Cereal disease update 2022

Take home messages

  • Proactive disease management, which combines options such as variety selection, paddock selection and appropriate fungicide use, provides proven sustainable and economic control of cereal diseases.
  • Septoria tritici blotch, an important disease in HRZ, caused yield loss in highly susceptible varieties in field trials in the MRZ (Wimmera) but not the LRZ (Mallee) during 2021. Fungicides did not provide an economic increase in grain yield in either environment.
  • Cereal rusts (especially wheat stripe rust) are likely to be very important during 2022 due to increased carry over of rust on volunteers growing over summer because of La Niña conditions in eastern Australia. A proactive strategy to manage cereal rusts in 2022 is essential.
  • An increasing range of decision support apps to assist with in-crop disease management are available for free. These apps assist with decisions around in-crop fungicide application.
  • Development of fungicide resistance is increasing in cereal pathogens but can be slowed through the adoption of integrated control strategies and prudent use of fungicides.

Background

Implementation of proactive strategies for the control of cereal diseases can prevent avoidable losses when seasonal conditions are suitable for disease. This paper provides an update on the latest research regarding cereal diseases for Victorian growers.

Septoria in wheat

Septoria tritici blotch (STB) (Zymoseptoria tritici) is a damaging disease of wheat with large losses known to occur in susceptible cultivars in high rainfall cropping zones (HRZ) of Victoria and South Australia. However, the impact of STB on yield in the medium (MRZ) and low rainfall cropping zones (LRZ) in Victoria and South Australia is less understood, even though this disease has become common in these regions. This increase in STB prevalence is associated with increased use of cultivars susceptible to STB and stubble retention practices. There is much uncertainty about the impact and control of this disease in medium and low rainfall zones.

To determine the impact of STB in Victoria and South Australia’s MRZ and LRZ, GRDC is supporting a new investment in research led by Agriculture Victoria, in partnership with the South Australia Research and Development Institute. This investment will investigate the epidemiological conditions required and the impacts of STB in these regions to inform disease management decisions.

In Victoria, seven field experiments were conducted during 2021: three each in the MRZ (Longerenong) and LRZ (Watchupga, with Birchip Cropping Group) and one experiment in the HRZ (Hamilton) of Victoria to understand the conditions that suit the disease progression, study the impact of STB on wheat with different resistance ratings and identify optimal timing for fungicide application during seasons at risk. Similar experiments are occurring in South Australia.

Conditions critical for STB development and spread

At three locations in Victoria (LRZ, MRZ and HRZ), susceptible wheat inoculated with infected stubble were grown. The plots were monitored for disease development and a Pessl weather station (courtesy of ADAMA) collected climatic data that influences disease progress including temperature, relative humidity, precipitation, and leaf wetness.

STB severity was different in each of the three locations. As expected, disease development was greatest in the HRZ and least in the LRZ (Figure 1). The three locations showed different weather conditions (Table 1). Low maximum temperatures combined with high growing season rainfall distributed evenly across the season and larger periods of leaf wetness at Hamilton provided ideal conditions for STB progress and the potential for maximum impact (Table 1). Conditions at Longerenong and Watchupga were only partially or not conducive respectively for STB development and wheat varieties with moderate resistance grown in these conditions are likely to escape the yield losses.

STB severity (% leaf area affected) across time in wheat (cv. Razor CL Plus, susceptible to STB) at three different locations in Victoria during 2021.

Figure 1. STB severity (% leaf area affected) across time in wheat (cv. Razor CL Plus, susceptible to STB) at three different locations in Victoria during 2021.

Table 1: Summary of weather at three locations in Victoria during the 2021 growing season.

Location

Growing season (April to October)

Total rainfall (mm)

Mean leaf wetness (hrs/day)

Mean maximum temperature (°C)

Mean number of rain days/month#

Watchupga (LRZ)

172

10

19

11

Longerenong (MRZ)

262

7

17

14

Hamilton (HRZ)

419

20

15

18

#A rainy day is defined as a day with a rainfall of at least 0.1mm or more rainfall.

