Managing wheat disease in the Wimmera

There are over 15 diseases that are known to attack wheat crops in the Wimmera.

In most seasons, these diseases are effectively controlled through the use of integrated disease management strategies by wheat growers. But in the absence of appropriate management these diseases can have serious impacts on grain yield and quality.

Integrated disease management

In the Wimmera, most of the important diseases of wheat can be effectively controlled when an integrated approach to disease management is implemented.

For some diseases, a single management strategy is effective, while for others a number of approaches need to be utilised together to ensure effective management. Many growers don't realise that they are already effectively controlling the important diseases of wheat using an integrated approach.

Good management of a range of wheat diseases can be achieved when growers adopt a strategy that includes:

• 'green bridge' management
• resistant varieties
• seed and fertiliser treatments
• crop rotation
• active monitoring of crops with a view to fungicide applications if required.

'Green bridge' management

Self-sown volunteer wheat plants that grow during the summer and autumn can carry over inoculum of some important diseases, particularly rusts. It is, therefore, important to remove these host plants before sowing.

Volunteer wheat can be removed by cultivation, heavy grazing or with herbicides. In seasons when wheat volunteers are widespread during the summer and autumn, increased emphasis should be placed on the management of rusts using resistant varieties, seed or fertiliser treatments, as well as on monitoring crops for disease to ensure the timely application of foliar fungicides.

Resistant varieties

Many of the important diseases of cereals can be effectively controlled by growing resistant varieties, while for some other diseases avoiding susceptible and very susceptible varieties may be sufficient.

Table 1 provides suggested minimum resistance levels for wheat varieties to be grown in the Wimmera. If a chosen variety does not meet the minimum recommended level, then increased emphasis must be placed on additional management strategies.

When choosing a wheat variety it is important to use a current Cereal Disease Guide, as variety ratings can change with changes in pathogen populations.

Table 1. Suggested minimum disease ratings for wheat varieties grown in the Wimmera

Seed and fertiliser treatments

Fungicide seed treatments (pickles) should be applied to wheat seed every year to protect the crop from bunt and smut outbreaks.

Bunt and smut diseases are economically severe because of their nil tolerance at receival sites. Good coverage of seed is essential for the best effect of these chemicals.

Fertiliser and some seed treatments can suppress important foliar and root diseases of wheat. The most suitable treatment for each situation should be selected. It should be noted that when using fertiliser treatments, which do not give control of bunts and smuts, a fungicide must also be applied to seed.

For more detailed information see SARDI Cereal Seed Treatments.

Crop rotation

Many important foliar and root diseases can carry over from one season to the next, on either crop residues or in the soil. Crop rotation is important to ensure wheat is not sown into paddocks with high levels of either soil or stubble-borne inoculum.

It is also important to control grass weeds during break crops to reduce disease carry over on host plants.

Monitor crops

Active monitoring of wheat crops during the growing season is advisable to detect any important foliar diseases that may be controlled using a foliar fungicide. There are a number of foliar fungicides available (see GRDC Fungicides registered in wheat) and further information on the target disease should be obtained to determine the best strategy for managing it with a foliar fungicide.

For most foliar diseases appropriate timing of fungicide application is critical. The best results are generally obtained from fungicide applications made early in the disease development. Growers, however, need to consider the costs of application, yield potential of the crop, and also remember that often low levels of disease late in the season (such as grain fill) have limited effects on yield.

Disease identification

Correct disease identification is the first step in the effective management of crops diseases. See 'Further references' for useful resources.

Foliar diseases of wheat

In general, most of the important foliar diseases of wheat in the Wimmera can be controlled by growing resistant varieties. However, when adequate resistance within the variety of choice is not available, additional protection from other control strategies is important.

Rusts

Leaf, stem and stripe rust of wheat are the most important foliar diseases of wheat in southern Australia. Where favourable conditions occur and susceptible varieties are grown, significant yield losses may be incurred.

Stripe rust is favoured by cool moist conditions and temperatures of between 8 and 15°C.

Stem and leaf rust are favoured by moist conditions and warmer temperatures (15-30°C for stem and 15-22°C for leaf rust).

Life cycle

Rust requires living plant material to survive and summer volunteer plants provide the bridge between one season and the next. In areas where there are long growing seasons, and summer rainfall, the 'green bridge' can be significant in determining the severity of rust in the following season.

Management

Rusts of wheat can be managed by using an integrated strategy that encompasses:

• 'green bridge' management to minimise inoculum carry over
• variety selection to avoid the more susceptible varieties
• seed or fertiliser treatment to suppress early infections of leaf and stripe rust
• active monitoring with a view to a timely fungicide application if necessary.

