Managing stripe rust and leaf rust in wheat in Western Australia

Identifying rusts

Rust in wheat appears as brown to orange dusty pustules on leaves. Stripe rust and leaf rust can be distinguished by the colour and shape of pustules and the location of the infection.

Stripe rust pustules are yellow-orange. Individual pustules are small and circular but develop into yellow stripes on upper leaf surfaces, leaf sheaths and awns and inside glumes. It can be seen from a distance in patches known as 'hotspots'. For further information refer to the Diagnosing stripe rust of wheat MyCrop page.

Figure 1 Stripe rust symptoms on wheat leaf

Figure 2 Stripe rust pustules on wheat leaves

Leaf rust pustules are orange-brown in colour, circular to oval in shape and chiefly found scattered on the upper surface of leaves (Figure 3). Colour depends on the freshness of the pustule. Because most spores are produced overnight, pustules are best observed in the morning. For further information refer to the Diagnosing leaf rust of wheat MyCrop page.

Figure 3 Leaf rust pustules on wheat

Stem rust pustules are a darker red-orange and are often found with ragged edges and going through to the other side of the leaf and present on the stem or leaf sheaths. For further information refer to the Diagnosing stem rust of wheat MyCrop page.

Risk factors

Wheat varieties

Very susceptible (VS) to susceptible (S) varieties increase the risk of rust infection in two ways. During the growing season, rust can develop and multiply rapidly on these varieties. During the summer and autumn non-cropping period, varieties in this category represent good volunteer wheat hosts that greatly enhance the survival opportunities for rust to carryover in the non-cropping period. Rust is more difficult to control with fungicides in varieties with low resistance ratings (very susceptible to moderately susceptible). When selecting varieties, consider responses to rust diseases and avoid varieties that are highly susceptible. Check disease resistance ratings on the Wheat disease ratings page. Please note that variety ratings have changed for some varieties due to the presence of new leaf rust pathotypes. For further information refer to Implications of known wheat leaf rust pathotypes present in WA.

Green bridge proximity

Summer rains permit the development of volunteer cereal hosts and autumn rains permit the early build-up of rust on these volunteers known as the 'green bridge'. This happens readily after wet summers. Cropping areas that receive summer rain resulting in self-sown green bridge cereals are at risk of early infection with stripe or leaf rusts. Wheat regrowth is the primary risk for carryover of both wheat leaf rust and wheat stripe rust. The amount of rust present in the previous season also determines the risk of leaf and stripe rusts. The more rust in a given year means there is more chance of carryover into the next season.

Weather

While resistance will influence individual crop risk, the overall risk of serious rust outbreaks is influenced by summer and winter weather factors (rainfall and temperature) which can be considered in your region each season. Both stripe and leaf rusts require moisture (rain or heavy dew) or high humidity for spores to germinate and infect leaves. Usually 4-6 hours of leaf wetness are required at optimum temperatures (warm days and dewy nights). Each rust has an optimum temperature for infection and growth (Table 1). Rust outlooks are provided as part of the plant pathology group's seasonal Crop diseases: forecasts and management page.

A stripe or leaf rust epidemic is more likely if the winter and/or spring is suitably wet. Seasonal outlooks are available on the Seasonal Climate Information page. Leaf rust has a warmer mean daily temperature optimum than stripe rust (Table 1). The mild winters in Western Australia result in leaf rust being relatively active in winter and into spring, particularly in the northern agricultural areas. Early sown crops, on which infections establish prior to the cooler winter months, are more at risk from early leaf rust which can develop rapidly in spring. The lower temperature optimum for stripe rust results in the disease being relatively more active in winter than later in the season. Warm spring conditions, particularly in the northern agricultural areas, can increase the time taken between infection and resultant new spores being produced (the latent period). Rust spores easily spread on wind.

How to monitor crops

The aim of crop monitoring is to detect infection at the earliest stage feasible. Inspect the most susceptible and earliest sown crops carefully over a wide area of the paddock. Examine leaves at the top and bottom of the canopy for scattered light infections. In green bridge areas also look for infrequent heavily-infected hot spots. Crops prone to infection at young stages (rated very susceptible to moderately resistant) should be inspected at seven to 10 day intervals from early stem elongation (growth stage Z31) or from early flag leaf emergence (growth stage Z37) if seeding fungicide treatments registered to control rust diseases have been used.