Yield loss due to STB

Two yield loss experiments, one each at Longerenong (MRZ) and Watchupga (LRZ), were conducted during 2021. Each experiment had six commercial wheat varieties with different resistance/susceptibility to STB, and six replications of plus and minus disease treatments (inoculated with infected wheat stubble) applied in a split plot design.

Good levels of STB developed at the Longerenong site (MRZ) in the susceptible varieties Scepter (S) and LRPB Impala (SVS) which caused 8% and 7% of yield loss, respectively (Table 2). This was due to suitable conditions during the season for the disease to progress with above average rainfall during the spring that supported infection of the top three leaves during grain fill, but also shows that partial resistance can provide adequate disease suppression in this environment.

Table 2: Septoria tritici blotch severity (% leaf area affected) and grain yield of six wheat varieties at Longerenong, Victoria with high (Max) and low (Min) disease, during 2021.

Variety

Rating

Disease severity (% leaf area affected)

Grain yield (t/ha)

Yield loss (%)

29-Jul, Z25-31

31-Aug, Z37

25-Oct, Z69-75

Max.c

Min.

Max.

Min.

Max.

Min.

Max.

Min.

Sunlamb

MR

0

0

5

2**

0

0

5.2

5.1

-

Orion

MRMS

1

0

1

1

8

0**

5.2

5.3

-

LRPB Lancer

MS

1

1

7

4**

6

0**

5.6

5.8

-

Hammer CL Plus

MSS

2

1*

12

7**

11

0**

5.3

5.3

-

Scepter

S

2

1**

24

12**

49

1**

6.2

6.8**

8

LRPB Impala

SVS

2

1**

27

14**

56

5**

5.3

5.8**

7

**=statistically significant at 1% lsd; *=statistically significant at 5% lsd, ns=not significant at 5%.

a Max. = Maximum disease; Min. = Minimum disease.

In contrast, conditions were not conducive for STB at Watchupga (LRZ) where only low levels of STB developed and yield loss was not measured, even in the susceptible varieties Scepter and LRPB Impala (Table 3).

Table 3: Septoria tritici blotch severity (% leaf area affected) and grain yield of six wheat varieties at Watchupga, Victoria with high (Max) and low (Min) disease, during 2021.

Variety

Rating

Disease severity (% leaf area affected)

Grain yield (t/ha)

28-July, Z25-31

2-Sep, Z39-49

06-Oct, Z61-71

Max.a

Min.

Max.

Min.

Max.

Min.

Max.

Min.

Sunlamb

MR

0

0ns

1

0ns

0

0ns

3.0

3.2ns

Orion

MRMS

0

0ns

0

0ns

1

1ns

2.7

3.0ns

LRPB Lancer

MS

0

0ns

2

1**

1

0**

2.7

2.5ns

Hammer CL Plus

MSS

0

0ns

4

2**

2

1**

2.8

3.0ns

Scepter

S

1

0ns

9

3**

4

1**

3.0

3.2ns

LRPB Impala

SVS

1

0**

10

4**

4

2**

2.9

2.8ns

**=statistically significant at 1% lsd; *=statistically significant at 5% lsd, ns=not significant at 5%.

a Max. disease = Maximum disease; Min. disease = Minimum disease.

Relative STB severity in each variety corresponded with resistance status at both locations. The suppression in STB levels in the partially resistant varieties demonstrates that avoiding susceptible varieties in these regions should be adequate to manage STB.

Fungicide timing for STB management

Two experiments, one each at Longerenong (MRZ) and Watchupga (LRZ), were conducted to determine the optimal fungicide timing for STB control. Six replications of six fungicide treatments consisting of either single or combinations of seed and/or foliar applied fungicide and an untreated control (UTC) were applied to a susceptible variety, Scepter (S). As limited disease developed at the low rainfall site, only the results from Longerenong will be discussed here.

