Wheat rusts, septoria and Tasmanian varieties - latest findings

Take home messages

• Few well adapted wheat varieties are currently available to Tasmanian growers that meet the minimum disease standard for leaf rust.
• To manage cereal diseases Tasmanian growers need to either accept lower yield potential with resistant wheat varieties or look to incorporate triticale into their rotations.
• Annual surveys have revealed that the Tasmanian septoria tritici blotch (STB) population has become dominated by isoforms (pathogens) with a high level of triazole resistance.
• To limit further fungicide resistance developing, fungicide resistance management practices must be implemented by growers.
• Integrated disease management complements fungicide resistance management.
• To slow fungicide resistance developing against the alternative modes of action, such as the succinate dehydrogenase inhibitors (SDHIs) and strobilurins, do not apply these more than once in a season.

Background

Wheat leaf rust

Wheat leaf rust is a disease caused by the pathogen Puccinia triticina and is found in all wheat growing regions of the world. Wheat leaf rust forms small, circular, orange-brown pustules that are raised above the leaf surface and are mainly found on the upper leaf surface and less frequently on leaf sheathes. Pustules become obvious on the leaf surface 7 to 10 days after infection and the disease spreads by wind-blown spores that can spread hundreds of kilometres. Like all rusts, Puccinia triticina requires a living plant host to grow and reproduce, with infection favoured by humid conditions. It prefers warm temperatures, ideally 15 to 20 ^oC. In worst-case scenarios, wheat leaf rust can cause up to 50% losses in grain yield in very susceptible varieties. Unlike many wheat diseases, leaf rust is easily controlled with either resistant varieties or fungicides.

Annual rust pathotype surveys have been conducted at the University of Sydney since the 1920s. Since 2000, 32 pathotypes of wheat leaf rust have been detected in Australia. New pathotypes have occurred as either new incursions from overseas, or as new step-wise mutants in existing pathotypes. The detection of wheat leaf rust pathotype 104-1,3,4,6,7,8,10,12 +Lr37 in South Australia in 2014 was the first occurrence of combined virulence for Lr27+Lr31 and Lr28. The distinctiveness of this pathotype clearly indicates that it was of exotic origin. It led to a downgrading of the resistance of the important wheat variety SQP Revenue, which was an important variety for the high rainfall growing regions of Australia. In the 2016 consensus ratings, the wheat leaf rust resistance ratings for SQP Revenue and Manning were very susceptible (VS) and moderately susceptible (MS), respectively. As a result, of these two varieties, Manning is the only one to meet the minimum disease resistance standards for wheat leaf rust.

Septoria tritici blotch

Septoria tritici blotch (STB) is a disease of wheat caused by the pathogen Zymospetoria tritici and is of global importance to wheat growers. This pathogen causes significant yield losses and is difficult to manage because it has developed resistance to fungicides in most major wheat production areas around the world. In Australia, STB resistance to triazole fungicides was first detected in samples collected in Victoria and Tasmania during the 2011/12 winter crop season. Our ongoing monitoring of this fungicide resistance is detecting more changes in the STB population and we are researching ways to manage this evolving fungicide resistance problem.

The STB symptoms observed by growers are necrotic leaf lesions with obvious small black fruiting structures on the surface called pycnidia. By the time these symptoms are visible on the leaf the infection has been developing unseen for a long time, typically 14 to 28 days. The initial infections within a paddock come from spores produced on old diseased wheat stubble. This is how the pathogen survives between seasons, on the stubble from the previous year. After the infected stubble has undergone a period of weathering in the field the pathogen grows fruiting structures that release spores that infect the young wheat crops.

Within the pathogen’s lifecycle, STB relies on spending several months sheltered within the wheat stubble. So as farming systems have progressed and adopted more and more stubble retention, STB has benefitted. In addition to creating an ideal environment for the survival of the pathogen between seasons we also give the pathogen a free go by choosing to sow a small number of wheat varieties across the farming landscape. This has the effect of allowing the pathogen to select the most adapted strains on those varieties eventually leading to resistant varieties giving the appearance of becoming susceptible overtime.

The large STB population that has built up in the high rainfall regions in recent years has required the use of fungicides to control disease levels and prevent yield loss. The use of these fungicides has had a direct impact on the pathogen by selecting strains that have lower sensitivity to triazole fungicides. Our research is focussed on identifying the different forms of the known genes involved in fungicide resistance present in the STB population.

Method

Wheat leaf rust

A survey of wheat growing regions around Launceston was conducted and wheat leaf rust when it was detected was collected. Samples were also submitted by staff from Peracto to the Australian Cereal Rust Survey. Pathotypes were identified at the Plant Breeding Institute, Cobbitty at the University of Sydney. The identification of pathotypes involves infecting seedlings of a set of cereal varieties, each carrying a different known rust resistance gene, with a field collected sample of rust. The ability or inability of the rust isolate to infect each variety allows the pathotype or pathotypes present to be identified.

