Keeping fungicide resistance in check and managing septoria tritici blotch in the low rainfall zone and medium rainfall zone

Take home messages

• Surveys for fungicide resistance are ongoing and agronomists are encouraged to notify SARDI of any suspect cases and submit samples for testing. Updated information is available from SARDI and the AFREN website.
• The efficacy of specific DMI actives where resistance is detected is currently not well understood and is an emerging issue that needs to be monitored.
• 2021 was not a conducive season to septoria tritici blotch (STB) development at the low rainfall zone (LRZ) and medium rainfall zone (MRZ) trial sites. No fungicide sprays were required to prevent yield losses and work in this area is ongoing.

Background

The Australian grains industry now has several cases of fungicide resistance in cereal foliar diseases that are either currently affecting or threatening South Australian growers. Additionally, there are fungicide resistance cases interstate that are likely to show up in South Australia in the near future. Up-to-date advice about the current Australian fungicide resistance situation, as well as management strategies, provides SA growers with the required tools to work with resistance in our systems.

Currently, the main concerns in South Australia are succinate dehydrogenase inhibitor (SDHI) and demethylation inhibitor (DMI) resistance to net form net blotch and strobilurin and DMI resistance in wheat powdery mildew. Strobilurin resistance has now also been detected in wheat septoria tritici blotch (STB) in South Australia, but current prevalence is thought to be localised to the Millicent region in the South East.

Interstate, the other fungicide resistance concerns are SDHI (WA) and DMI (WA and VIC) resistance to spot form net blotch and DMI resistance to barley powdery mildew in WA, QLD, NSW, VIC and TAS.

The Australian Fungicide Resistance Extension Network (AFREN) consists of regional plant pathologists, fungicide resistance experts and communications specialists. This grouping coordinates the provision of up-to-date information and advice as the fungicide resistance situation changes in Australia.

In addition to fungicide resistance tracking and more general management advice, more research is required to understand the specifics of the resistances present. For example, current DMI resistance testing often only uses tebuconazole as an indicator molecule for the DMIs. More thorough testing using the actives used by growers would add significantly greater value. In some instances, further testing is done using plate cultures. Whole plant or in-field testing would further our understanding of resistances and product efficacies relevant to growers. In South Australia for wheat powdery mildew, SAGIT have funded Trengove Consulting to conduct some of this in-field work in the Bute area, where the disease and fungicide resistance are having substantial impacts. Further information on this work will be published in a separate GRDC update paper by the respective authors.

An integral part of managing fungicide resistance is reducing unnecessary fungicide applications and using fungicides in a more targeted approach. Understanding which seasons are conducive to disease development that causes yield loss is an important tool in this decision making. The first year of data on the septoria tritici blotch trials in South Australia have been included in this paper as they demonstrate a non-conducive season for disease development in low and medium rainfall zones.

GRDC have invested in the research on septoria tritici blotch of wheat in the low and medium rainfall zones in the Southern Region (GRDC project: DJP2104-004TRX). Agriculture Victoria and SARDI are working together with input from FAR Australia and NSWDPI to conduct this work. The research aims to better understand the disease outside of the high rainfall zone (HRZ) to enable smarter integrated disease management strategies and to lower unnecessary chemical inputs.

The integrated disease management (IDM) work in STB includes spore trapping and stubble monitoring to better understand the epidemiology of the pathogen. This monitoring requires multiple seasons of data before results can be meaningful. These data will therefore be presented in future years. Plot trials have focused on better understanding the interaction between variety disease resistance rating and yield loss, as well as optimal fungicide timing. These trials are targeted at medium and low rainfall zones.

Methods

Fungicide resistance prevalence

Data contributing to our understanding of the prevalence of fungicide resistance is obtained from the collection of diseased leaf samples sent to the Centre for Crop and Disease Management (CCDM) Fungicide Resistance laboratory for analysis. Samples are collected by growers, agronomists, and pathologists either when fungicide resistance is suspected in a paddock, or through targeted surveys. CCDM conduct the analysis for fungicide resistance by both molecular and culture phenotype methods. Sample results are reported as either lab detection (detected in lab but no evidence of field failure), reduced sensitivity (for DMIs and SDHIs only) or resistance (indicating complete fungicide resistance).