At Longerenong, all fungicide treatments reduced STB severity compared to the UTC, but the early applications of seed only or a single spray at Z31 were not as effective as any of the three treatments that included a fungicide application at Z39 (Table 4).

Table 4: Septoria tritici blotch severity (% leaf area affected) and grain yield of wheat (cv. Scepter (S)) in response to different fungicide treatments in the Victorian medium rainfall zone (MRZ), Victoria during 2021.

Treatment

Active ingredient

Disease severity (% leaf area affected)

Grain yield (t/ha)

29-July, Z31*

31-Aug, Z37*

25-Oct, Z73#

Seed

Fluquinconazole

1a

13a

34c

6.0

Foliar at Z31

Benzovindiflupyr + Propiconazole

2b

13a

17b

6.1

Foliar at Z31 and Z39

Benzovindiflupyr + Propiconazole at Z31

and Epoxiconazole at Z39

2b

13a

1a

6.2

Seed and Foliar at Z39

Fluquinconazole as seed

+ Epoxiconazole at Z39

1a

11a

3a

6.2

Foliar at Z39

Epoxiconazole

2b

23b

3a

6.1

Untreated control

-

2b

23b

52d

6.1

P

0.00

<0.001

<0.001

0.47

lsd (0.01)

1.14

2.72

4.65

ns

Means with one letter in common are not significant

*Average of single plot assessments; #Average of the top three leaves of ten tillers per plot.

Although fungicide application reduced STB severity significantly, no significant yield benefit was observed with any of the fungicide treatments (Table 4).

These trials showed that fungicide applications may not be economical in either the LRZ or MRZ during years with below average to average rainfall conditions. Economical returns may be possible, particularly in the MRZ should above average rainfall conditions occur, and the fungicide strategy will need to reflect seasonal conditions.

Cereal rust

The risk from cereal rusts (stripe rust, leaf rust and stem rust in wheat, leaf rust in barley and crown rust and stem rust in oats) is expected to be very high during 2022. Growers should adjust their rust management strategies to reflect this risk and avoid losses associated with cereal rusts.

Historically, rust pressure is highest in seasons following wet summers that support the widespread growth of volunteer cereals that enable rust to survive and build up over summer (often referred to as the “green bridge”). Following higher levels of rust in cereal crops in eastern Australia compared with recent years, and the late spring and early summer rainfall events in NSW and parts of Victoria, along with predicted La Niña conditions, we expect high rust levels in 2022.

Cereal rusts can only survive from one season to the next on living plant material, which is why rust is more important in seasons following a green bridge over summer. Cereal rusts do not survive on seed, stubble or in the soil.

Another consideration is that cereal rusts are dispersed by wind across very large distances. This means that a widespread green bridge in another area, such as NSW, can increase the risk for Victorian crops and this needs to be considered in management plans.

Strains of stripe rust and disease ratings

Surveillance of cereal rust strains (pathotypes) by the University of Sydney provides us with good information on the strains of rust affecting cereal crops nationally. This knowledge of the strains present nationally is critical for the development of resistance ratings that are published in annual disease guides.

In recent years, there have been changes in the relative prevalence of the strains of stripe rust in eastern Australia. The two stripe rust strains first detected in Australia in 2017 (the 239 strain) and 2018 (the 198 strain) have now become the dominant strains, each representing about 40% of the population, replacing the “Western Australian” family of strains (134) which were dominant since 2003. The ratings in the Agriculture Victoria Cereal Disease Guide (Cereal Disease Guide) represent the worst case rating for these three strains. Resistance ratings for varieties against each of these individual strains can be found on the NVT website (National Variety Trials). Unfortunately, it is not possible to predict which strains will dominate in any season, and rapid testing to determine strains is not possible. Therefore, monitoring of crops is still an important component of disease management.

