Take Home Messages
- STB caused significant grain yield (5–28%) losses in wheat varieties grown in the Wimmera (MRZ) and Mallee (LRZ).
- Foliar fungicide applications were required to protect grain yield in susceptible varieties at the MRZ (one each at stem elongation and flag leaf emergence) and LRZ (stem elongation only) sites.
- Use of varieties with at least MS (Wimmera) and MSS (Mallee) rating reduced grain yield losses significantly and negated the need for fungicide applications.
Aim
To understand seasonal conditions critical for STB development and spread.
To determine the impact of varying stubble amounts on in-crop STB risk.
To determine yield loss caused by STB in the MRZ and the LRZ in different seasons.
To identify optimal fungicide application timing should STB control be required in either the MRZ or the LRZ.
Background
Septoria tritici blotch (Zymoseptoria tritici), is a stubble-borne fungal disease of wheat, that is now widespread and damaging in many parts of Victoria. In recent years, it has become the most prevalent disease across Victoria’s medium (MRZ) and low (LRZ) rainfall zones with grain yield losses of greater than 20 per cent in susceptible crops. During 2022, Agriculture Victoria and BCG trials demonstrated up to ~43 per cent yield loss and quality reductions in susceptible varieties, showing losses can be greater in seasons when conditions are wet.
Large areas sown to susceptible varieties and high inoculum carryover from 2022 increased the disease risk in Victoria in 2023. Wet conditions in winter further increased the disease pressure resulting in a damaging outbreak of STB in wheat crops which resulted in grain yield losses in the Wimmera and Mallee despite the dry spring. Losses were reported for the first time in Mallee conditions, demonstrating this is an important disease in lower rainfall environments.
Agriculture Victoria in partnership with the South Australia Research and Development Institute (SARDI), with supporting investment from GRDC, has conducted several field experiments in the past three seasons to determine the best management practices for STB control suited to LRZ and MRZ. This research found yield losses can be significantly reduced by avoiding susceptible varieties, rotating wheat with other crops for a minimum of at least one season, and using foliar fungicides. This article reports on experiments conducted in different regions in Victoria during 2023.
Paddock Details
Trial Details
Crop type: Wheat
Treatments: Refer to Table 1
Target plant density: 150 plants/m²Â
Seeding equipment: Knife points, press wheels, 30cm row spacing.
Sowing date: Kinnabulla 1 May, Wallup 2 May, Longerenong 26 April, Hamilton 1 May
Replicates: Six (stubble load, variety selection and fungicide timing)
Harvest date: Kinnabulla 21 December, Wallup 17 December, Longerenong 4 December, Hamilton N/A
Trial Inputs
Fertiliser: Trials managed as per best practice reflecting the region and so nutrients are not limiting
Herbicide/insecticide: Trials kept weed and pest free
Fungicide: Refer to Table 1
Seed treatment/inoculant: Refer to Table 1
Method
Experiment 1: Conditions critical for STB development and spreadÂ
At three locations in Victoria (LRZ, MRZ and HRZ), susceptible wheat inoculated with STB infected stubble was grown. The plots were visually monitored for disease development and a Pessl weather station (supplied by ADAMA) collected site climate data (temperature, relative humidity, precipitation and leaf wetness) to relate to disease progress. Â
Experiment 2: Impact of inoculum load on in crop disease risk from STBÂ Â
One field experiment was conducted in the MRZ at Wallup, during 2023. This replicated field trial was sown in a randomised complete block design, with six treatments (Table 1a) including different levels of inoculum and a control with minimum disease applied to a susceptible to very susceptible variety LRPB Impala (SVS). Evaluation included visual assessment of STB severity in-season, grain yield and quality. Â
Experiment 3: Variety selection –yield lossÂ
At both the LRZ and MRZ sites, replicated field trials were sown in a split plot design with disease control as the whole plot and variety as the subplot, with six replications at the two sites (Table 1b). Evaluation included visual assessment of STB severity in-season, grain yield and quality.Â
Experiment 4: Fungicide timing
At both the LRZ and MRZ sites, replicated field trials were sown in a randomised complete block design with six replications at the two sites. Fungicide was applied at different timings (Table 1c). Evaluation included visual assessment of STB severity in-season, grain yield and quality.Â
Results & Interpretation
Conditions critical for STB development and spread (Experiment 1)
Impact of inoculum loads on in-crop disease risk from STB (Experiment 2)
Stubble application significantly increased STB severity and caused grain yield loss (11%) in wheat variety LRPB Impala (Table 2) demonstrating the importance of stubble management in wheat as part of an integrated STB control strategy. Stubble retention and quantities influence how much inoculum is available to infect wheat crops from one year to the next. As shown in this study, yield losses in wheat can increase if higher stubble loads are present, so wheat-on-wheat should be avoided where possible to reduce yield losses. When wheat is sown into stubble, stubble management techniques that aim to reduce stubble load provides a potential option to manage early epidemics, noting STB infection will also come from wind-borne spores from adjacent paddocks.