Barley net form net blotch underwent targeted surveys during 2019 when SDHI resistance was first detected on the Yorke Peninsula and samples from other regions have been sent in subsequent seasons.

Wheat powdery mildew has undergone targeted surveys through Trengove Consulting’s SAGIT project in 2020 and 2021 on the northern Yorke Peninsula.

In 2021, CCDM confirmed the detection of strobilurin resistance to wheat septoria tritici blotch in the Millicent area and put an open call out for STB samples in the 2021 season.

Septoria tritici blotch IDM

Results in this report will focus on the yield loss by variety trials. These trials will be run for the three years of the project, but only the first year of trials have been completed so far.

Six varieties were selected based on their disease resistance ratings to STB. Ratings for stripe rust and powdery mildew were taken into consideration as well. Varieties and STB resistance ratings are listed in Table 1.

Table 1: Varieties and STB disease resistance ratings used in 2021 yield loss by variety trials.

Trials were designed by Statistics for the Australian Grains Industry (SAGI) South and included disease-inoculated plots and disease-controlled plots to develop plus and minus disease for each variety. Trials included six replications and were blocked by disease treatment, plots were 10m x 1.5m. In South Australia, trials were located at Hart Field Site and Booleroo Centre.

Plus-disease plots were inoculated at seedling and mid tiller stages using a conidial suspension in water applied as a spray. Fungicides were applied to minus-disease plots at GS 31 and 39. The GS 31 spray consisted of Elatus Ace (250 gai/L propiconazole + 40 gai/L benzovindiflupyr) @ 500mL/ha and GS 39 epoxiconazole (500 gai/L) @125mL/ha. Disease assessments were conducted at flowering time by assessing percentage of disease severity on each leaf of 10 plants/plot. Trials were harvested and yield values calculated based on harvest weight/plot. Preliminary single site statistical analysis was conducted with Genstat 20^th Edition.

Results and discussion

Fungicide resistance prevalence

Fungicide resistance is now prevalent across all grain growing states in Australia as can be seen in Figure 1. This figure demonstrates which states have recorded fungicide resistance and the associated crop, disease and fungicide group. The legend also indicates if the detection is lab, reduced sensitivity or complete resistance. In cereal crops, South Australia has now detected fungicide resistance to barley net form of net blotch in SDHIs and DMIs, wheat powdery mildew in strobilurins and DMIs and wheat septoria tritici blotch in SDHIs and DMIs.

Figure 1. Prevalence of fungicide resistance across Australia in grain crops. Figure produced by AFREN.

Barley net form net blotch (NFNB)

In 2019, SDHI fungicide resistance to barley net form net blotch was detected on the southern Yorke Peninsula in the Minlaton area. Targeted surveying by CCDM and SARDI determined the resistance to be prevalent throughout the mid and southern Yorke Peninsula. Additionally, DMI resistance was detected in many of the samples, identifying the presence of dual fungicide resistance. The SDHI resistance was detected from the use of fluxapyroxad as a seed treatment. In the paddock of original detection, the crop had been barley for the previous two years with fluxapyroxad used as a seed dressing in both years.

Since 2019, SDHI resistance has been detected in multiple locations on the Eyre Peninsula, in the Mid North and the South-East regions of SA. At the end of the 2021 growing season, SDHI resistance had also been detected in the North-West of Victoria. There are currently three separate mutations present causing the SDHI resistance in NFNB, indicating there are multiple separate occurrences of resistance development.

Although the current testing for DMI resistance to NFNB is confined to tebuconazole in laboratory tests, field reports from affected paddocks indicate commonly used chemistries such as propiconazole and prothioconazole are still effective. Further field research on individual actives within the DMIs for their efficacy to NFNB would be beneficial to industry.