The University of Sydney’s rust surveillance team provides a map of cereal rust strains during the season on the web (see link under “useful resources”). This service is useful for an idea of strain distribution nationally. It takes about 3 weeks from when a sample is collected to when these results are available.

Cereal rust management for 2022

All rusts of cereal crops (stripe rust, leaf rust and stem rust in wheat, leaf rust in barley, and crown rust and stem rust in oats) will most likely require proactive management in 2022 due to increased risk compared to recent years. The summer rain, associated with the La Niña, will support carry over of rust spores on volunteer cereals growing over summer, and combined with the susceptibility of many cultivars, contribute to the heightened risk. It is therefore important that growers take the following steps to reduce their risk:

  • Remove the green bridge (volunteer cereals) by mid-March
  • Use a current cereal disease guide to revise resistance ratings as there have been many changes due to new strains
  • Where possible, avoid susceptible varieties
  • Develop a fungicide management plan
  • Download the StripeRustWM App for iPads and tablets to support with wheat stripe rust management (see below).

Fungicide resistance in Victoria

Resistance to fungicides is becoming an increasing threat to cereal crops across Australia. The status of resistance to fungicides in important cereal diseases is summarised in Table 5 and is based on work by the Fungicide Resistance Group (FRG) at the Centre for Crop and Disease Management (CCDM at Curtin University).

Table 5: Fungicide resistance and reduced sensitivity cases identified in Victorian and South Australian wheat and barley crops.

Disease

Pathogen

Fungicide Group

StatusA

Industry implications

Barley powdery mildew

Blumeria graminis f.sp. hordei

3 (DMI)

Lab detection

May see some reduced efficacy in field. Field resistance detected in WA

Wheat powdery mildew

Blumeria graminis f.sp. tritici

3 (DMI)

Field Resistance

Some Group 3 DMIs will be ineffective in field

11 (QoI)

Field Resistance

Group 11 fungicides ineffective

Barley net-form of net blotch

Pyrenophora teres f.sp. teres

3 (DMI)

Reduced sensitivity

Expect to see reduced field performance with time

7 (SDHI)

Field resistance (SA)

Lab detection (Vic)

Increasing occurrences of field failure expected

Barley spot-form of net blotch

Pyrenophora teres f.sp. maculata

3 (DMI)

Lab detection

Products still effective but will decline as resistance develops

7 (SDHI)

Not detected

Field resistance detected in Western Australia but not eastern Australia

Wheat septoria tritici blotch

leaf blotch

Zymoseptoria tritici

3 (DMI)

Reduced sensitivity

May see some reduced efficacy in field

11 (QoI)

Field resistance (SA)

Two detections in SA during 2020, but not detected in 23 locations across Victoria and South Australia in 2021

Lab detection - Measurable differences in sensitivity of the pathogen to the fungicide when tested in the laboratory. Detection of resistance in the lab can often be made before the fungicide’s performance is impacted in the field; Reduced sensitivity – Some reduction in fungicide performance which may not be noticed in the field. Serves as a warning that resistance is developing in the pathogen. Increased fungicide rates as per registered labels may be necessary. Field Resistance - Fungicide fails to provide an acceptable level of control of the target pathogen at full label rates.

Following are the latest findings on fungicide resistance in Victoria:

Barley net blotches

On the South Australian Yorke Peninsula during 2019, a mutation in the net form of net blotch (NFNB) pathogen was identified that conferred field resistance to SDHI (Group 7) fungicides which includes key actives such as fluxapyroxad and bixafen. Limited testing during 2021 detected this mutation in six barley samples from Victoria suggesting that this resistance may be widespread across the southern region (Table 6). Mutations in NFNB that confer reduced sensitivity (partial resistance) to SDHI fungicides were also common in the Victorian samples.