A chargeable service is available to assist with disease diagnosis, send ~10 infected leaves to:

Diagnostic Laboratory Services (DDLS)
Department of Primary Industries and Regional Development
Locked Bag 4, Bentley Delivery Centre WA 6983

Post in a paper envelope (no plastic) with date, location, name and contact details. Broadacre diagnostic submission forms are available from your local office or from the Diagnostic Laboratory Services (DDLS) page.

Rust pathotype testing

Leaf rust and stripe rust occur as different strains because they can readily mutate and strains can easily move around the country and the world on the wind or people's clothing. Possible new strains need to be continuously monitored in order to understand the implications for existing varieties and to assist wheat breeders in developing new resistant varieties.

To monitor rust strains in Western Australia, growers and consultants are encouraged to send rust samples at no cost to the Australian Rust Survey, particularly from varieties showing unusually high levels of rust. Post leaf samples in paper envelopes to: University of Sydney, Australian Rust Survey, Reply Paid 88076, Narellan NSW 2567. Further instructions on submitting samples and printable dispatch forms are available from the University of Sydney website.

Biosecurity measures

Because there are different strains of leaf, stem and stripe rusts, care must be taken when travelling interstate or receiving interstate or overseas visitors, since spores carried on clothing could introduce new strains of rusts. Implement biosecurity measures to minimise rust becoming established or spreading on your farm. Rust spores are small, light and may survive for several days without a host. Rust spores can spread long distances by wind, on machinery/vehicles, on tools, clothing and footwear. Remember that if you walk through an infected crop, follow biosecurity protocols and thoroughly clean your boots, hands and trousers before entering another paddock or travelling as rust spores can be unknowingly transferred via people locally and also from overseas. Also check biosecurity measures taken by your visitors and agronomists.

Be particularly vigilant when returning from eastern Australia or internationally, as rust pathotypes with different virulences exist outside WA.

If entering a paddock suspected to be infected with rust, biosecurity suggestions include:

• Wear protective overalls and rubber boots
• After crop insprection clean any material off boots with a brush. Prepare footbath of bleach (10% household bleach, 90% water) and spray bottles of methylated spirits brew (95% metho, 5% water) for use to disinfect footwear, pants and hands
• Decontaminate vehicles, tools and machinery
• Walk instead of driving through crops
• Ask visitors/agronomists to leave their vehicle at the gate and only travel on your property in your vehicle.

Yield losses

Yield loss will depend on the disease resistance of the variety and on how early the disease starts in the crop (Table 2). In general, yield losses are greatest in susceptible varieties and with early infection. Severe disease will cause grain shrivelling (but grain is otherwise sound). For stripe rust, quality reductions such as lower hectolitre weights and grain staining (rare), can add to high yield loss impacts.

Disease management

Utilise variety resistance

Resistance to one rust is normally independent of resistance to other rusts. Varieties may express a range of resistance to stripe or leaf rust in three broad categories:

• Resistant at all plant stages from seedling to adult (MR-R). Fungicide is not required. Normally based on a single gene resistance, experience around the world has shown this resistance can be rendered ineffective (break down) through rust mutation.
• Partially resistant (MRMS), susceptible at young crop stages and gradually increasing in resistance as the crop develops during late stem elongation, expressing maximum adult plant resistance around heading/flowering. Varieties usually develop rust slowly unless they become infected early.
• Very susceptible to susceptible (VS-S) throughout all stages, rapid rusting causing significant yield losses. Promotes epidemic development and pathogen mutation.
• Variety ratings are available on the Wheat disease ratings page. There have been some recent changes due to the presence of two new leaf rust pathotypes in WA (one in 2013 and one in 2015). For further information see Implications of the known wheat leaf rust pathotypes in WA.

Avoid early infection

Destroy green bridge well in advance of seeding

The overlap of summer volunteer or autumn sown susceptible wheat with conventional wheat plantings in winter is a crucial factor in establishing early severe infection of stripe or leaf rust. Destroy self-sown wheat (particularly the most susceptible varieties) well in advance of seeding (4-6 weeks recommended) as occurrence of rain leading into a cropping season increases susceptible regrowth and allows very early sowing opportunities. Further information on green bridge managment is available at Control of green bridge for pest and disease managment.

