STRIPE RUST OF WHEAT

Stripe rust is currently the most common of the three rusts in Victoria, with suitable conditions for rust occurring in most seasons. The importance of stripe rust is usually related to the proportion of susceptible cultivars being grown and the opportunity for carry over of inoculum during the summer.

Historically, there have been two introductions of wheat stripe rust into Australia. These introductions may have entered Australia on clothing. The first introduction occurred in Victoria in 1979 and then rapidly spread across eastern Australia. This original rust mutated, with a number of pathotypes (also known as races or strains), developed enabling the rust to attack more wheat varieties over time. This first introduction quickly moved to New Zealand, but did not move to Western Australia against the prevailing wind.

The second introduction of stripe rust into Australia occurred in Western Australia, (WA) in 2002. In 2003, this pathotype was detected in eastern Australia, most likely spread on wind currents. This second introduction, now known as the WA pathotype, quickly became dominant in eastern Australia. Since 2003 the WA pathotype has undergone several mutations in eastern Australia, but not WA. There are now many pathotypes of wheat stripe rust that are common in Victoria. The resistance ratings provided in Victorian Cereal Disease Guide (AG 1160) often represent the dominant and most virulent of the pathotypes.

What to Look For

Stripe rust is easiest to identify in the morning. Examine leaves, especially the older leaves, low in the canopy and look for yellow stripes of pustules. These pustules are raised above the leaf surface and can be easily wiped off onto a white cloth or tissue leaving a yellow stain, (Figure 2.3). Also watch for hot spots in the crop. Hot spots are 1-10 metres in diameter and are generally well developed just before the disease becomes widespread in the crop.

Figure 2.3 Stripe rust on a MS-S wheat variety

Disease Cycle

Stripe rust is caused by the fungus Puccinia striiformis. Stripe rust survives from one season to the next predominantly on volunteer self-sown cereals, but can also survive to a lesser extent on other cereal and grass weeds that grow over summer. Stripe rust epidemics are usually more severe in seasons following wet summers that have supported the ‘green bridge’, (Figure 2.4).

During the season, conditions suitable for epidemic development occur from April to December in Victoria and stripe rust can be expected in crops by late August or early September in most years. For an infection of a plant to occur the spore requires temperatures of less than 18°C (optimum 6-12°C) with a minimum of three hours of leaf-wetness (for example, dew). Once an infection is established within the leaf the fungus can survive short periods of temperatures as high as 40°C.

Following infection of the leaf the fungus grows within the leaf for 14 to 28 days (or longer depending on temperature) until the spores appear on the leaf surface as pustules. The time between infection and symptom development is known as the latent period, with the time reducing as temperatures warm during the spring. The spores are then dispersed by wind, allowing the cycle to repeat multiple times during the season.

Figure 2.4 Disease cycle of stripe rust on cereals. Illustration by Kylie Fowler

Stripe Rust Management

Stripe rust can cause significant loss to wheat yield and grain quality, given appropriate environmental conditions and susceptible varieties. However, farmers have shown that by planning to manage this disease they can effectively minimise its effects.

The most appropriate stripe rust management strategy for a given farm will vary from one farm to another, from region to region and from season to season.

There has been much discussion regarding the merits of various approaches to stripe rust management, such as choice of seed or fertiliser fungicide treatment, versus reliance on fungicide sprays alone. Either strategy, provided it is implemented in a timely fashion, can be effective. Therefore, it is important that growers choose a strategy that is appropriate for their situation and follow it during the growing season.

Management Strategies to Minimise the Impact of Stripe Rust

The following management strategies are recommended to minimise the impact of stripe rust:

  • remove volunteer wheat plants (the ‘green bridge’) that will support stripe rust inoculum in the 6 weeks prior to sowing
  • avoid Very Susceptible (VS) and Susceptible (S) varieties by selecting more resistant varieties
  • use a seed or fertiliser treatment to suppress early infection
  • monitor crops during the growing season and apply a foliar fungicide early in the epidemic, if required.

Each of these approaches is discussed in the following pages.

The Green Bridge

Stripe rust can only survive from one season to the next on living plants; mostly wheat and to a lesser extent barley, triticale, barley grass, brome grass and phalaris. This is called the ‘green bridge’.

Stripe rust does not survive on seed, stubble or soil. Therefore, the greater the number of susceptible volunteer wheat plants growing during summer/ autumn the greater the risk of a stripe rust epidemic and the reason that stripe rust is often more severe following wet summers.

The susceptibility of the volunteer wheat plants over summer influences the quantity of inoculum generated by the green bridge. If most varieties in a district are resistant there will be considerably less inoculum than if the majority of plants are susceptible or very susceptible.