Table 6: Mutations associated with resistance and reduced sensitivity to demethylase inhibitor (DMI, Group 3) and succinate dehydrogenase inhibitor (SDHI, Group 7) fungicides detected in net blotch samples from Victoria and South Australia in 2021.

Location

State

Disease present

DMI Reduced Sensitivity

 

SDHI Resistance

 

SDHI Reduced Sensitivity

Promoter lindel

F489L

 

PtSdhC-H134R

 

PtSdhD-D145G

PtmSdhC-N75S

SFNB

NFNB

 

NFNB

 

NFNB

NFNB

n/a

Vic

NFNB+SFNB

-

+

 

+

 

-

-

Banyena

Vic

NFNB

-

+

 

+

 

-

+

Banyena

Vic

NFNB

-

+

 

+

 

-

+

Minyip

Vic

NFNB+SFNB

-

+

 

+

 

+

-

Minyip

Vic

NFNB+SFNB

-

+

 

+

 

-

-

Horsham

Vic

NFNB+SFNB

-

+

 

-

 

-

-

Horsham

Vic

NFNB

-

+

 

+

 

+

-

Minlaton

SA

NFNB+SFNB

-

+

 

-

 

-

-

Minlaton

SA

NFNB+SFNB

-

-

 

-

 

-

-

Minlaton

SA

NFNB+SFNB

+

-

 

-

 

-

-

Minlaton

SA

NFNB+SFNB

-

-

 

+

 

-

+

Minlaton

SA

SFNB

-

-

 

-

 

-

-

A mutation that confers reduced sensitivity to the DMI (Group 3) fungicides was also common in the limited testing conducted on Victorian NFNB samples during 2021 (Table 6). Previously, this mutation was observed in NFNB from the Yorke Peninsula of SA and is associated with reduced sensitivity to DMI compounds. These types of mutations provide an early warning that the pathogen is accruing mutations that will eventually enable the pathogen to be resistant to the fungicide.

Within the spot form of net blotch testing, a mutation that confers partial resistance to DMI (Group 3) fungicides was identified in South Australia but not Victoria. This mutation has been detected in Western Australia since 2016 and is associated with reduced sensitivity to some DMI compounds. This detection in eastern Australia provides an early warning that practices are required to slow the development and spread of fungicide resistance.

Septoria tritici blotch

Based on studies conducted several years ago by NSW DPI, it is known that reduced sensitivity (partial resistance) to the triazole (DMI, Group 3) fungicides is well established within the Victorian STB pathogen population. Work is required to establish the current status of resistance to these fungicides in Victoria.

The finding of resistance mutations to strobilurin fungicides (Qol, Group 11) in two Septoria tritici samples collected in South Australia during 2020 was very concerning for growers in the southern region. In vitro testing of these isolates showed a 200-fold increase in resistance to azoxystrobin compared with the susceptible strain. Fortunately, testing of 32 samples collected from Victoria, Southern Australia and New South Wales during 2021 did not detect this mutation suggesting that this mutation is still at a low prevalence. However, this re-enforces the need to adopt strategies to protect the limited range of fungicides that we have available (see below).

Wheat powdery mildew

Resistance to both DMI (Group 3) and Qol (Group 11) fungicides has been detected in Victoria. Analysis of samples from seven paddocks in north-east Victoria were all positive for the mutation associated with resistance to Qol fungicides. This means there will be increasing occurrences of field failure in areas prone to powdery mildew and where susceptible varieties are grown.

Fungicide resistance management

There are five strategies that growers can adopt to slow the development of resistance in pathogen populations and therefore extend the longevity of the limited range of fungicides available:

  • Avoid susceptible crop varieties. Where possible, select the most resistant crops suitable and/or avoid putting susceptible crops in high-risk paddocks
  • Rotate crops. Avoid planting crops back into their own stubble or adjacent to their own stubble
  • Use non-chemical control methods to reduce disease pressure. Delaying sowing, early grazing are examples of strategies that can reduce disease pressure
  • Spray only if necessary and apply strategically. Avoid prophylactic spraying and spray before disease gets out of control
  • Rotate and mix fungicides/mode of actions. Use fungicide mixtures formulated with more than one mode of action, do not use the same active ingredient more than once within a season and always adhere to label recommendations.

