By Hari Dadu, Grant Hollaway (Agriculture Victoria), Tara Garrard (SARDI) and Genevieve Clarke (BCG)
Take Home Messages
- Septoria tritici blotch (STB) development was directly related to seasonal rainfall.
- STB caused significant grain yield (17–43 per cent) and quality losses in wheat varieties grown in the MRZ but not in the LRZ.
- Fungicides were required to protect grain yield in the MRZ, and reduced yield loss in susceptible cultivars by ~35 per cent with two sprays (Z31 and Z39).
Background
Septoria tritici blotch (Zymoseptoria tritici), a stubble-borne fungal disease of wheat, is widespread and damaging in the Victorian and South Australian HRZ where susceptible cultivars are grown. In recent years, it has also become widespread across Victoria’s medium (MRZ) and low (LRZ) rainfall zones. During 2021, Agriculture Victoria and BCG trials demonstrated less than 10 per cent yield loss in the MRZ and no yield loss in the LRZ in susceptible cultivars.
STB development depends on suitable growing season conditions, especially rainfall. Rainfall is required to initiate disease infection, and disperses pathogen spores from stubble to help the disease spread. Disease pressure was extreme during 2022 due to a combination of the quantity and frequency of rainfall from August to November, which provided ideal conditions for disease development. This caused a damaging outbreak of STB in all regions of Victoria and yield losses where control was inadequate.
To determine the impact of STB and identify best management practices in LRZ and MRZ, GRDC has invested in research led by Agriculture Victoria, in partnership with the South Australia Research and Development Institute (SARDI). This report discusses the benefits of variety selection and proactive management strategies during a high-pressure year. This article reports on three experiments conducted in different regions in Victoria during 2022.
Aim
- To determine yield loss caused by STB in the MRZ and the LRZ.
- To understand how seasonal conditions influence STB development and spread.
- To identify optimal fungicide application timing should STB control be required.
Paddock Details
Location: Nullawil
Crop year rainfall (Nov–Oct): 497mm
GSR (Apr–Oct): 384mm
Soil type: Clay
Paddock history: Lentil
Location: Longerenong
Crop year rainfall (Nov–Oct): 651mm
GSR (Apr–Oct): 547mm
Soil type: Clay
Paddock history: Faba bean
Location: Hamilton
Crop year rainfall (Nov–Oct): 749mm
GSR (Apr–Oct): 636mm
Soil type: Clay loam
Paddock history: Lentil
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: Nullawil: 4 May, Longerenong: 28 April, Hamilton: 28 April
Replicates: Six (Variety selection and Fungicide timing)
Harvest date: Nullawil: 15 December, Longerenong: 17 December, Hamilton: N/AÂ
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
1. Experiment 1: Seasonal conditions influence on STB development and spreadÂ
Susceptible wheat inoculated with STB infected stubble was grown at three locations in Victoria (LRZ, MRZ and HRZ). 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
2. Experiment 2: Variety selection – yield lossÂ
At 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. Assessments included scoring for STB in-season, yield and grain quality.Â
3. Experiment 3: Fungicide timing
At 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 1b). Assessments included scoring for STB in-season, yield and grain quality.
Results & Interpretation
1. Seasonal conditions influence on STB development and spread (Experiment 1)
STB severity was monitored at three locations (LRZ, MRZ and HRZ) during 2022 (Figure 1). Unlike 2021, initial disease development was delayed until stem elongation at both the LRZ and MRZ sites, due to late outbreak. Disease severity levels increased rapidly during the season at the MRZ location (Figure 1), and were similar to the HRZ, which was directly related to high rainfall in the region. During 2022, both the amount and frequency of rainfall (Table 2) favoured STB, enabling it to progress to the top of plants, causing severe disease and potential yield and quality impacts.
Limited STB developed at the LRZ site, but was still more than in 2021. Relatively moderate severity was recorded later in the season at milk development stage. This was higher than 2021 at the same time of the year (Figure 2). Wheat varieties with moderate resistance grown in these conditions are more likely to escape yield losses.Â
2. Variety selection: yield loss (Experiment 2)
Severe STB developed at the Longerenong (MRZ) site in all varieties, reducing grain yield by 17–43 per cent (Table 3). Losses were greater than the <10 per cent measured during 2021, highlighting the role of rainfall in STB infection. Disease development and yield loss correlated with varietal resistance, demonstrating the benefits of avoiding highly susceptible cultivars in the management of STB. In the susceptible to very susceptible varieties (S or SVS) grain yield losses of around 35–43 per cent, or ~1.7t/ha were measured. Losses in less susceptible varieties (LRPB Lancer (MS) or Hammer CL Plus (MSS)) were 21 per cent or less. STB infection also reduced grain quality of most varieties with increased screenings, reduced retentions and lower seed weight when not controlled (Table 4).
A heavy infestation of stripe rust at the Nullawil site compromised the ability to draw conclusions on the yield impact of STB in the LRZ during 2022 (for more details, see ‘Stripe rust management during a high-pressure season’ p 92). However, based on the extent of STB development measured at this site it is unlikely significant yield losses due to STB would have occurred there during 2022.Â
3. Fungicide timing (Experiment 3)
Fungicides provided variable control of STB in a field experiment conducted at Longerenong (MRZ) (Table 5). All fungicide treatments reduced STB severity compared to the untreated control (UTC). Early applications to seed or a single foliar application at Z31 were not as effective as the treatments that included a fungicide application at Z39. Within this trial, the best economic control of STB was achieved by two foliar fungicide applications at Z31 and Z39 (with or without a seed treatment) which increased grain yield 37–39 per cent. The two-spray strategy also improved Scepter grain quality by reducing screening percentage (~5 per cent) and increasing retention percentage (~17 per cent) compared to untreated control (Table 6).
Commercial Practice and On Farm Profitability
STB has become a common foliar disease of wheat in both the medium (Wimmera) and low (Mallee) rainfall zones.
Historically, losses due to STB were most common in susceptible cultivars in the high rainfall zone. However, Agriculture Victoria trials demonstrated large losses (i.e ~35–43 per cent in the MRZ) are possible in exceptionally wet conditions. In the MRZ, 2022 was the wettest August to November period in more than 100 years which created highly favourable conditions for STB development. This season clearly demonstrated avoiding highly susceptible cultivars can greatly reduce the potential impact of this disease.
Fungicides have been shown to effectively suppress STB infection but 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. However, if susceptible varieties are grown during seasons with wet conditions — like 2022 — fungicide strategies are most effective when applications are at both growth stages Z31 and Z39. This cangive up to 37 per cent more grain than the untreated control. It is also worth noting the STB fungal population has the potential to develop resistance to fungicides and unnecessary use should be avoided.
This study has so far shown STB rarely causes losses in the Mallee environment, and this was evident at Nullawil where low-moderate severity developed, despite the trial site receiving above average rainfall. However, high stripe rust pressure during 2022 in the Mallee had a confounding effect on results, and more research is needed.
In summary, avoid highly susceptible varieties (especially those rated S and SVS) to minimise losses from STB. If conditions are suitable for STB, fungicide strategies should include applications at both growth stages Z31 and Z39. Avoid sowing wheat into paddocks with one- or two-year-old wheat stubble, noting early STB infection will also come from wind-borne spores out of 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 BCG team for technical and scientific support. Thanks also to our research collaborators Andrew Milgate (NSW DPI), Julian Taylor (University of Adelaide) and Tara Garrard (SARDI).
