Loose smut in 2016

Take home message

• Annual seed treatment with an effective fungicide is the best means to control loose smut.
• Loose smut infects heads at flowering and survives inside the seed.
• Some varieties are more susceptible to loose smut than others.
• Fungicides differ in their efficacy against loose smut and 100% control is difficult to achieve.
• Resistance to loose smut is not considered a high priority of barley breeding programs in Australia and control relies on the use of effective seed treatments.
• Effective seed treatment depends on choice of product, thorough application of fungicide and treatment of planting seed annually or not less than biannually.

Background

Loose smut (Ustilago nuda) was detected in a number of barley crops in the northern region in 2016.  

Low levels of infection were reported in several varieties with crops of the Hindmarsh lineage e.g. Hindmarsh, La Trobe and Rosalind, often infected.

Loose smut is most conspicuous at around flowering when infected heads bearing a mass of dark brown to black sooty spores are visible among the green heads of unaffected plants. This stark contrast in colour can lead to exaggerated estimates of infected heads. Losses in yield equate to the percentage of infected heads and the detection of greater than 0.1g of loose smut particles in a half litre harvest sample will result in rejection of grain deliveries.

Several fungicides are registered for the control of loose smut; but the levels of control vary among products.

Smuts of barley

In Australia, barley is host to two species of smut – loose smut and covered smut (Ustilago hordei) - where infection results in florets producing thousands of spores in individual florets instead of grain. In both species the resultant spore masses are encased in a membrane. This membrane is quite fragile in loose smut but much more persistent in covered smut.

In plants infected with loose smut, the membrane ruptures soon after heading, releasing spores which are carried on the wind to infect surrounding florets. Infection occurs under moist conditions at temperatures around 16 – 22°C. Florets are susceptible to infection from flowering to about one week after pollination.

Germinating spores infect the ovary and the fungus then survives as mycelium within the embryo of the infected seed. It can persist for extended periods in this state. Once infected seed is sown, it germinates and carries the fungus in the growing point of the plant until it manifests as the symptomatic black spore masses at head emergence. Loose smut is well adapted for survival in that infected plants are usually slightly earlier than healthy plants, ensuring an adequate supply of inoculum when the bulk of the crop is flowering.

Loose smut is exclusively internally seed-borne while covered smut is either externally seed-borne or survives in the soil. The life cycle of loose smut in barley is the same as in wheat; however barley loose smut will not infect wheat and vice versa. See Figure 1.

Figure 1. Life cycle of loose smut of barley (and wheat). (Image Courtesy of CropPro).

Control

The level of loose smut in a crop is a function of

• Varietal susceptibility
• The number of grains infected in the previous seed crop
• The efficacy and rate of the seed treatment applied and
• The precision of the seed treatment process

Resistance to loose smut is available; but has never been viewed as a priority objective of Australian barley breeding programs. Seed treatment has provided economical control of the disease for the past 50 years and is likely to continue to do so. As the loose smut fungus is internally seed-borne, systemic fungicides are necessary for control. Products containing carboxin, difenoconazole, flutriafol, fluxapyroxad, ipconazole, penflufen, tebuconazole and triticonazole are registered for the control of loose smut; but their efficacies against the disease vary.

A selection of smuticides was evaluated by the Department of Agriculture Western Australia and Bayer Australia in 2013 and demonstrated superior control by EverGol®Prime (penflufen), Jockey® Stayer® (fluquinconazole) + Raxil®T (tebuconazole) and Vibrance® (difenoconazole+metalaxyl+sedaxane). Their results are summarised in Table 1.

Table 1. Loose smut control from a range of fungicides

Source: Hills (2015) Controlling loose smut in 2016.

The precision of seed treatment can also impact on fungicide performance. Application machinery must be well calibrated, fungicide suspensions continuously agitated and seed mixed thoroughly to ensure even distribution of chemical on the seed.

Conclusion

The reappearance of loose smut in barley in 2016 is a reminder to maintain effective fungicide treatment of planting seed. If seed is sourced from a crop known to have been infected with loose smut, it would be wise to treat seed at the higher recommended rate. The increased incidence of loose smut may be due to good infection conditions during flowering in the parent seed crop; it may be due to application of lower rates of fungicide or it may simply be that new varieties are more susceptible. Whatever the cause, vigilance in routine, quality seed treatment should continue to provide effective control of the disease into the future.

References

Hills, Andrea (2015) Controlling loose smut in 2016. 

Wallwork, Hugh (2000) Cereal leaf and stem diseases pp70 -73.

Wallwork, Hugh (2015) Cereal seed treatments 2016.

Mathre DE (1997) Compendium of barley diseases pp 45-47. American Phytopathological Society, St. Paul, Minnesota.

Acknowledgements

Results quoted in this paper were made possible by the support of the GRDC. The authors thank them for their support. The authors wish to thank Dr Steve Simpfendorfer NSW DPI and Mr Trevor Klein, Syngenta Australia for their input.   

Contact details

Greg Platz
Department of Agriculure and Fisheries
Warwick, QLD
Ph: 0408 733 055
Email:  Greg.Platz@daf.qld.gov.au

Lisle Snyman
Department of Agriculure and Fisheries
Warwick, QLD
Ph: 0428 324 932
Email:  Lisle Snyman@daf.qld.gov.au

Ryan Fowler
Dept. of Agriculture and Fisheries
Warwick, QLD
Ph: 0433 406 669
Email:  Ryan.Fowler@daf.qld.gov.au

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