Wheat powdery mildew

Strobilurin fungicide resistance to wheat powdery mildew is prevalent on the northern Yorke Peninsula in the Bute area and has now been detected around Cummins on the Eyre Peninsula, as well as near Naracoorte in the South-East. Laboratory detections for DMI resistance have also been recorded in these areas. These detections are based on what is known as a gateway mutation that has been associated with reduced sensitivity, although on its own it does not confer much resistance.

Wheat septoria tritici blotch

Previous surveys by NSWDPI conducted in 2016 and 2017 identified the prevalence of a resistance mutation in the Cyp51 gene in the pathogen. Results of the survey found the mutation to be present in a high proportion of crops sampled in SA, VIC, TAS and NSW. The mutation significantly reduces the efficacy of DMI fungicides such as tebuconazole, propiconazole and flutriafol.

The occurrence of DMI resistance in STB has also favoured the use of SDHIs and strobilurins. These chemistries are prone to higher levels of resistance arising from a single mutation as opposed to the stepwise mutations seen in DMIs. At the beginning of the 2021 growing season, the CCDM confirmed that there had been a detection of strobilurin resistance in STB in the Millicent region of SA in 2020. This was only a single detection that was taken from a plot at a trial site and was not the result of an in-field failure.

Given the significance of losing this chemistry, CCDM put out an open call for samples in the 2021 growing season to assess whether the mutation was more widely distributed. As of December 2021, CCDM have not detected the mutation elsewhere and the current risk is thought to be confined to the lower South-East region of SA. Growers in this region are advised to avoid the use of strobilurins for STB control in 2022.

Barley spot form net blotch (SFNB)

In Western Australia, DMI resistance in spot form net blotch has been present for several years. In 2020, SDHI fungicide resistance in SFNB was also detected in WA. The resistance has currently not been detected outside of WA and at this stage, SA growers should be aware the resistance has developed but need not be concerned for the 2022 season.

Barley powdery mildew

Resistance to DMIs in barley powdery mildew is now present in every state of the grain growing regions other than South Australia. Currently in SA, DMIs are still very effective at controlling barley powdery mildew to the extent that SARDI have had no reports of the disease in recent years. This reduces the chance of the resistance developing independently in the state and the main cause for concern is interstate resistant strains entering SA.

Fungicide resistance management

Managing fungicide resistance will continue to play an increasingly important role in on-farm decision making as the situation develops in Australia. Growers need to have a good understanding of the disease pressures in their area, as well as any fungicide resistance present or posing a major threat of entering their region.

Fungicide resistance develops when a fungus is repeatedly exposed or over-exposed to the same mode of action group. (No, fungicide resistance occurs all the time and is simply correlated with the population size of the pathogen (See COVID). It is the repeated exposure to the same selection pressure that determines whether a mutation survives and the rate at which it multiplies). Therefore, fungicide resistance management is focused on reducing the over-exposure, managing disease with other strategies, as well as reducing the chances of additional cases of fungicide resistance developing.

The implementation of integrated disease management is the core of this approach. Figure 2, developed by AFREN, summarises how disease management decision making should be weighed up. The largest emphasis should be placed on variety selection as varieties that contain some level of resistance to the disease provide the strongest and most economical protection.

Figure 2. Fungicide resistance management pyramid produced by AFREN.

Following this, non-chemical farm management should be considered and appropriate IDM strategies for your farming system implemented. It is important to note that many endemic diseases such as net blotches, STB and powdery mildew are stubble borne and stubble retention systems in combination with the use of susceptible varieties heavily contribute to high inoculum loads.

Once these factors have been considered and appropriate strategies implemented, then fungicide management should be the last consideration. Fungicide use should be targeted and only used when necessary, rotate modes of action and only use the same active once per season, and use mixtures where possible.

Once fungicide resistance is present within a paddock, it may not be apparent in the first one to two seasons and the affected active may continue to be effective at a paddock scale. Pathogens causing diseases such as net blotch, powdery mildew and STB can reproduce sexually. When fungicide resistance first affects a paddock, it may only be present in a small percentage of the pathogen population, allowing the rest of the sensitive pathogen population to respond to the fungicide. It can take two or more years for the resistant portion of the population to become the dominant portion and for field failure of the fungicide to be observed.