It is critical that all volunteer wheat plants are removed either by spraying, cultivation or heavy grazing by the end of March. Particular care should be taken to destroy plants around sheds and silos, as stripe rust often survives on these plants.

Variety Selection

Selecting wheat varieties for rust resistance is an extremely important part of rust management. Select varieties with the highest levels of rust resistance possible, keeping in mind other agronomic and disease traits of the variety. The actual disease response that occurs in the field will depend on many factors including the amount of inoculum carry over, the timing of the rust outbreak in the crop and the pathotypes (races/strains) of stripe rust occurring in a region.

The resistance ratings to stripe rust are based on data collected from around Australia. However, since stripe rust pathotypes are known to change over time, it is critical that only the current Victorian Cereal Disease Guide (AG 1160) is used. The current guide is available on the DEDJTR website www.vic.gov.au/ cerealdiseaseguide.

Based on field data collected during 2005 the yield loss in wheat varieties with different stripe rust resistance ratings is shown in Figure 2.5. This graph clearly shows the importance of replacing susceptible cultivars with more resistant cultivars.

Varieties rated as Very Susceptible (VS) or Susceptible (S) to stripe rust should be avoided. In such varieties stripe rust is more difficult to manage, especially if the season is favourable for stripe rust. VS and S varieties have the potential to rapidly lose all leaf area to stripe rust.

The build up of rust on these varieties can lead to infection of other crops in the district and increase the chance of resistance break down occurring due to the large amount of spores they produce.

If VS or S varieties are grown it is critical that seed or fertiliser is treated with a fungicide before sowing. Crops of VS and S varieties should be monitored regularly for the first sign of rust and a fungicide applied when necessary.

Varieties rated as Moderately Resistant to Moderately Susceptible (MR-MS) or Moderately Susceptible (MS) generally have adult plant resistance. These varieties are unlikely to lose all their flag leaf to disease, but may need a fungicide spray if rust is detected early (before flag emergence).

Varieties rated Moderately Resistant (MR) show only limited rust symptoms on their flag leaves under ideal rust conditions. Varieties rated as Resistant are those with resistance which persists for the duration of the plant’s life. Even varieties rated as MR and R should be monitored with a view to fungicide application as mutations in the rust can occur.

STRIPE RUST RATING

Figure 2.5 Yield loss due to stripe rust in wheat cultivars with differing resistance/susceptibility to stripe rust

 

Seed/Fertiliser Fungicide Treatments

Seed and fertiliser fungicide treatments play an important role in stripe rust management. In the Wimmera, Western, Central and North East districts all varieties with a stripe rust rating of MR-MS or lower should be treated with either a seed or fertiliser treatment to suppress early stripe rust. In the Mallee, growers should use a seed or fertiliser treatment that suppresses early infection in crops when there is a high carry over of inoculum on the green bridge and when susceptible varieties are grown.

These treatments will be most effective when adopted across a region as they will greatly reduce the inoculum levels in a district. The length of protection varies depending on the product selected. For a comprehensive list of products, see the current SARDI Cereal Seed Treatments Guide for more information.

Be aware that some seed treatments effective against stripe rust (e.g. products containing triadimenol and flutriafol) may reduce coleoptile length, and this should be considered at sowing time. Also note that fertiliser treatments do not control bunts and smuts, so a seed treatment still needs to be applied to the seed. Where crops are sown early for anticipated grazing benefits, issues such as withholding period will need to be considered.

Products containing triadimenol or triticonazole give suppression of stripe rust for about 4 weeks after sowing and can help reduce early development of the disease in the crop. However, crops must continue to be monitored during the growing season with a view to foliar fungicide application.

Longer season protection can be provided by applying fluquinconazole to seed, or flutriafol or triadimefon to fertiliser. These products can give protection up to flag leaf emergence or later in some cases. Often these products will reduce the need for follow up foliar fungicide sprays. However, crops should still be monitored with a view to foliar sprays, if necessary, especially when susceptible cultivars are grown.

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

Fungicide Sprays

Effective fungicides for controlling stripe rust are available, however, these should be regarded as a support and not a substitute for growing resistant varieties.

The requirement for fungicide sprays will depend on the carry over of rust inoculum on the ‘green bridge’, the timing of the epidemic (in relation to crop growth stage) and the level of resistance in the variety. For example, in 2011 where stripe rust was detected early (i.e. tillering to flag), a fungicide spray was required in many varieties to protect green leaf area until the onset of adult plant resistance (APR), which starts around ear emergence. Varieties without effective APR may have required sprays beyond ear emergence. Sprays are generally more effective when applied early in an epidemic.

It is likely that the onset of a rust epidemic will be different in different years. The timing of the first occurrence in the crop may be different and the area where it first occurs may also be different. Therefore, it is important that the decision to apply fungicides is based on current seasonal and crop conditions and is not based on previous experiences alone.

