Cereal disease update 2020

Take home messages

• The proactive use of different management options in combination (such as variety selection, paddock selection and appropriate fungicide use), provides proven sustainable and economic control for both root and foliar diseases.
• A new strain of stripe rust is important to durum wheat and some bread wheats with new ratings published in the 2020 Victorian Disease Guide.
• A new decision support tablet-based app will help in the management of stripe rust.
• Recent reports of fungicide resistance in cereal pathogens highlight the importance of avoiding fungicide use that increase the likelihood of resistance development.

Background

The good start to the 2019 season favoured early foliar disease development but did not continue as dry spring conditions mostly limited further disease progress. Despite this dry finish to the season, field experiments by Agriculture Victoria demonstrated yield losses of 8 to 18% due to net form of net blotch (NFNB) in the Wimmera and Mallee when highly susceptible cultivars were planted, infected stubble was present, and fungicides not applied. This highlights the importance of implementing disease management strategies to minimise yield losses.

Cereal disease management in 2020

Cereal diseases will require proactive management prior to and during the 2020 season. A disease management plan should consider the variety disease rating (consult a current disease guide) and inoculum loads within a paddock (stubble and soilborne diseases) and the district (consider any green bridge). A fungicide strategy should be developed for each crop, based on identified risks and being mindful of avoiding over-reliance on, or overuse of, fungicides. Diseases can be cost effectively controlled when a proactive management approach is used.

Wheat stripe rust

During 2019 there was a late outbreak of stripe rust in wheat crops, particularly in the variety LRPB Trojan due to the occurrence of a new pathotype. This late outbreak caused concern about the merits of fungicide applications late in the season. A new stripe rust app, StripeRust Wheat Management (StripeRustWM), will help with difficult decisions around in-crop fungicide control.

Rust pressure is expected to be relatively low during 2020 given the hot and dry conditions during December. These conditions have provided an effective control of the green bridge thus reducing rust carry over into the 2020 season. This reduced risk from rust can be factored into 2020 rust management plans.

A new stripe rust pathotype

During 2019, there were reports of stripe rust in the wheat varieties LRPB Trojan and DS Bennett at higher than expected levels. These reports were most likely related to a new stripe rust strain (pt. 198 E16 A+ J+ T+ 17+) that was first detected in 2018. It was isolated from Victoria and Tasmania later in 2018, and in 2019 from NSW (four isolates), Victoria (two isolates), and Queensland (one isolate). This pathotype is a simple mutational derivative of pt. 134 E16 A+ J+ T+ 17+, that is, from the ‘Western Australian’ family of stripe rust pathotypes, with added virulence for the differential Suwon 92/Omar. Given that the resistance of Suwon 92/ Omar has not been fully characterised, The University of Sydney is currently undertaking studies to fully understand the implications of this new pathotype.

Data collected from the field during 2019 by NSW DPI and AgVic, as part of the GRDC’s National Variety Trials (NVT) disease rating project, indicated that this new pathotype has implications for several wheat varieties, such as DS Bennett and LPB Trojan and to a lesser extent Devil, Illabo, DS Darwin, Emu Rock and Hatchet CL Plus. There were also implications for several durum varieties, such as DBA Spes, DBA Lillaroi, DBA Vittaroi and EGA Bellaroi.

The ratings published in the 2020 Victorian cereal disease guide have been updated and consider this as well as the other pathotypes known to occur in Victoria. It is important to always consult a current disease guide due to changes such as these.

Decision support for stripe rust control: StripeRustWM

A new tablet-based app, StripeRustWM, has been developed to support in-crop decision making for the management of stripe rust of wheat. The app is based on the already successful BlacklegCM and SclerotiniaCM apps that are widely used in canola. StripeRustWM estimates potential losses using information including variety resistance rating, plant growth stage, fungicide history, presence of rust either within the crop or the district, climatic conditions, expected yield and economics. StripeRustWM was developed using data and information from the last 30 years’ national pathology research projects. The app will be updated annually with the latest research findings so that new information can be utilised by industry as soon as available.

StripeRustWM is designed to be quick to use in the field, to guide profitable decisions about stripe rust management. It has a straight-forward user interface that asks for inputs that can be readily estimated by agronomists.

Figure 1 illustrates the StripeRustWM app interface, where a trace of stripe rust has been detected in a crop with a resistance rating of moderately susceptible (MS), at the booting growth stage. The output shows that a marginally higher net return would be expected if fungicide was applied to the crop once, compared with not spraying. The net return would probably be lower if the crop were sprayed twice.

