Key messages

• Flag smut, caused by the fungus Urocystis tritici, occurs sporadically in most growing seasons in the medium and lower rainfall regions of the Western Australian grainbelt. Flag smut incidence and severity were higher than usual in 2024, particularly in more susceptible varieties and crops sown without registered seed dressing fungicide.
• The fungus is externally seed borne. Spores of the fungus are carried on the seed coat or survive in soil for 3–7 years.
• Foliar fungicides are totally ineffective and unregistered for flag smut control.
• For at least the next three years, wheat crops sown into paddocks affected by 2024 flag smut should be of more resistant varieties or be sown with a registered fungicide seed dressing (particularly susceptible varieties).

Aims

This paper outlines the life cycle of flag smut and the key environmental factors influencing its occurrence, incidence of disease in Western Australia (WA) from Department of Primary Industries and Regional Development (DPIRD) surveillance, variety responses from 2024 DPIRD trial observations and disease management including recent and historical fungicide seed dressing impacts.

Introduction

Flag smut, caused by the fungus Urocystis tritici, occurs sporadically in most growing seasons predominantly in medium and lower rainfall regions of the WA grainbelt. It occurs across the grainbelt, being reported from Esperance to Geraldton but has appeared most frequently in the Kwinana port zone. Current farming systems that include crop rotation, fungicide seed dressings and resistant varieties have significantly reduced its potential impact, however in 2024 the incidence and severity of infection within paddocks was significantly higher than normal. Infection in the 2024 season may have impacted yield in some paddocks and will have contributed to increased concentration of soil-borne inoculum and greater disease risk for coming years.

Symptoms and impact

Flag smut primarily affects the leaves of wheat plants, unlike other smut diseases such as loose smut and common bunt that target the heads. The disease is characterised by long, raised, black streaks on the leaves, leaf sheaths, and sometimes stems. These streaks eventually break open, releasing masses of sooty grey-black spores. Symptoms are most likely to become evident around booting to head emergence.

Infected plants often exhibit stunted growth, curled and distorted leaves, and excessive tillering. Not all tillers show symptoms, but those that do generally fail to produce grain. Flag smut can have significant impact on the yield of individual plants and crop yield loss is closely related to disease incidence. In WA, the disease is generally at trace levels in paddocks, in the 2024 season in some paddocks incidence of infection was greater than 5% and would have impacted yield.

Flag smut life cycle

Spores are released from affected leaves and stems during harvest, contaminating both grain and soil. Spores can be transferred on infected seed, contaminated machinery, infected plant material or by soil movement. Spores can survive in soil for between 3–7 years making it difficult to avoid once established in a paddock. Infection occurs when spores germinate and penetrate the coleoptile of seedlings before and during emergence. The spores can survive under a wide range of environmental conditions, they are resilient to temperature fluctuations and can withstand both dry and moist soil conditions. Following penetration, the fungus grows systemically within the plant leading to the characteristic symptoms of flag smut, including black streaks on leaves and stems often becoming evident around booting to head emergence.

Environmental and soil conditions at time of planting are reported to impact disease risk. Warm (18–24°C), dry soil conditions favour spore germination and plant infection. Deeper and earlier sowing have also been linked to increased disease risk in affected paddocks.

Disease incidence in WA

Flag smut occurs sporadically across medium and lower rainfall regions of the WA grainbelt in most seasons. Since 2020, DPIRD surveillance has reported flag smut in all seasons except 2021 but generally occurring in less than 5% of paddocks visited and less than 1% plants affected within the paddock. Noticeably, in the 2024 season 10% of wheat paddocks visited had flag smut present with incidence in some paddocks potentially reaching yield impacting levels (>5% incidence).

Factors contributing to high disease incidence in 2024 include, warmer than median temperatures in most regions during wheat crop germination and emergence, increasing adoption of S to VS varieties in lower rainfall environments, reduced deployment of fungicide seed dressings in lower rainfall environments and regular wheat on wheat plantings exposing emerging seedlings to soil borne inoculum pressure.

Resistant varieties are available

Wheat varieties grown in WA vary in disease resistance ranking from very susceptible (VS) to resistant (R) (2025 CVSG). Variety rankings were assessed by DPIRD as part of National Variety Testing services until 2020.

In the 2024 season >45% of crop area was sown to varieties ranked susceptible (S) or worse and only 7% of the crop area was sown to varieties ranked resistant to moderately resistant (RMR). The most commonly grown variety Scepter (>35% crop area) is ranked as moderately susceptible to susceptible (MSS).

