Barley leaf rust 2016 – what lies ahead?

Take home message

Resistant varieties are the most practical way of controlling barley leaf rust.  Yields of S and VS varieties are impacted most by the disease and increase the amount of inoculum exerting pressure on available resistance genes. If using S and VS varieties, be pro-active in terms of removing the green bridge, monitor crops regularly and apply fungicides early.

Introduction

Barley leaf rust is caused by the obligate parasite (Puccinia hordei) and spread by means of airborne spores that can travel long distances.  The pathogen has the ability to spread rapidly when conditions are favourable and large areas planted to susceptible varieties create the perfect scenario for epidemic development.  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 pathotypes with increased virulence.

Leaf rust is considered one of the five major barley diseases in Australia and can cause significant yield loss up to 60% ($25/ha) and a reduction in grain quality (Murray & Brennon, 2009).  Despite varying greatly between production areas, it is widely distributed and occurs regularly in some regions.

What have we learned so far?

In 2010 a barley leaf rust epidemic occurred in Queensland with major contributing factors being susceptible varieties sown over large areas, producing ample inoculum combined with favourable conditions for disease development.  Favourable conditions during the previous summer provided inoculum for early sown crops.  Foliar fungicide applications provided mixed results that could be contributed to many factors, including high inoculum pressure and late application of fungicides.

The identification of new pathotypes virulent to the single, major Rph3 gene has rendered some varieties susceptible, including Bass and Compass. This virulence has been identified in all major production areas.

In looking at the resistance levels of barley varieties to leaf rust it is evident that over 50% of current commercial varieties carry no or very little resistance to leaf rust.  The distribution of leaf rust resistance levels in northern region barley varieties is indicated in Figure 1.

Figure 1. The distribution of resistance levels to leaf rust in barley varieties in the northern region

Figure 1. The distribution of resistance levels to leaf rust in barley varieties in the northern region

Currently large areas are sown to varieties in the S or VS categories.  The area sown to Compass (VS) is expected to increase Australia wide in the 2016 season, increasing the risk of epidemics caused by high inoculum levels and will put additional pressure on the currently effective resistance genes available.

Some varieties carry adult plant resistance (APR) genes such as Rph20 and Rph23. Unless combined with major resistance genes, varieties carrying APR can be quite susceptible during tillering and may require fungicide protection during early growth stages. Regular monitoring of crops can reveal the need for early intervention with fungicides, particularly in susceptible varieties.

Yield loss response trials

A significant amount of research has been done in an effort to quantify losses caused by barley leaf rust.  A pilot study by DAF QLD in 2013 identified the need for fungicide application on susceptible varieties.

The study included the barley varieties Shepherd (MR), Flagship (MS), Commander (MSS) and Grout (VS).  A single fungicide application on Shepherd (GS72-74) increased yield by 314 kg/ha compared to the untreated plot.  There was however no significant difference between a single spray and two sprays.  Two sprays in both Flagship and Grout resulted in a significantly higher yield than a single spray and both single and double sprays had an increased yield advantage over the nil treatment.  In Commander a single and double spray resulted in similar yields, which were significantly better than the nil treatment.  In addition, both spray treatments resulted in test weights acceptable for malt quality, in comparison to the seed treatment and nil application where test weights were below 65 kg/hl.  The seed treatment did not seem to have any advantage over the nil treatment in any of the varieties.  Results (Table 1) indicated that the more susceptible a variety, the more benefit can be gained by foliar fungicide application.

Table 1. Yield advantage in barley varieties of a single spray compared to nil fungicide treatment

Variety

1 spray (kg/ha)

Nil treatment (kg/ha)

Yield advantage (kg/ha)

Shepherd

4566.80

4252.63

314.17

Flagship

4166.92

3790.20

376.72

Commander

4606.95

3851.45

755.49

Grout

3947.08

3059.68

887.40

Following the 2013 trials, more detailed trials were performed in 2014 and 2015.  Yield and quality loss were calculated in 6 varieties, ranging from MR to VS.  Plots were inoculated with leaf rust to represent infection levels ranging from nil disease (fungicide treated) to high.

Yield responses (Figure 2) in all varieties, except Shepherd (MR), indicated a significant yield loss for the high disease plot compared to nil disease.  All treatments had significantly higher yields than the high disease treatments in the susceptible varieties Grout and Compass, with the biggest yield loss observed in Compass.  Yield loss in the susceptible variety Scope was lower than expected.  Test weights and protein for all treatments in La Trobe were acceptable for malt quality, whereas none of the Scope treatments, including the nil disease, had test weights or protein acceptable for malt quality.  Only the Nil treatment of Compass (not yet malt accredited) was above 65 kg/hl.

Figure 2. Yield response of barley varieties at different disease levels in 2014

Figure 2. Yield response of barley varieties at different disease levels in 2014

Results from 2014 led to variety changes in 2015 with Mackay replaced by malt variety Commander and Fathom included as a MS type. Compass replaced Grout as a VS type.

Results from 2015 confirmed observations from 2014.  In all varieties (Figure 3) the nil disease treatment had significantly higher yield than the high disease treatment and in all varieties, except Shepherd and Commander, all treatments were significantly better than the high disease treatment.  Similar to 2014, the biggest yield loss was observed in Compass.

Figure 3. Yield response of barley varieties at different disease levels in 2015

Figure 3. Yield response of barley varieties at different disease levels in 2015

Conclusions

Trials conducted in 2014 and 2015 confirmed observations made in 2013 that the more susceptible a variety, the bigger the yield and quality losses due to leaf rust.  Using resistant varieties is the most practical means of disease control.  In avoiding S and VS varieties, the amount of inoculum can be reduced.

According to the Bureau of Meteorology above average rainfall is expected for most of southern Queensland and parts of northern NSW for the first three months of 2016, providing ideal conditions for the over summering of rust on volunteers.  Leaf rust was observed in the 2015 season with some crops requiring fungicide spraying.  Regular crop monitoring, particularly in susceptible varieties is essential for timely intervention with fungicide.  Keep in mind that rust diseases are difficult to control once established.  In some instances more than one fungicide application may be needed for rust control.

Even though barley leaf rust can cause significant yield and quality losses, data indicated that by being pro-active it is possible to manage the disease.  We however need to be able to adapt to ever changing pathogens and climatic conditions.

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 author would like to thank Greg Platz for guidance and the technical - and farm staff at Hermitage Research Facility for their assistance.

Contact details

Lislé Snyman
DAF QLD
Hermitage Research Facility, 604 Yangan Rd, Warwick, Qld
Ph: 07 4660-3661
Fx: 07 4660-3600
Email: lisle.snyman@daf.qld.gov.au

Greg Platz
Ph: 07 4660-3633
Email: greg.platz@daf.qld.gov.au

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

GRDC Project code: DAW00245