Figure 1. The summary view from StripeRust Wheat Management (StripeRustWM) comparing expected yield, loss to stripe rust and net return for the cases where fungicide is not applied, is applied once, or is applied twice.

An alternate presentation of the StripeRustWM app is shown in Figure 2, illustrating the range of possible outcomes, as a probability, from a single fungicide application. This reflects the variable nature of a biological system where a range of outcomes are possible. It shows the relative likelihood of positive and negative returns from the spray based on different environmental conditions and yield potentials.

Figure 2. The net return view from StripeRust Wheat Management (StripeRustWM) showing the distribution of expected net return resulting from a single fungicide application.

StripeRustWM is available at no cost for iPads or Android tablets from the Apple App Store or Google Play — search for ‘StripeRustWM’.

Net form of net blotch

Net form of net blotch (NFNB) is becoming a common foliar disease of barley in Victoria due to the adoption of susceptible varieties such as RGT Planet and moderately susceptible varieties such as Compass. A survey of 80 barley crops across Victoria during 2019 found NFNB in 11% of paddocks. Severity was relatively low, ranging between 1 to 15% of leaf area infected, which was due to proactive fungicide strategies and/or dry spring conditions not favouring NFNB development. This level, however, serves as a warning for potential damage in conducive seasons.

During 2020, the risk of loss due to NFNB will be greatest where susceptible varieties are sown into barley stubble from either of the last two years. NFNB is seed and wind-borne, and therefore can establish in crops where there is no recent paddock history of barley. A new NFNB strain virulent on Spartacus CL has been found in SA and is likely to spread and become established in Victoria. In SA, the Centre for Crop and Disease Management (CCDM) researchers have identified resistance to fluxapyroxad (a member of the Succinate Dehydrogenase Inhibitors (SDHI) group of fungicides), the active ingredient in the seed treatment Systiva^®, and tebuconazole. Subsequently these fungicides may no longer be reliable options for NFNB control (see section on fungicide resistance below for more details). While the initial discovery of resistance occurred on the Yorke Peninsula, SA, the extent of the distribution is likely to increase with time.

Fungicides will be an important part of NFNB control in susceptible varieties. Previous research has shown that two fungicide applications can be effective for NFNB management. Either seed applied Systiva^® followed by foliar application at Z39-55, or two foliar applications at Z31 and Z39 or Z55 are effective. Earlier applications tend to be more effective in shorter season environments and later applications in longer, high rainfall environments. It is important to rotate fungicides with different modes of action to minimise the chance of resistance developing.

In general, susceptible crops with a yield potential of 5t/ha or more are at risk of substantial economic losses, especially during a wet spring. Experiments conducted in the Victorian Mallee and Wimmera during 2019 demonstrated that NFNB was severe, with up to 70% of leaf area affected at grain fill (Figure 3), up to 18% grain yield loss and reductions to grain plumpness (Tables 1, 2 and 3) in the very susceptible breeding line VB9613. The partially resistant varieties had less infection and grain yield loss than VB9613. RGT Planet is rated susceptible to very-susceptible (SVS), and had up to 11% infection and 5% grain yield loss while all other varieties had less than 5% leaf area infected at the end of the season. This illustrates that avoiding growing highly susceptible varieties, such as those rated as susceptible to very-susceptible (SVS), or worse, to NFNB, will significantly reduce potential losses and the need for fungicide intervention. In general, NFNB should be managed in susceptible varieties where disease levels are moderate and yield potential is above 5t/ha. It is also likely that NFNB can cause economic losses where yield potential is between 3 to 5t/ha and there are wet spring conditions. Experiments are being conducted during 2020 to investigate this further.

Figure 3. Severe net form of net blotch infection developed in breeding line VB9613, rated susceptible to very-susceptible (SVS), compared to moderate and low infection in RGT Planet (SVS) and Fathom moderately resistant to moderately susceptible (MRMS) at Birchip during 2019.

Table 1. Net form of net blotch severity (% leaf area affected) and grain yield of eight barley varieties grown at Birchip during 2019.

^ADis. = Disease - 1kg infected stubble, no fungicides; Fung. = no stubble, Systiva^® + Prosaro^® at Z31 and Z39.

* = Significant at 5%; ns = not statistically significant when the fungicide and disease treatments are compared.