Yellow leaf spot (yellow spot)

Yellow leaf spot is a stubble-borne disease that can cause serious yield losses when susceptible varieties are sown into infected wheat stubble. Yield loss from this disease has typically been minimal but, given suitable conditions, yellow leaf spot is a potentially important disease in the Wimmera.

Life cycle

The yellow leaf spot fungus survives from one season to the next on stubble.

Rain early in the season stimulates the release of spores that infect nearby wheat seedlings. Damage from this disease is worse in wet seasons. If wet weather occurs throughout spring, the infected seedling leaves produce more spores that infect leaves higher up the plant which can result in significant yield loss.

Management

Not sowing wheat into wheat stubble, and avoiding very susceptible varieties will help manage this disease.

Foliar fungicides are available, but their use is not economic unless disease pressure is high.

In many cases plants will grow out of seedling infections, unless the season is continually wet favouring more disease development.

Septoria tritici blotch

During the 1970s, Septoria tritici blotch was the most important foliar disease of wheat in the Wimmera.

With the adoption of partially resistant varieties since the 1980s this has become a minor disease in the region. It is important to note that a return to growing susceptible varieties will see Septoria tritici blotch re-establish as an important disease in the Wimmera.

Life cycle

Septoria is predominantly a stubble-borne disease.

Rains in early autumn release airborne spores which infect susceptible wheat varieties. These infections produce fruiting bodies which release spores during warm wet weather causing secondary infection higher up the plant. Severe damage can occur after heavy and frequent rain which can be experienced in some springs.

Management

Growing varieties with partial resistance to septoria will continue to effectively manage this disease. If susceptible varieties are adopted in the future, growers will need to:

• remove infected stubble
• avoid continuous cropping with wheat
• delay sowing to avoid early spore release
• use foliar fungicides as required.

Other foliar diseases of wheat

Minor diseases of wheat such as Halo spot and Wirrega blotch are known to occur but their impacts are usually minimal.

When identifying diseases of wheat it is necessary to note that not all disorders are caused by a disease. Nutritional disorders are often confused with foliar diseases and this should be considered when identifying diseases of wheat.

Soil-borne diseases of wheat

In general, most soil-borne diseases can be effectively controlled with the use of crop rotation. Resistant varieties are available for the control of cereal cyst nematode. Chemicals are available to assist with the management of take-all, rhizoctonia and pythium root rot.

To effectively prevent yield losses from root diseases it is important for growers to identify paddocks at risk. If a grower is uncertain of the root disease risk because of the presence of symptoms within a paddock in previous seasons, a PredictaB soil test can be used prior to sowing to identify the disease and determine inoculum levels (consult your local agronomist).

Cereal cyst nematode (CCN)

Cereal cyst nematode (CCN), also known as eelworm, is historically the most important root disease of wheat in the Wimmera.

In recent times this disease has been well controlled with the use of pulse and oilseed crops in rotation, and the use of CCN resistant cereals. CCN decreases root function as nematodes feed and reproduce within the roots causing the characteristic knotting of the roots.

Life cycle

The nematode survives from one season to the next as eggs inside protective cysts that form on the roots of host plants. The eggs hatch in response to soil moisture when soil temperature drops below 15°C.

Nematodes then invade the host plant's roots. After about eight weeks these nematodes produce eggs. Each year, about 85% of eggs hatch (lower in dry seasons). Therefore, a 2 year break from susceptible hosts is required to effectively reduce egg reserves. There is only 1 cycle per season.

Management

In paddocks with medium to high CCN levels, susceptible cereals should not be grown for 2 years.

Grass weeds in break crops, particularly wild oats, should be controlled within 10 weeks of crop emergence.

Paddocks at risk of CCN can be identified by taking a soil test or monitoring cereal crops for the presence of symptoms of CCN.

Take-all

Take-all is a sporadic disease of Wimmera wheat crops, and is more common in seasons following wet springs. Take-all causes blackening of the sub-crown internode and roots. Above ground, dead heads are the most obvious symptom.

Life cycle

The take-all fungus can only survive between susceptible crops in the root and tiller bases of previously infected plants. Following autumn rains the take-all fungus grows out of this material, and attacks the roots of susceptible plants.

In the absence of hosts (cereals and grasses), the take-all fungus is unlikely to survive for more than 1 year.

Management

To effectively control take-all grasses must be removed before the end of July the year prior to a cereal crop.

Delayed sowing can help reduce the level of disease pressure to a crop.

In paddocks at risk of take-all infection there are some seed and fertiliser treatments available that can suppress take-all.