During the season crops should be monitored regularly (at least every 2 weeks) for the presence of stripe rust. The earlier that rust occurs within a crop the greater the potential loss, though the easier it is to control.

It is important to note that in susceptible (S), susceptible – very susceptible (SVS) and very susceptible (VS) cultivars, rust can develop very quickly (Figure 2.6). Consequently, fungicides should be applied as a preventative. In varieties rated as S, SVS or VS don’t wait until rust is obvious.

If stripe rust is present before ear emergence, then crops must be sprayed before the level of infection reaches 1 per cent leaf area affected (i.e. when approximately 35 leaves per 100 have stripe rust). It is better to spray sooner rather than later.

When stripe rust is first detected at ear emergence, only the most susceptible (S, SVS and VS) crops or longer season crops may need spraying. After a fungicide application crops should continue to be monitored as fungicides only provide between 2 to 4 weeks protection.

There is often an apparent increase in stripe rust for a few days after spraying. This is caused by the development of symptoms of infections that occurred just before spraying. Control becomes apparent within a week of spraying, and the period of protection is normally about four weeks.

Figure 2.6. Stripe rust progress at Horsham during 2013 in 5 cultivars with different resistance/susceptibility to stripe rust (data collected by J. Fanning)

Early Season Protection Vs. Foliar Sprays

There has been much discussion as to the relative merits of early season protection (i.e. applying early season seed or fertiliser treatments, with follow up fungicide spray if required) versus relying only on applications of foliar fungicides. Both approaches can effectively manage stripe rust, with similar costs to the grower, if used appropriately.

The disadvantage of early season protection is that expense is incurred before knowing if rust will be an issue or the yield potential of the crop. The advantage of the early applied long season protection is that in the presence of rust, the likelihood that a fungicide will be required before flag leaf emergence is reduced. This minimises the need for timely fungicide applications during the season.

The disadvantage of the foliar spray option alone is that crops must be sprayed early in the rust epidemic, in a timely fashion, keeping in mind the difficulty of spraying during periods of continuous wet weather. The advantage of this approach is that expense is only incurred when, and if, stripe rust is an issue within the crop. Both methods are effective if used appropriately. To determine which approach is the most suitable, growers need to consider rust carry over on the green bridge, variety selection, local conditions and the ability to spray for stripe rust in a timely fashion.

Resistance to Stripe Rust in Wheat

In general, there are two types of resistance to stripe rust deployed in Australian wheats. They are major gene resistance and adult plant resistance. These resistance sources may be used either alone or in combination.

Major Gene Resistance

Major gene resistance is a race specific resistance that is very effective against some strains of rust, though ineffective against others. Typically when these major genes are first deployed they are completely effective, but through mutation of the rust these resistances are often short lived in wheat as they are overcome or ‘broken down’ by the pathogen. An example of this is the acquisition of virulence toward the Yr17 gene deployed in many varieties.

When a major resistance gene is ‘broken down’ the level of resistance in a variety will depend on the other genes present in that variety.

Adult Plant Resistance (APR)

APR is a resistance that is widely used in Australian wheats. APR genes are often partial resistance genes that work by slowing down the rate of epidemic development. They do not stop the disease progress completely.

There are a number of APR genes used in commercial wheats. The relative effectiveness of APR genes can be influenced by factors such as the:

  • temperature; APR often working better at higher temperatures
  • crop nitrogen status; there may be a delayed onset in high nitrogen status crops
  • wheat variety that they are deployed in
  • number of APR genes present; their effects are often additive
  • pathotypes of stripe rust present, in some instances.

Even though APR genes are widely used in Australian wheat varieties they are often not well understood. Some APR genes may also be pathotype specific making them prone to being overcome by new pathotypes of stripe rust, while other APR genes are regarded as ‘durable’ and less likely to be overcome by new pathotypes. In general, APR becomes effective at around ear emergence and works best if rust levels are not excessive in the crop at this time.

In varieties that have APR as their only source of resistance it may be important to protect the earlier growth stages of the crop with seed or fertiliser treatments and/or fungicide sprays. In general, varieties rated as MS and above with effective APR will rarely lose all their flag leaf to disease, whereas varieties rated S to VS are at risk of losing 100 per cent of their leaf area to disease.

Many cultivars with APR can be very susceptible as young plants. Growers using such varieties must plan to protect their young crops from stripe rust before the APR becomes effective to minimise rust build up. The level of susceptibility of young crops will vary from one variety to another. This early susceptibility of young crops can result in build up of rust in some years.

Further Information

More detailed information can be obtained from:

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

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

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

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

Wallwork H (2015) Cereal Seed Treatments (SARDI), 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

RUST OF WHEAT

LEAF RUST OF WHEAT