# rating = moderately resistant (MR), moderately susceptible (MS), moderately resistant – moderately susceptible (MRMS), moderately susceptible – susceptible (MSS), susceptible (S), susceptible – very susceptible (SVS), very susceptible (VS).

Table 2. Effect of net form of net blotch on grain quality for eight barley varieties grown at Birchip during 2019.

^ADis. = Disease - 1kg infected stubble, no fungicides; Fung. = no stubble, Systiva^® + Prosaro^® at Z31 and Z39.

* = Significant at 5%; ns = not statistically significant when the fungicide and disease treatments are compared.

# rating = moderately resistant (MR), moderately susceptible (MS), moderately resistant – moderately susceptible (MRMS), moderately susceptible – susceptible (MSS), susceptible (S), susceptible – very susceptible (SVS), very susceptible (VS).

Table 3. Net form of net blotch severity, frost damage and grain yield of eight barley varieties grown at Horsham during 2019.

^ADis. = Disease - 1kg infected stubble, no fungicides; Fung. = no stubble, Systiva^® + Prosaro^® at Z31 and Z39.

** = statistically significant at 5%, * = Significant at 5%; ns = not statistically significant when the fungicide and disease treatments are compared.

# rating = moderately resistant (MR), moderately susceptible (MS), moderately resistant – moderately susceptible (MRMS), moderately susceptible – susceptible (MSS), susceptible (S), susceptible – very susceptible (SVS), very susceptible (VS).

Foliar diseases of oats

Foliar diseases are a major constraint on milling oat production in south eastern Australia. Surveys identified red leather leaf (RLL), caused by the fungus Spermospora avenae, as the most common and severe foliar disease in south eastern Australia. It is restricted to the medium and high rainfall zones as it is favoured by wet and cool conditions.

During 2019, bacterial blight was common across all rainfall zones but was generally at low levels that were unlikely to cause significant loss. However, given the right seasonal conditions there is potential for it to cause losses.

Experiments conducted in the Wimmera during 2019 demonstrated that RLL caused up to 13% (0.5t/ha) grain yield loss in susceptible varieties (Table 4). Yallara was the worst affected, while Mitika and Williams were also affected. Growers should grow moderately susceptible (MS) or better rated varieties to reduce losses. Most of the infection was on the flag -2 leaves and lower. Greater losses are possible during wet weather conditions that favour greater infection on the top two leaves.

There are no fungicides registered for control of RLL in oats. We investigated fungicides registered for use in oats for potential suppression and control. We found that fungicides suppress RLL but do not provide complete control. Previous research showed that each foliar fungicide application provided 5-7% reduction in RLL severity in the Wimmera. Greater response has been observed in the high rainfall zone. This was illustrated by the fungicide treatment in Table 4 which shows up to 11% RLL infection following three applications of propiconazole. Previous research has shown that foliar fungicide applications at Z25 and Z31 are most effective as they coincide with early disease development, while application at Z39 can provide benefits during seasons with wet springs.

Growers should avoid sowing oats into paddocks with oat stubble from previous years as this is the main source of infection. RLL can also be seed-borne, so it is important to monitor all crops and apply fungicides if necessary.

Table 4. Red leather leaf (RLL) severity and grain yield of six milling oat varieties in response to disease and fungicide treatments near Horsham during 2019.

^ADis. = Disease - 1 kg infected stubble, no fungicides; Fung. = no stubble, propiconazole at Z25, Z31 and Z39.

# rating = moderately resistant (MR), moderately susceptible (MS), moderately resistant – moderately susceptible (MRMS), moderately susceptible – susceptible (MSS), susceptible (S), susceptible – very susceptible (SVS), very susceptible (VS).

Fungicide resistance

Fungicide resistance is an important issue for the management of diseases in cereals. During the last 20 years fungicides have provided cheap and reliable control of many fungal diseases, but their frequent use has resulted in a selection pressure which favours pathotypes that have mutations for fungicide resistance. Subsequently, there are increasing reports of diseases displaying reduced fungicide sensitivity and fungicide resistance. It is important that the agricultural industry adopts strategies that reduce reliance on fungicides to ensure their longevity.