In a paddock with soil-borne inoculum at the DPIRD Merredin Research Station, a trial investigating wheat time-of-sowing and varieties had severe infection, affecting whole plants and large areas of some plots. Greater infection was evident in plots sown in late-April compared to mid-May. Varieties rated MSS, (e.g. Scepter) had about 20% of plants affected, while varieties rated susceptible to very susceptible (SVS), (e.g. Vixen) or VS, (e.g. Rockstar) had about 50% of plants affected in the earliest sowing times. Some newer varieties, without current disease ratings for flag smut, such as Vortex, Calibre, Shotgun, Dozer CL Plus and Tomahawk CL Plus, had disease incidences that ranged between Scepter (MSS) to Rockstar (VS). Varieties ranked RMR, (e.g. Ninja, Catapult, Kinsei) to R, (e.g. Denison, Illabo) had an estimated 0–5% infection, even under extremely high disease pressure (Table 1).

Table 1. Visual estimate of Flag Smut incidence in wheat variety by time of sowing experiment at DPIRD Merredin Research Station 2024. Average of three replicates from plots sown 24 April 2024

Fungicide seed dressings are an effective management strategy

Fungicide seed dressings effectively control flag smut and a range of registered products are available. These have been collated into tables by DPIRD, available from (https://www.agric.wa.gov.au/barley/seed-dressing-and-furrow-fungicides-cereals-western-australia) or through the APVMA (https://portal.apvma.gov.au/pubcris).

Limited recent local experimental data is available for this disease, however a DPIRD seed dressing fungicide trial in 2016 at Dalwallinu was sown with Corack wheat (S) into a paddock known to have soil borne flag smut inoculum present. Untreated Corack showed incidence of 0.8% plants affected by flag smut, all seed treatments (fluxapyroxad, triadimenol, fluquinconazole) were 100% effective and reduced flag smut infection to undetectable levels (0%). Foliar fungicide applications were ineffective in reducing flag smut expression and there are no in-crop control options registered.

DPIRD experimental data from the 1980s (Loughman 1989) shows that seed dressing fungicides containing a range of active ingredients including currently registered active ingredients triadimenol, carboxin and flutriafol provided 82–100% reduction of flag smut from incidence levels of 6.4% and 7.3% at two sites.

Conclusion

Flag smut of wheat is relatively easy to manage either through the use of more resistant varieties or application of registered fungicide seed dressing on more susceptible varieties. Unlike other smut diseases such as loose smut of barley, which is only seed borne, flag smut is both seed- and soil-borne and, as such, both seed source and paddock history are relevant factors in current season disease risk. Given the extended period that spores can survive in soil and the high level of disease that occurred in 2024 season, it is recommended that for at least the next 2–3 seasons any susceptible variety be protected with a registered fungicide seed dressing, particularly when being sown into paddocks that were in wheat in 2024.

Acknowledgments

The research and surveillance data presented in this paper is made possible by the significant contributions of Department and Primary Industries and Regional Development (DPIRD) Grains Directorate staff and funding support of DPIRD and the GRDC. Research and surveillance data presented are sourced from the following projects: Wheat agronomy - Capturing the best sowing opportunities for wheat (DPIRD), DAW2404-005RTX - DPIRD Seasonal status of pests and diseases delivered to growers (GRDC / DPIRD), DAW2104-003RTX - Disease surveillance and related diagnostics for the Australian grains industry (Western region; GRDC / DPIRD), DAW00229 – Improving grower surveillance, management, epidemiology knowledge and tools to manage crop disease (GRDC / DPIRD)

Contact details

Geoff Thomas
Department and Primary Industries and Regional Development (DPIRD)

Ph: 0428 947 287
Email: geoff.j.thomas@dpird.wa.gov.au

References

Fuentes-Dávila, G., Goates, B. J., Thomas, P., Nielsen, J. & Ballantyne, B. (2002). Smut diseases. In Bread Wheat. Improvement and Production, ed. Curtis, B. C., Rajaram, S. & Macpherson, H. Gómez. Rome: Food and Agriculture Organization of the United Nations, pp. 251–71. (https://www.fao.org/4/y4011e/y4011e0h.htm)

Loughman, R. (1989). Chemical control in Western Australia of soilborne flag smut caused by Urocystis agropyri. Australasian Plant Pathology 18, 94–95. https://doi.org/10.1071/APP9890094

Power, S., Shackley, B., Paynter, B., Seymour, M., Dhammu, H. & Wackett, B. (2024).  2025 DPIRD Crop Variety Sowing Guide. Bulletin: 4935, ISSN: 1833 7236 (https://www.agric.wa.gov.au/barley/2025-wa-crop-sowing-guide)