Early season fungicide protection is important in high risk situations

For S-MRMS varieties, if there is an increased risk of localised rust carryover associated with green bridge cereals in your region, use of a seed dressing or in-furrow fungicide at seeding or fungicide spray at stem elongation can limit early prior to flag leaf emergence. Risk of early season infection can not be assessed until autumn. If possible, delay general seed treatment of susceptible to intermediate varieties until autumn to determine the risk of rust associated with high early rainfall and regional green bridge cereals. Highly effective options include long-acting seed dressings or in-furrow fungicides that can provide protection until around flag leaf emergence stage depending on rate of application and disease pressure. Expenditure decisions (including product choice) should be made according to risk, yield potential and presence of other diseases. For details on registrations refer to the Seed dressing and in-furrow fungicides for cereals in WA page.

Foliar fungicides can also protect from early infection. In the absence of fungicide at seeding, crops at high risk of early infection can be treated with a foliar fungicide spray at first node (Z31) to protect the crop from early infection until around flag leaf emergence. This strategy is applicable to varieties with ratings S-MRMS and can be used instead of fungicide at seeding. This can delay and sometimes avoid costs if the disease risk does not eventuate. This early spray will only protect the leaves which are emerged at the time of the foliar application and may require a follow up spray if seasonal conditions favour continued rust development. For details on registrations refer to the Seed dressing and in-furrow fungicides for cereals in WA page.

Trials in 2014 with Mace and Lr 76-1, 3, 5, 7, 9, 10, 12 +Lr37 pathotype showed infection and yield losses were greatest when disease was present from Z31, infection after Z39 had less impact.

Control late infection in susceptible to intermediate varieties with fungicide

Foliar fungicide retards disease development for about 3-6 weeks after application, depending on product and rate. These diseases can be controlled effectively and economically if fungicide is applied shortly after infection commences but follow-up application will probably be required in long season environments or on very susceptible varieties. If crops have not received early fungicide, commence monitoring for stripe or leaf rust at first node stage, Z31 and apply foliar fungicide at the first sign of infection according to the stage of crop development and variety susceptibility. If crops have received early fungicide, commence monitoring for stripe or leaf rust at early to full flag leaf emergence and apply fungicide at the first sign of infection according to the stage of crop development and variety susceptibility. When taken up by the leaf, the fungicide can stop development of early infections but more established infections can continue such that rust pustules may persist for several days after fungicide application. A list of registered foliar fungicides is available to assist choosing a product.

Foliar fungicide guidelines

Fungicide timing

• Apply fungicide as soon as possible after the first detection of stripe or leaf rust taking into consideration the stage of crop development and variety susceptibility. The value of prompt fungicide application after detection was show in Foliar fungicide application timing for managing leaf rust in wheat, Mingenew 2016 trial report.
• Economic responses are reduced with later fungicide application.
• Spraying after crop flowering is normally not economic for stripe or leaf rusts. Late rust infection should be carefully inspected to check it is not stem rust. For more information refer to Managing stem rust of wheat.
• Optimise control of stripe rust on leaves is important to reduce risk of infection of heads by applying fungicide at or before crop heading. Although spores may adhere to seed, they rarely induce grain discolouration and do not become seed bourne. Head infection will shrivel grain, so screenings will increase with severe infections.

Fungicide rate

Use high rates of fungicide if application is delayed or if infection is advanced. Use high rates of fungicide for longer duration of protection, for example, when season conditions favouring infection are likely to persist or for more susceptible varieties. Cost should be tuned to crop yield potential and crop season length.

Fungicide products

A list of registered foliar fungicides including registered rates is available. Consider what other diseases are present or the crop is at potential risk of when choosing which fungicide product to use.

Figure 4 Stripe rust infected wheat glumes

Reporting to PestFax

For the benefit of industry; growers, agronomists and consultants are encouraged to submit samples of rust finds to their local DPIRD office or directly to the National Cereal Rust Survey and to PestFax. Reports to PestFax can be made online to PestFax Reporter or PestFax map. To receive the weekly PestFax newsletter of disease and pest finds across the Western Australian wheatbelt, subscribe by emailing pestfax@agric.wa.gov.au.