A new approach to managing loose smut on barley

Key messages

• Loose smut is a seed-borne disease in barley, generally controlled using seed dressings that prevent infected seed from expressing the disease in-crop.
• This study demonstrates that another effective way to reduce loose smut in subsequent seasons is to apply foliar fungicides from ear emergence to anthesis to prevent seed infection occurring. This approach can significantly reduce seed infection rates compared to unprotected crops.
• An added advantage is that these fungicides are registered in barley and used for management of other fungal diseases. Therefore, this approach can be integrated into current disease management practices or used specifically in seed production crops to maintain seed of the highest quality.

Aims

This study investigated the possible use of a foliar fungicide to control loose smut in barley.

Introduction

Loose smut in barley is caused by the fungus Ustilago nuda (Jens.) Rostra. The disease can cause significant yield loss in susceptible varieties.

Understanding pathogen behaviour is important to successful disease management. The infected plants produce smutted heads consisting of several million spores encased in a fragile membrane.  At this stage, loose smut symptoms are conspicuous. The spores are released from the fragile membrane and are dispersed by wind, landing on the crop and infecting flowering heads. The spores germinate in the florets; the mycelia spreads to the embryo and surrounding tissue and survives dormant until the seed is sown. This process is repeated in the following season. Infected seeds are asymptomatic although diagnostic laboratory testing (ISTA 7-013a, 2014) can detect the disease.

The disease is presently managed by: (1) avoiding the use of susceptible barley varieties that belong to the Dash/Hindmarsh family, (2) use of healthy seed and (3) using broad spectrum systemic seed dressing fungicides. In Australia, seed dressing fungicides have been widely adopted in control loose smut but these vary in their level of control (Hills et al. 2014; 2019). Evidence suggests that foliar application of triazole-based fungicides can reduce loose smut infection in the new crop (Jones 1997).

Method

Field experiments 2017 to 2018

This study examined the application of foliar fungicides to barley plants to reduce loose smut infection of grain produced from an infected paddock. Certified (<0.05% loose smut) Hindmarsh seed (without seed dressing) was grown during 2017 and 2018 at Kojaneerup South (-34.456309, 118.432918). Seed was sown in one-metre rows 50cm apart using an Earthway1001-B Precision Garden Seeder. Trial design was replicated in triplicate using a randomised block design. To facilitate high loose-smut spore infection levels in the trial area, heavily infected Hindmarsh seed produced at Esperance Downs Research Station (EDRS) was sown around the trial two weeks before row plots were established.

The choice of fungicide for the experiment took into consideration that the triazole product is registered in Western Australia for loose smut control as a seed dressing and as a foliar fungicide for the control of other barley diseases. Therefore, tebuconazole (e.g, Orius^® 430SC) was selected for the field experiments. The foliar fungicide used, rates and spray timings are shown in Table 1 and Table 2.

In 2017 and 2018, micro plots were hand harvested and seed tested to determine the loose smut infection levels of embryos according to the International Seed Testing Association, seed health testing protocol (ISTA 2014).

GenStat 16th Edition (2013 Lawes Agricultural Trust, VSN International Ltd) was used to analyse data with a one-way analysis of variance (ANOVA).

Spore trapping

In 2017 and 2018, airborne spores were monitored by placing a passive, wind-vane spore trap adjacent to the trial site.  A gelatine coated slide was placed in the spore trap and replaced weekly and then examined for the presence of smut spores under a compound light microscope in the laboratory.

Results

Spore trapping

In 2017, smut spores started to appear in the lower great southern (LGS) in late August and at the trial site around mid-September. In 2018, initial detection in the region was in early September and at the trial site in mid-September. These observations were used as a guideline along with plant growth stages (ear emergence) to initiate foliar fungicide spraying.

Field experiments from 2017 to 2018

The results of the trials are shown in Tables 1 and 2. Foliar fungicide sprays commenced with the detection of loose smut spores in the spore traps. Unprotected Hindmarsh had 4.3% and 0.2% embryo infection in 2017 and 2018, respectively. Application of foliar fungicide in 2017 and 2018 between Zadoks growth stages Z53, Z59 and Z65/Z69 significantly reduced loose smut embryo infections compared to unprotected plots (Tables 1 and 2).

Table 1. Effects of foliar fungicide spraying to prevent loose smut infection on Hindmarsh barley at Kojaneerup South, 2017.

*Folicur 430SC @ 290mL/ha at Z59 (13 Oct)), Hindmarsh was sown on 15 June.
**Values with same letter in each column not significant at p = 0.05 level.

In 2017, a single spray at Z59significantly reduced embryo infection compared to unprotected plots (Table 1). In 2018, there was no significant difference between single application of systemic foliar fungicide at Z53 or Z65/69 in reducing embryo infection (Table 2). All application treatments significantly reduced the level of loose smut infection in seed.

Table 2. Effects of foliar fungicide spraying to prevent loose smut infection on Hindmarsh barley at Kojaneerup South, 2018.

*Folicur 430SC @ 290mL/ha spray at Z53 (19 Sept) or Z65/Z69 (26 Sept), Hindmarsh was sown on 12 June. **Values with same letter in each column not significant at p = 0.05 level.

Conclusion

Applications of systemic foliar fungicide containing the active ingredient tebuconazole at ear emergence to late flowering can reduce loose smut infection in subsequent forming seed. While spore trapping in the region can provide information on when the loose smut spores are in the air, foliar fungicide spraying should be based on when the crop is most vulnerable (ear emergence). Since foliar fungicide can reduce loose smut levels in the following barley crop this application method could be integrated into the loose smut management plans for barley. When using fungicide at late foliar stages, the minimum withholding period of the product should be considered to avoid residue violations in delivered grain.

It is also important to be mindful of fungicide resistance issues in certain foliar fungal pathogens on barley. If the fungicide applied to the crop before flowering contains different fungicide groups, such as an SDHI or QoI, then the use of tebuconazole (DMI) as foliar spray is less likely to select for fungicide resistance in other diseases.

Acknowledgments

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

We are grateful to Mark Slattery (from the Stirling to Coast Farmers group) for allowing us to conduct the research at Bundeera Farm at Kojaneerup South for two years. Folicur 430SC was provided by Bayer Crop Science Australia.

References

Hills A, Thomas G and Horbury R (2014). Seed dressings to control loose smut in Hindmarsh barley GRDC Crop Research Update 24-25 February, Crown Perth, Perth.

Hills A, G Thomas and K Jayasena (2019) Barley loose smut – control, variety susceptibility and effects on grain yield. GRDC Crop Research Update 25-26 February, Crown Perth, Perth.

International Rules for Seed Testing (2014) Annexe to Chapter 7: Seed Health Testing Methods 7-013a: Detection of Ustilago nuda on Hordeum vulgare (Barley).

Jones P (1997) Control of loose smut (Ustilago nuda and U. tritici) infections in barley and wheat plants by foliar application of triadimefon Plant Pathol. 1997, 46:946-951.

Zadoks JC, Chang TT and Konzak CF (1974). A decimal code for the growth stages of cereals. Weed Research. 14:415-421.

Reviewed by: Andrea Hills, DPIRD

Contact details

Contact

Dr. Kithsiri Jayasena,  DPIRD
444 Albany Hwy, Albany, WA 6330
(08) 9892 8477
kithsiri.jayasena@agric.wa.gov.au