Impact of inoculum loads on in-crop disease risk from STB (Experiment 2)
Severe STB developed at the Longerenong (MRZ) and Kinnabulla (LRZ) sites in all the varieties, reducing grain yield by 5–28 per cent (Table 3). While losses in Mallee conditions were measured for the first time in 2023, losses in the Wimmera were greater than the ~10% measured in 2021 and less than ~40 per cent measured in 2022. At the Longerenong site, 2023 (growing season rainfall: 331mm) was relatively wetter than 2021 (262mm) and drier than 2022 (547mm), demonstrating the role of rainfall in STB damage. Consistent with previous seasons, disease development and yield loss were related to varietal resistance, demonstrating the benefits of avoiding highly susceptible cultivars for the management of STB. Losses in less susceptible varieties (LRPB Lancer (MS) or Hammer CL Plus (MSS)) were 5 per cent or less in the Mallee while minimal losses (5 per cent) were found with LRPB Lancer (MS) in the Wimmera. This indicated that varieties with a rating of at least moderately susceptible (MS) or moderately susceptible to susceptible (MSS) should be considered in Wimmera and Mallee respectively, for STB management.
STB infection also resulted in reduced grain quality of most varieties in the Mallee, with a small increase in screenings not affecting protein and test weight (data not shown). Grain quality of Wimmera trials had not been analysed at the time of this report.
Impact of inoculum loads on in-crop disease risk from STB (Experiment 2)
Fungicides provided variable control of STB in field experiments conducted at Longerenong (MRZ) and Kinnabulla (LRZ) (Table 4). At Longerenong, two foliar fungicide applications, one each at Z31 and Z39 (with or without a seed treatment) were required to reduce STB severity and increase grain yield by 16 per cent. However, complete control of STB was not achieved even with two foliar applications, which indicated a possible reduction in efficacy levels of fungicides applied or the need for a more intensive fungicide strategy. Testing of 25 samples collected from Victoria in 2023 did not detect resistance to strobilurins (Group 11) while reduced sensitivity to DMIs (Group 3) is known to exist in Victoria. It is recommended to rotate fungicides with different modes of action to slow fungicide resistance development. At Kinnabulla, a single foliar application at Z31 was as effective as dual applications at Z31 and Z39 to protect yield (7–10 per cent) and grain quality (reduced screenings and improved test weight) from STB (Table 5).
Commercial Practice and On Farm Profitability
STB has become the most prevalent foliar disease of wheat in both the medium (Wimmera) and low (Mallee) rainfall zones of Victoria. Grain yield losses due to STB are now common in these regions when susceptible varieties are grown. Agriculture Victoria trials demonstrated that by not growing highly susceptible varieties, yield and quality reductions due to STB can be avoided. This also eliminated the need for fungicide use and associated fungicide resistance complications.
Continuous cultivation of susceptible varieties produces large volumes of inoculum, which has implications not just for the current season but for the next. Disease severity and grain yield losses in 2023 were similar to 2022, despite being comparably drier. There are significant benefits in stubble retention systems but on the other hand they encourage carryover of inoculum and increase the risk of in-crop disease. In seasons with significant wet conditions, management of stubble loads might prove beneficial for reducing inoculum carryover and early disease epidemics in the following season. Lowering stubble quantities reduced grain yield losses due to STB in 2023 and should be considered as part of integrated control strategy.
Fungicides have been shown to effectively suppress STB infection, but their application can usually be avoided in seasons with below average rainfall, such as in 2021 when application in either the MRZ or LRZ was not economical (Table 6). When susceptible varieties are sown into wet seasons, fungicide strategies should include applications at both growth stage 31 and 39 (Table 6). A two-spray strategy at Z31 and Z39 protects yield contributing flag leaves and has shown ~10–16 per cent yield increase compared to untreated controls at Longerenong and Kinnabulla in 2023. It is also worth noting that STB populations have potential to develop resistance to fungicides and their unnecessary use should be avoided.
In summary, the most effective strategy for minimising losses from STB is to avoid highly susceptible varieties (especially those rated S and SVS). If conditions are suitable for STB, fungicide applications are needed to minimise losses. Also, avoid sowing wheat into paddocks with one- or two-year-old wheat stubble, noting that early STB infection will also come from wind-borne spores in adjacent paddocks.
Acknowledgements
This research was funded by the Victorian Government (Agriculture Victoria) and GRDC as part of the GRDC project ‘Epidemiology of Septoria Tritici Blotch in the low and medium rainfall zones of the Southern region to inform IDM strategies’ (DJPR2104_004RTX).
Thanks to Agriculture Victoria’s field crops pathology team at Horsham and the Birchip Cropping Group team for technical and scientific support. Thanks also to our research collaborators Dr Mark Mclean (Project Platypus), Dr Andrew Milgate (NSW DPI), Dr Julian Taylor (University of Adelaide) and Dr Tara Garrard (SARDI).