For more information on the management of fungicide resistance consult the “Fungicide Resistance Management Guide” available from AFREN.

Apps to support in-crop disease management decisions

Apps are available to assist in making decisions around disease management in crop. The apps use models that produce predictions based on information on variety resistance rating, plant growth stage, yield potential and the presence of disease inoculum. The model predictions are compared and validated with field trial data to ensure accuracy and reliability. The models were developed by DPIRD and GRDC with input from AgVic. The apps are available free from the Apple App Store and Google Play.

StripeRustWM App

The StripeRustWM App is available to download on tablets. The app can support decision making around fungicide use for stripe rust management during the season. The app uses information that is specific to a local area or paddock to improve accuracy. Comparisons to field trials have demonstrated a high level of accuracy and reliability.

YellowSpotWM App

The YellowSpotWM App was released during 2021 and is available to download on tablets and smartphones. Its predictions are being developed in line with data from the 2021 season, ready for the 2022 growing season. This app will support in-crop fungicide decisions.

NetBlotchBM App

The NetblotchBM tool will be tested during 2022 and the app will be released during 2023 to help support in-crop fungicide management decisions for the spot and net forms of net blotch in barley. If you would like to help with testing of NetBlotchBM, please contact Anna Hepworth (anna.hepworth@dpird.wa.gov.au), or any of the lead authors of this paper.

Conclusion

In the absence of proactive disease control, yield losses due to diseases can be greater than 20%. The risk from rust diseases is likely to be greater with a wet summer (La Niña) supporting volunteer cereals that carry rust inoculum from one season to the next. It is, therefore, important that plans are developed to effectively manage cereal diseases this season. Disease management plans should consider paddock and variety selection and, where the risk warrants it, the proactive and prudent use of fungicides that avoid overuse to protect their longevity.

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.

Funding for this work was provided by the Victorian Government (Agriculture Victoria) and the GRDC through the GRDC projects: DJPR2104-004RTX, DJP2103-005RTX, DJP2003-011RTX, DAW1810-007RTX, University of Sydney (9175448), CUR1905-001SAX, CUR00023.

Thanks to Agriculture Victoria’s Cereal Pathology Team: Jordan McDonald, Glenn Sluggett, Joshua Fanning, Jon Baker, Melissa Cook, Luise Fanning, Rajandeep Singh, Zoe Nicholson, Bhanu Kalia and Andrew Hallett. Thanks also to the Birchip Cropping Group for field trials within the Victorian Mallee and to our research collaborators Andrew Milgate (NSW DPI), Julian Taylor (University of Adelaide) and Tara Garrard (SARDI).

Useful resources

Agriculture Victoria cereal disease guide

Australian cereal rust survey

Field Crop Diseases Victoria

Contact details

Grant Hollaway
Agriculture Victoria
Private Bag 260, Horsham VIC 3401
03 4344 3111
grant.hollaway@agriculture.vic.gov.au
@Grant_Hollaway

Mark McLean
Agriculture Victoria
Private Bag 260, Horsham VIC 3401
03 4344 3111
mark.s.mclean@agriculture.vic.gov.au
@msmclean777

Hari Dadu
Agriculture Victoria
Private Bag 260, Horsham VIC 3401
03 4344 3111
Hari.Dadu@agriculture.vic.gov.au
@Imharidadu

GRDC Project Code: DJPR2104-004RTX, DJP2103-005RTX, DJP2003-011RTX, DAW1810-007RTX, UOS1707-003RTX, CUR1905-001SAX, CUR1403-002BLX, DJP2104-004RTX,