STEM RUST OF WHEAT

Stem rust is an occasional, but devastating disease of wheat. Conditions that favour stem rust epidemics are rare and occur on average once every 16 years in Victoria. However, when conditions are conducive, the disease can cause complete crop loss in susceptible varieties.

Historically, the most severe epidemics in Victoria occurred (in descending order of severity) in 1973, 1947, 1934 and 1955. In 1973, stem rust reduced the Victorian wheat harvest by 25 per cent. It is unlikely that stem rust losses will ever be as severe as in 1973 due to the increased cultivation of stem rust resistant varieties and the greater availability of effective foliar fungicides. In recent years, there have been few localised occurrences of stem rust.

Following the exceptionally wet January of 2011 there was a large amount of inoculum carry over that resulted in widespread stem rust in Victoria during 2011. In spite of this, the widespread preventative use of chemicals helped minimise losses from this disease.

Disease Cycle

Stem rust (caused by the fungus Puccinia graminis ) can only survive from one season to the next on a living host. It does not survive on stubble, seed or soil. The most important hosts are susceptible wheat, but it can also survive on barley, triticale and some grasses. Carry over on wheat from one season to the next is greatest during wet summer/autumns, (Figure 2.10).

Rust spores are wind-blown and can be spread over large areas in a short time. Wet conditions and temperatures of approximately 15-30°C favor the establishment of stem rust within crops. Stem rust usually becomes evident later in the season than stripe rust.

Figure 2.10 Disease cycle of stem rust on cereal. (Illustration by Kylie Fowler)

What to Look For

Stem rust is characterised by reddish-brown, powdery, oblong pustules. The pustules have a characteristic torn margin that can occur on both sides of the leaves, on the stems and the glumes. Stem rust spores are much darker in colour than leaf or stripe rust spores, (Figure 2.2). As the plant matures, the pustules produce black spores known as teliospores. They occur mainly on the leaf sheaths and stem.

Figure 2.9 Symptoms of stem rust on wheat

Conditions Favourable to Stem Rust

Stem rust can occur in all regions of Victoria where susceptible varieties are grown. However, the likelihood of a stem rust epidemic is increased by several factors:

  • the build up of stem rust inoculum on volunteer wheat before sowing, both locally and in neighbouring states
  • the widespread planting of susceptible varieties
  • favourable weather conditions, which includes good spring rains and warm (15-30°C) humid conditions.

If the first two conditions above are met and there is a wet spring, an outbreak is likely to occur.

Pre-Season Management of Stripe Rust

Stem rust can be managed using an integrated approach. This includes reducing the inoculum in a district by managing the green bridge, avoiding susceptible cultivars and close monitoring to enable timely fungicide sprays.

The Green Bridge

Rust can only survive from one season to the next on living plant material (mainly self sown cereals). Therefore, the removal of the green bridge is essential to reduce the amount of inoculum present to infect a new crop. This is why stem rust epidemics have been worse following wet summer/autumns that favour volunteer cereal growth.

Variety Selection

Sowing resistant varieties provides the best protection against stem rust. In most parts of Victoria stem rust has been controlled because of the use of resistant varieties.

Stem rust occasionally produces new pathotypes (races) which are capable of attacking resistant varieties. These new pathotypes occur when a chance mutation occurs in this asexually reproducing fungus. Use of resistant varieties minimises the amount of rust in a district, thus reducing the chance of new pathotypes occurring. It is important that growers are aware of a variety’s resistance reaction to stem rust. For a comprehensive list of varieties, consult a current Victorian Cereal Diseases Guide (AG1160).

In Crop Management of Stem Rust

The effects of stem rust can be minimised with the timely application of foliar fungicides. As there is limited information on the management of stem rust in Victoria, the following recommendations for the in-crop management of stem rust are based on experience in Western Australia (Jayasena et al 2015).

Monitoring

Stem rust is most severe in susceptible varieties when it begins to develop in the crop before flowering when crop losses of 50 per cent are possible. Yield losses from later infections are possible, but not as severe.