Crown rot

Crown rot is a sporadic problem, depending on the season. It often shows up in seasons with a dry finish. After flowering, white heads occur scattered throughout the crop. The disease is often observed near trees where there is moisture stress.

Life cycle

Crown rot is a stubble-borne disease, and the fungus survives for up to 2 years on infected stubble from previous cereals and grass weeds.

Survival is enhanced by dry summers, and stubble management is important to reduce the impact of crown rot. The disease is favoured by moist humid conditions, with temperatures between 15° and 20°C.

Management

Grow varieties less susceptible to this disease.

Where this disease has occurred grow non-host crops for 2 years such as:

• grass-free pulses
• oilseeds
• oats
• pasture
• fallow.

Burn or burry stubble to speed up the breakdown of stubble which carries the disease.

Root lesion nematodes

Yield losses caused by root lesion nematodes correlate with:

• the population of these nematodes present in the soil at sowing
• the tolerance of the wheat variety
• the date of sowing.

As the symptoms caused by root lesion nematodes are not as distinct as for other cereal root diseases, it is advisable to have soil tested to determine the likely impact of this disease.

Life cycle

Root lesion nematodes survive in the soil from one season to the next either as eggs or in a dormant state. During the growing season they multiply and feed within the roots of the host. They have a number of generations per season.

Management

In paddocks with high levels of root lesion nematodes, resistant crops should be grown to reduce nematode levels. If wheat is to be sown into an at risk paddock avoid late sowing, and select a tolerant variety.

Find out more about managing cereal cyst nematodes and root lesion nematodes.

Rhizoctonia root rot (bare patch)

Rhizoctonia is favoured when weed growth is not controlled before sowing, there is no soil disturbance before sowing and soil fertility is low (especially in the nutrients phosphorus, nitrogen and zinc).

Symptoms of rhizoctonia become evident as bare patches in the crop appearing at an early growth stage and forming sharply defined areas of stunted plants. Bare patches may vary in size from a few centimetres to several metres in diameter.

Diseased plants are usually stunted and sometimes appear purple in colour. If plants are severely infected, the root systems will be small, discoloured and have characteristic brown 'spear points'.

Life Cycle

The Rhizoctonia fungus survives between crops in particles of plant residue in the soil. The fungus grows out of this material after autumn rains and spreads rapidly through the soil, infecting the roots of germinating plants.

The fungus is most active between of 10° and 15°C, and can survive and grow even under extremely dry conditions.

Rhizoctonia has a wide host range and all crops, pasture and weed species grown in rotations are susceptible. There are no resistant cereal varieties, and crop rotations cannot be used to control this disease.

Management

Rhizoctonia multiplies on weed roots so it is imperative to control weeds from the first autumn flush. If weeds are established, a 3 to 4 week chemical fallow, prior to sowing, will reduce disease damage. Weeds must be dead for 2 to 3 weeks before sowing, but it is inadvisable to delay sowing past the optimum date to ensure chemical control of weeds.

Soil disturbance has, in some instances, proved effective in controlling rhizoctonia. However, multiple cultivations before sowing are not necessary provided weeds are controlled and there is a single deep cultivation (50mm under the seedbed) in the fortnight prior to sowing.

For direct drilling, a depth-modified combine that disturbs the soil 50 m under the seedbed while sowing at the correct depth will provide enough soil disturbance to reduce disease damage.

A seed treatment has recently become available that gives suppression of Rhizoctonia.

Bunts and Smuts

Due to the nil tolerance for bunt and smut in wheat at receival centres these diseases are economically very important. Fortunately these diseases can be effectively controlled with the application of fungicides (pickles) to seed every year.

Life Cycle

These diseases carry over from one season to the next, predominantly on seed and to a lesser extent in soil. During the growing season infected plants are apparently healthy, with no symptoms appearing until ear emergence.

Management

Annual seed treatments are very effective, provided good seed coverage is achieved.

In the advent of an outbreak of bunt, all machinery that handled the infected grain must be thoroughly cleaned.

The affected paddock should be kept wheat-free for several years, prior to sowing with a clean source of seed.

Further references

• Crop diseases
• Cereal root diseases
• Cereal disease guide
• Victorian Winter Crop Summary
• Winter Cereal Nutrition: The Ute Guide
• SARDI Cereal Seed Treatments
• Wallwork H (2000) Cereal Leaf and Stem Diseases. (Book) GRDC
• Wallwork H (2000) Cereal Disease: The Ute Guide (Book) GRDC

Contact

Dr Grant Hollaway
Dr Mark McLean
Cereal Pathologists – Horsham
03 5450 8301

Field Crops Pathology
Grains Innovation Park
110 Natimuk Rd
Horsham 3400
03 5450 8301

Acknowledgments

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