Septoria tritici blotch

Surveys were conducted across the winter cereal high rainfall regions of Australia and collections were made where STB occurred. Two types of analysis were conducted on the isolated strains of STB. A molecular genetic approach where the cyp51 gene is sequenced to look for changes in the DNA code. Changes in the sequence of this gene can lead to altered folding of the protein that is targeted by triazole fungicides which in turn leads to loss of efficacy. The identified changes have then been compared to those associated with fungicide resistance in other parts of the world. In addition a wet chemistry approach has also been used to determine the level of resistance in Australian strains of STB. This is where the strains are exposed to a range of fungicides and concentrations then their growth is measured to determine the effective concentration where 50% of growth is inhibited (EC50).

Results and discussion

Wheat leaf rust pathotype results

A summary of the results from Tasmania from 2016 are shown in Table 1. From 12 samples submitted to the Australian Cereal Rust Survey, nine yielded a viable sample. The occurrence of pathotype mixtures in wheat leaf rust samples is common, and in this case mixtures of two pathotypes of wheat leaf rust were detected in seven of the samples.

The varieties Manning and SQP Revenue were both found to be infected by pathotype 104-1,3,4,6,7,8,10,12+Lr37 and pathotype 104-1,3,4,5,7,9,10,12+Lr37. Pathotype 104-1,3,4,6,7,8,10,12+Lr37 was first detected in South Australia in 2014 and had spread to all wheat growing regions of Australia within 12 months. Pathotype 104-1,3,4,5,7,9,10,12 +Lr37 was first detected in late June 2016 from a sample collected at Port Neill, South Australia off a crop of Mace. It was subsequently identified from all eastern states and increased rapidly in frequency to be the second most commonly isolated leaf rust pathotype from eastern Australia (48 isolates). While this pathotype appears to be a derivative of the Race 76 lineage which was first introduced to Australia in 2002, it shares some features with pathotype 104-1,3,4,6,7,8,10,12+Lr37. While the full impact of this pathotype on commercial wheat varieties is not yet completely understood, it does not appear to carry any new virulence that would be expected to change their leaf rust responses.

Septoria tritici blotch

Triazoles

We have identified 11 changes in the cyp51 gene that result in changes to the protein it encodes. These result in 10 altered isoforms (different types) of the protein which interact with the triazole fungicides. All of these changes to the gene sequence have previously been identified in other regions of the world. However a number of the isoforms have not been previously described. Since our recent collections started in 2011, we have not identified any of what is considered the original sensitive (wildtype) form of the gene present in the Australian population.

The wet chemistry tests support the molecular evidence that these changes do have an impact on the resistance to triazoles. The testing has revealed that there are a number of combinations which have elevated levels of resistance that will be noticeable to growers. The results are also demonstrating that if an isoform has high resistance to one triazole it will be high for all other currently available triazoles. However this does not mean that all triazoles will be ineffective in the field, this will be further discussed in the sections of the paper to follow.

Regional patterns have emerged in the incidence of isoforms and the level of triazole resistance they convey. Tasmania has the highest levels of resistance when compared to Victoria and South Australia. This is due to the detection of a number of isoforms which are not prevalent on the mainland at this time. Tasmania has the greatest diversity of isoforms and is showing signs of more intensive selection for ones with higher levels of resistance.

The 2016 STB survey results show that Cyp51 isoform 11 which has a high level of triazole insensitivity is now the dominant form in Tasmania. This means growers need to choose triazole fungicides carefully for disease control. Product formulations relying only on flutriafol, tebuconazole, propiconazole and cyproconazole will not provide full protection against STB Cyp51 isoform 11.

Strobilurins

No changes have been identified to the resistance status to strobilurins in Australia. This mode of action is still effective and can be part of a fungicide management strategy for STB. It is worth noting that Australia is one of the few places in the world where strobilurins are still effective. Therefore use of the strobilurin class of fungicides should be carefully managed to maintain its effectiveness on STB. The strobilurins are at high risk of selecting for STB resistance when used repeatedly within a growing season.

Succinate DeHydrogenase Inhibitors (SDHIs)

No changes have been identified to the resistance status of SDHI (Succinate DeHydrogenase Inhibitors) in Australia. With SDHI products becoming available in Australia it is worth noting that STB has developed resistance to this mode of action in the UK^ɸ. This mode of action requires proactive management in order to sustain its useful life. Avoid applying an SDHI more than once in a season.

^ɸCurrently no SHDI products registered for control of STB in Australia.

Fungicide resistance and disease management for Septoria tritici blotch

To minimise fungicide resistance development and disease build-up there are three important steps growers need to implement:

1. Stubble removal.
   1. Stubble is the source of infection each year. By removing stubble before sowing there is a substantial reduction of pathogen population size.
   2. This reduces all isoforms irrespective of resistance and reduces the initial establishment of disease.
   3. But to be effective the removal must reduce infected stubble to very low levels, ideally below 100kg/ha infected stubble remaining within a paddock.
   4. Do not sow wheat on wheat.
   5. Variety choice.
      1. Under high disease pressure a variety rated moderately resistant – moderately susceptible (MRMS) can reduce the leaf area loss by as much as 60% compared to a susceptible – very susceptible variety (SVS).
      2. Host resistance reduces all isoforms irrespective of resistance and reduces the need for multiple canopy fungicide applications.
      3. But resistance ratings do change so crops must still be monitored in season for higher than expected reactions and check annually for updates to disease ratings.
2. Fungicide choice and use.
   1. Do not use the same triazole active ingredient more than once in a season. Do not use a strobilurin or SDHI more than once in a season^ɸ.
      ^ɸ As per registered product labels; no more than two applications per season.
   2. Aim for early control of disease. STB spreads up the leaf layers of the canopy through rain splash and direct leaf contact. Reducing the disease in the lower canopy slows the upward movement of disease and ultimately the leaf area lost.
   3. Follow registered product label instructions at all times.