As stem rust requires warmer conditions than stripe rust for development, it is advisable to begin monitoring for stem rust from flag leaf emergence onwards. Monitoring will be necessary in seasons when stem rust has been detected locally or on volunteer plants before sowing.

Guidelines for Monitoring for Stem Rust in Wheat Crops

Inspect wheat crops every 7 to 10 days from flag leaf emergence to early dough grain development. However, if stem rust is detected within a region, then increase inspection frequency to every 4 to 7 days.

Carefully inspect different plant parts, especially the lower stems, for symptoms of stem rust. Spend at least 15 minutes walking through each wheat crop.

If stem rust is detected, walk through the paddock in a ‘W’ pattern and collect 10 stems from 10 random locations (total 100) to determine the percentage of stem rust infection. See Table 2.7 for control options.

When to Spray

The information in Table 2.7 is a guide for the application of foliar fungicides. Note: this table is not based on Victorian data, but on limited experimental data from Western Australia (Jayasena et al 2015). Fungicides will give better control of stem rust when applied early in the epidemic. A late, low level occurrence of stem rust (i.e. after mid-dough) will have little impact on yield.

In 2011, when there were paddocks of self sown wheat heavily infected with stem rust at sowing, the prophylactic application of fungicides to susceptible varieties was important for the area-wide control of this disease. Such an approach would not be warranted in most seasons.

Table 2.7 A guide for timing fungicide control of stem rust (adapted from Jayasena et al 2015)

CROP GROWTH STAGE STEMS INFECTEDA

%

RESISTANCE RATINGB

VS, S, MS-S MR-MS

Before ear emergence 1-5

>5

Spray Monitor

Spray Spray

Ear emergence / mid dough >5

>50

Spray Monitor

Spray Spray

ABased on 100 stems selected in a W pattern across crop.

BR= Resistant, MR = Moderately Resistant, MS = Moderately Susceptible, S = Susceptible, VS = Very Susceptible

Choice of Fungicide

In Victoria, there are a number of active ingredients (available in a range of products) registered for the control of stem rust.

It is always important to read the chemical label before use. In particular, check that the product is registered and use the maximum recommended label rate for stem rust control in wheat.

Note: products containing tebuconazole break down relatively slowly in plants and users must observe the product label restrictions regarding the total amount that can be applied to one crop per season. This will ensure harvested crops don’t exceed the tebuconazole maximum residue limit (MRL) in cereal grains. See Taking Care with Foliar Fungicides for more information. As sprays for stem rust may be applied late in the season, it is extremely important to know the harvest withholding period for the chemicals, which can vary from 4 to 6 weeks.

Further Information

More detailed information can be obtained from:

DEDJTR AgNotes Seriesw, website: www.vic.gov.au/graindiseases

Victorian Cereal Diseases Guide (AG 1160), website: www.vic.gov.au/cerealdiseaseguide

Victorian Taking Care with Foliar Fungicides, website: http://www.depi.vic.gov.au/agriculture-and-food/farm- management/chemical-use/publications/taking-care-with-foliar-fungicides

Victorian Winter Crop Summary, website: www.vic.gov.au/victorianwintercropsummary

Jayasena et al (2015) Managing Stem Rust of Wheat. Department of Agriculture and Food https://agric.wa.gov.au/n/395

Wallwork H (2000) Cereal Leaf and Stem Diseases (SARDI) www.grdc.com.au/bookshop

Wallwork H (2015) Cereal Seed Treatment Guide http://www.pir.sa.gov.au/, Direct link:http://www.pir.sa.gov. au/ data/assets/pdf_file/0005/237920/cerealseedtreat2015_web.pdf

For rust identification, send rusted plant samples in a paper envelope (do not use plastic wrapping) to: Australian Cereal Rust Survey. Plant Breeding Institute. Private Bag 4011, Narellan NSW 2567

LEAF RUST OF WHEAT

YELLOW (LEAF) SPOT