatments
• Wallwork H (2000) Cereal Leaf and Stem Diseases. (Book) GRDC.

Contact

Dr Grant Hollaway
Cereal Pathologist - Horsham
03 4311 3111

Field Crops Pathology
Grains Innovation Park
110 Natimuk Rd
Horsham 3400
(03) 4344 3111

Acknowledgments

Grant Hollaway, Frank Henry. Support by the Grains Research and Development Corporation is gratefully acknowledged.