The suite of fungicide products available on the market specifically registered for the control of STB in the canopy or as up front treatments are limited. At the time of writing this paper the modes of action that are registered are confined to the triazoles (epoxiconazole, tebuconazole, flutriafol, fluquinconazole, propiconazole, cyproconazole and triadimefon) and the strobilurins (azoxystrobin) and some are only available in combinations. With the STB Cyp51 isofrom 11 dominant in Tasmania there are now two groupings within the triazoles available for STB control. Based on our testing, the first group will appear less effective in Tasmania and they are; triadimefon, tebuconazole, propiconazole, flutriafol and cyproconazole. While the second group epoxiconazole and fluquinconazole have shown smaller shifts in resistance and will still control disease. Timing of application in the disease epidemic is critical to getting the most out of these products.

Triticale as an alternative crop

Tasmanian growers are facing tough decisions for disease management in wheat. Some triticale varieties could provide a useful addition to the cereal rotation in Tasmania. Triticale has better resistance to both wheat leaf rust and STB than the wheat varieties currently available to Tasmanian growers.

Conclusion

The Tasmania STB population has continued to shift towards higher levels of triazole fungicide resistance. Understanding the lifecycle of STB presents opportunities for growers to be proactive about fungicide resistance and disease control. STB is a pathogen that survives from one crop to the next on the stubble left after harvest. During this period is an ideal time to take action and reduce the overall disease burden.

The incursion of the new wheat leaf rust pathotype into South Australia in 2014 led to the downgrading of resistance of the variety SQP Revenue. When combined with higher levels of triazole fungicide resistance in the STB population, Tasmanian growers need to make some difficult decisions about managing two important diseases. The introduction of stripe rust resistant triticale varieties into the farming system and managing the summer survival of both diseases could lead to a total reduction in the severity of disease epidemics for all Tasmanian growers.

Submitting samples to fungicide resistance screening;

The more samples of STB received the better informed growers are about the status of fungicide resistance.

To collect and submit samples please follow these instructions:

1. Sample collection: collect up to 20 leaves in total by walking in two parallel transects 10m apart collecting leaves which have STB lesions every 10m.
2. Place the leaves in a paper bag and label with the following: name, contact details, any fungicides applied to the crop and location
3. Then post to
   
   Dr. Andrew Milgate
   
   WWAI, Pine Gully Rd, Wagga Wagga, NSW, 2650

Submitting samples to the Australian Cereal Rust Survey;

To collect and submit samples please follow these instructions:

1. Sample collection: collect at least 10cm² of rust infected leaves.
2. Place the leaves in a paper bag and label with the following: name, contact details, variety and location.
3. Then post to
   
   University of Sydney
   Australian Rust Survey
   Reply Paid 88076
   
   Narellan NSW 2567

Alternatively, contact Will Cuddy for reply paid envelopes to be sent to you.

Acknowledgements

The authors of this paper would like to acknowledge members of the cereal pathology team at WWAI who contributed to the fungicide resistance work namely Melanie Renkin, Merrin Spackman and Beverly Orchard. The agronomists and growers across the high rainfall zone who have submitted samples to the STB survey. Keshab Kandel, Margerita Pietilainen and Ashlea Grewar from the University of Sydney who have contributed to the wheat leaf rust pathotyping work. Rust samples submitted by Peracto were appreciated. The assistance of Terry Horan in conducting the 2016 survey is also greatly appreciated.

Useful resources

GRDC Septoria tritici blotch Fact Sheet

Minimising fungicide resistance in septoria tritici

Fungicide resistance talks at AARM 2015 – Session 1

Septoria tritici blotch experiments – southern NSW 2015 pp 148-150. In Southern NSW research results 2015.

The University of Sydney Institute of Agriculture - Plant breeding institute, cereal rust - reports forms

The Rust Bust Website

Wheat Rust Northern, Southern and Western Regions

Contact details

Will Cuddy
NSW Department of Primary Industries
Elizabeth Macarthur Agricultural Institute, Woodbridge Road, Menangle NSW 2568
02 4640 6515
will.cuddy@dpi.nsw.gov.au

Robert Park
Plant Breeding Institute, University of Sydney
107 Cobbitty Rd, Cobbitty, NSW, 2570
02 9351 8806

Andrew Milgate
WWAI, Pine Gully Rd, Wagga Wagga, NSW, 2650
02 6938 1990
andrew.milgate@dpi.nsw.gov.au