Septoria tritici blotch IDM

Conditions at both Booleroo Centre and Hart Field Site were not conducive for extensive disease development in the 2021 growing season. As a result, disease levels were barely detectable at the Booleroo site and at Hart, the SVS variety Impala had only 11.3% disease (Tables 2 & 3).

Table 2: STB mean disease severity of whole plants at Hart Field Site in 2021.

Table 3: STB mean disease severity of whole plants at Booleroo Centre in 2021.

Mean yields at the Hart site were numerically higher in the minus-disease (fungicide treated) plots than in the plus-disease (disease inoculated) plots (Figure 3). However, preliminary statistical analysis found no significant differences. The Booleroo mean yields were very variable and minus-disease plot yields were numerically slightly lower than plus-disease plot yields in all varieties except Impala (Figure 4). There were no significant differences in yields at the site.

Figure 3. Mean yield losses associated with STB at Hart Field Site in 2021, no significant differences were detected.

Figure 4. Mean yield losses associated with STB at Booleroo Centre in 2021, no significant differences were detected.

These trials provide growers with an example of STB disease development in low and medium rainfall zones, how disease development alters depending on variety rating and associated yield losses. In the 2021 season, conditions were not conducive to disease development at these locations and resulted in no significant yield losses. This is important data to inform decision making, as in 2021, fungicide sprays would not have been economical in these areas as yield losses were not significant.

These trials are also being run at medium and low rainfall sites in Victoria. It is expected that after three years of trials, there will be multi-environment data that is able to give growers information about which seasons are conducive for STB yield losses.

Conclusion

IDM plays a crucial role in fungicide resistance management and a better understanding of how endemic diseases interact with our environment. This paper provides growers and advisers with up-to-date information on which fungicide resistances are present in South Australia, as well as which interstate resistances are currently posing threats. Resistances of most concern in South Australia are NFNB resistance to SDHI and DMIs, wheat powdery mildew resistance to strobilurins and STB resistance to DMIs.

Growers can manage fungicide resistance by knowing which resistances are present in their region, selecting varieties with genetic resistance, using IDM strategies and strategically applying fungicides. Fungicide applications should be limited to spraying only when necessary, avoiding actives if they are resistant in your region, rotating modes of action, only using any one active once per season, and using mixtures where possible.

Septoria tritici blotch field data from the 2021 growing season showed that, at the low and medium rainfall sites tested, seasonal conditions were not conducive for enough disease development to result in significant yield loss, even in SVS and S varieties. Further years of data will better develop our understanding of which years provide conducive disease development so that fungicide use can be targeted to these seasons.

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 authors would like to thank them for their continued support. The authors would also like to acknowledge the rest of the AFREN team and the Fungicide Resistance team at CCDM as well as collaborators on the STB project including Agriculture Victoria, FAR Australia, Hart Field Site Group, Upper North Farming Systems, Birchip Cropping Group and AgXtra.

References

McDonald MC, Renkin M, Spackman M, Orchard B, Croll D, Solomon PS, Milgate A (2019) Rapid parallel evolution of azole fungicide resistance in Australian populations of the wheat pathogen Zymoseptoria tritici. Applied and Environmental Microbiology 85(4), e01908-18. doi: 10.1128/AEM.01908-18. PMID: 30530713; PMCID: PMC6365823.

Milgate A (2020) Septoria tritici blotch of wheat, management strategies for the medium and low rainfall zones of south east Australia. Proceedings GRDC Grains Research Update, Adelaide, February 2020, pp. 231-235

The Australian Fungicide Resistance Extension Network

Cereal variety disease guide 2021

Cereal seed treatments 2021

Contact details

Tara Garrard
SARDI
2A Hartley Grove, Urrbrae SA 5064
0459 899 321
tara.garrard@sa.gov.au
@TaraGarrard