Leaf rust in Compass

Take home messages

• Compass is very susceptible to leaf rust and crops should be closely monitored for the disease
• Barley varieties differ in yield response to leaf rust across resistance ratings
• Resistance ratings of current commercial varieties are available on the NVT website
• Growing resistant varieties is the most practical and economical way of controlling barley leaf rust
• Varieties characterised as S to VS are impacted most by disease and also contribute to inoculum increase, leading to pathogen mutations putting available resistance genes at risk
• The use of resistant varieties forms part of a bigger disease management plan, which also includes green-bridge control, regular crop monitoring and the timely application of fungicides.

Introduction

Leaf rust of barley is widely distributed and occurs regularly in the northern region.  It is considered one of the five major barley diseases in Australia and can cause significant yield loss and a reduction in grain quality (Murray & Brennan, 2009). Barley leaf rust was widespread in Queensland in 2016 on Compass and other vulnerable varieties.  In most instances, timely fungicide sprays were able to avoid epidemic infection levels.

The disease is caused by the obligate parasite (Puccinia hordei), spreading by means of airborne spores that have the ability to travel long distances.  The pathogen spreads rapidly when conditions are favourable and large areas are planted to susceptible varieties, creating favourable conditions for epidemics to develop.  In the presence of a green bridge, the pathogen can survive over summer and be present at high levels early in the growing season.  High inoculum levels put pressure on major resistance genes and can lead to the development of new, more virulent pathotypes.

Why the concern about Compass?

Compass is a high-yielding, broadly adapted, mid-season maturing variety expected to complete Barley Australia malt accreditation by March 2018. In Queensland, it is rated VS to pathotypes virulent for the Rph3 gene.  This virulence is present in all major production areas.

Compass currently accounts for 15-20% of barley production in Australia. Due to the high yielding ability over a range of environments, the area sown to Compass in 2016 was estimated to be approximately 20 000 ha in WA, 400 000 ha in SA, 120 000 ha in VIC, 60 000 ha in NSW and 20 000ha in QLD, totalling at 620 000 ha of the barley production area (Seednet – pers. comm.).  Widespread cultivation of a variety as susceptible as Compass to barley leaf rust ensures a continuous supply of rust inoculum and contributes to the breakdown of valuable resistance genes.

Some of the major factors contributing to the barley leaf rust epidemic in Queensland in 2010 was the widespread sowings of susceptible varieties, which in turn led to an increase in the inoculum load and conditions favourable for disease development.  High inoculum pressure and application of fungicides after establishment of the disease contributed to mixed results from the application of foliar fungicides.

The area sown to Compass (VS) is expected to increase across production areas in 2017. Large areas sown to a VS variety across a range of environments almost ensures that leaf rust will be a problem in some areas contributing to high inoculum levels causing epidemics whilst adding selection pressure on the pathogen to mutate and acquire new virulences.

Yield response curve trials

Since 2013 trials have been conducted by DAF QLD in an effort to quantify losses caused by barley leaf rust.  A pilot study in 2013, on barley varieties ranging in resistance levels from MR to VS, showed the benefits of fungicide application on susceptible varieties.  Results indicated that the more susceptible a variety, the more benefit can be gained by foliar fungicide application.

Following the 2013 results, more detailed trials were performed annually from 2014. The relationship between yield and disease severity was examined in six varieties, with resistance levels ranging between MR and VS.  Plots were inoculated to represent infection levels ranging from nil disease (fungicide treated) to high disease levels.

After the 2014 trial, Compass (included in S category) was clearly a VS type based on yield loss (38.5%) when compared to Grout with a 25.1% yield loss.  Similarly, the biggest yield loss (28.8%) was observed in Compass in 2015 (Figure 1).

Figure 1. Yield response of barley varieties to different disease levels in 2015.
(Shepherd, Fathom, La Trobe, Commander, Scope and Compass are all protected under the Plant Breeders Rights Act 1994.)

In 2016, yield loss of 45.6% was recorded in Compass (VS) and 57.6% in Bass (SVS) (Figure. 2).  Yield losses in Shepherd(MRMS) were 8.1% and 10.5% for the two years respectively and Fathom (MS) 8.3% and 7.7%.

Figure 2. Yield response of barley varieties to different disease levels in 2016.
(Shepherd, Fathom, La Trobe, Commander, Scope, Bass and Compass are all protected under the Plant Breeders Rights Act 1994.)

Results from these trials confirmed earlier reports that barley leaf rust infection can result in quality losses.  From the retention results displayed in Figure 3 it is obvious that susceptible varieties suffered huge losses in terms of quality.  Retention in all six varieties were significantly better in the nil disease treatment than in all other treatments.  In the varieties Commander, Scope and Compass, the high disease treatment had a significantly lower retention percentage than the other diseased treatments (medium, low and very low).  For the malting variety Commander, retention percentages in all treatments were higher than 58%, whereas none of the treatments in Scope and Bass would be acceptable as malt quality.  For Compass only the nil treatment would qualify for malt quality when accredited.  Similar observations were made with regards to other quality characteristics such as test weight.

Figure 3. Response in retention percentage of barley varieties to different disease levels in 2016.
(Shepherd, Fathom, La Trobe, Commander, Scope, Bass and Compass are all protected under the Plant Breeders Rights Act 1994.)

Conclusions

Results from the yield response curve trials conducted between 2014 and 2016 indicated that disease has a significantly bigger impact on the yield and quality characteristics of varieties in the S to VS categories, than on more resistant varieties.  In all three years, Compass suffered significant losses ranging between 28.8% and 45.6%.  The only variety tested to suffer bigger losses was Bass in 2016.

Therefore it can be concluded that the more susceptible a variety, the bigger the yield and quality losses due to leaf rust.  The area planted to Compass is expected to increase in 2017 and with the increase in area of susceptible, especially very susceptible varieties grown, the risk of epidemics increase, particularly if conditions are favourable for disease development.

The best strategy for barley leaf rust control is to grow resistant varieties and if not, to remain vigilant with crop management decisions.  It is important to ensure that there are no green bridge for the rust pathogen to over summer on.  Regular crop monitoring is essential for timely fungicide control and to decide if follow-up applications are needed.

From these results it can be concluded that growing a susceptible variety such as Compass increase the risk of yield and quality loss and requires dedicated effort towards persistent monitoring and decision making.

Reference

Murray, GM & Brennan GP, 2009.  Estimating disease losses to the Australian barley industry.  Australasian Plant Pathology 39, 85-96.

Acknowledgements

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.

The authors would like to thank the technical and farm staff at Hermitage and Leslie Research Facilities for their assistance.

Contact details

Lislé Snyman
DAF QLD
Hermitage Research Facility, 604 Yangan Rd, Warwick, Qld
Ph: 07 4660 3661
Email: lisle.snyman@daf.qld.gov.au

Greg Platz
DAF QLD
Hermitage Research Facility, 604 Yangan Rd, Warwick, Qld
Ph: 07 4660 3633
Email: greg.platz@daf.qld.gov.au

Clayton Forknall
DAF QLD
13 Holberton St, Toowoomba, Qld
Ph: 07 4529 1218
Email: clayton.forknall@daf.qld.gov.au

​ Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994