During 2019, there were several reports of fungicide resistance in cereal diseases across Australia. The most significant was the identification of fungicide resistance in barley net form of net blotch (NFNB) to fluxapyroxad (a member of the SDHI group of fungicides) in the seed treatment Systiva^®. This resistance was confirmed by the fungicide resistance researchers from the Centre for Crop and Disease Management at Curtin University in collaboration with SARDI in samples taken from multiple paddocks across the Yorke Peninsula in South Australia. Whilst testing for resistance to SDHI fungicides, a very high level of resistance to tebuconazole (used as an indicator of resistance within the DMI group of fungicides) was also detected in all 15 paddocks tested across the YP. Testing so far has focused on the YP but given the widespread dispersal of airborne spores it is possible that spores of the dual-resistant pathotype of NFNB will have been dispersed during 2019 and may be present across a wider area, albeit at a low level during 2020. Further field testing is planned for 2020.

This development and spread of fungicide resistance for NFNB is likely to have been enhanced with the sowing of susceptible barley varieties into infected barley stubbles and the repeated use of fluxapyroxad (Systiva^®) and a narrow range of DMI fungicides. This incidence highlights the importance of not becoming over-reliant on a single option for disease control.

The agricultural industry can slow the development of fungicide resistance and thus protect the longevity of the limited fungicides available by adopting the following disease management strategies;

• Use a range of control strategies to minimise disease development, including;

- avoid growing highly susceptible cultivars

- use crop rotation to avoid planting into paddocks with disease present

- manage the green bridge for diseases such as mildew and rust.

• Use seed and/or fertiliser treatments, if available, to suppress early disease development.
• Avoid unnecessary fungicide use.
• Use fungicide mixtures formulated with more than one mode of action.
• Do not use the same active ingredient more than once within a season.
• Adhere to label recommendations.

New fact sheets

Several new fact sheets have been released recently that provide current and useful information for the management of diseases in cereals. Links to each of the new fact sheets can be found in the ‘useful resources’ section at the end of this report.

Cereal disease guide

The 2020 Cereal disease guide published by Agriculture Victoria provides current variety ratings for all recently released and commonly grown cereal varieties in Victoria, using the latest disease resistance ratings from the NVT. The ratings reflect the disease strains of importance in Victoria. As the strains of disease do change over time it is always important to consult a current version of the guide.

Crown rot

A new GRDC fact sheet has been published on the identification and control of crown rot. In seasons conducive to crown rot (such as those with a dry spring) losses greater than 20% are common in wheat crops grown in paddocks with medium to high crown rot levels. As there are no in-crop control options available, the fact sheet recommends the use of soil testing to identify risk prior to planting.

Root lesion nematode

The latest information on the management and control of root lesion nematodes has been published in a GRDC fact sheet. It highlights the advantages of using soil testing to determine nematode levels within paddocks to enable the planning of rotations to minimise losses.

Spot form of net blotch in barley

A fact sheet on the identification and management of spot form of net blotch in barley is due for release by GRDC in early 2020. This disease can cause yield losses greater than 20% in susceptible varieties during wet seasons, when crops are planted into paddocks with infected stubble. The fact sheet highlights the potential yield losses from the disease and the important control strategies to minimise losses.

Seed treatment guide

SARDI has published the 2020 guide to cereal seed treatments, which lists all the registered seed and fertiliser treatments available to assist with disease control.

Conclusion

In the absence of proactive disease control, yield losses due to diseases can be greater than 20%. Therefore, it is important to develop plans to effectively manage cereal diseases this season. Disease management plans should consider paddock and variety selection and, where the risk warrants it, the proactive use of fungicides that avoid overuse to protect their longevity.

Acknowledgements

This research is a collaborative project between the GRDC and the Victorian Government (Agriculture Victoria). 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.

Thanks to Agriculture Victoria’s cereal pathology team; Graham Exell, Jordan McDonald, Glenn Sluggett, Joshua Fanning, Jon Baker, Melissa Cook, Hari Dadu, Luise Sigel, Jennifer Cutajar and Winnie Liu Heang. Thanks also to the Birchip Cropping Group for field trials within the Victorian Mallee.

Useful resources

Current Victorian cereal disease guide:

Cereal-Disease-Guide_Ag Victoria

Root lesion nematode fact sheet:

Root-Lesion-Nematode-Southern_GRDC

Crown rot fact sheet:

Crown-Rot-Southern_GRDC

Cereal seed treatment guide, 2020:

Cereal_seed_treatments_2020.pdf_SARDI

Spot form of net blotch fact sheet:

Check the GRDC web site for an early 2020 release.

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