Take Home Messages
- Crown rot and Rhizoctonia root rot risk can be identified through PREDICTA®B soil testing before sowing.
- Risk levels can change with seasonal conditions.
- Management of Crown rot and Rhizoctonia root rot cannot be achieved with in-season foliar fungicides and requires an integrated approach to reduce inoculum build up and yield damage.
- Crown rot can infect seeds, as seen in 2022. Where this has occurred, avoid using the seed and find a source of clean seed.
Background
Greater retention of stubble in farming systems is known to increase the risk of certain diseases, including crown rot (Fusarium pseudograminearum) and Rhizoctonia root rot (Rhizoctonia solani) (Fischer, 2009). Yield loss from these diseases is variable across seasons and crop types but crown rot infection can cause up to 50 per cent loss in Durum wheat (Agriculture Victoria, 2020) and rhizoctonia infection can cause up to 50 per cent loss in barley (GRDC Gupta, 2016).
Disease inoculum survives in soils and plant material, requiring a longer-term approach to management, with foliar fungicides not available as an option for in-season control.
Crown rot is a stubble-borne disease and inoculum levels are greatest at the crown of a plant, which extends below the soil surface, making practices such as stubble burning insignificant for reduction.
Rhizoctonia is a soil-borne disease with inoculum surviving in plant roots as well as fungal hyphae in the soil (GRDC Gupta, 2016). Infection can be first identified as bare patches within paddocks. Closer inspection shows plants have spear tipped, pruned and browning roots.
Paddocks at risk of crown rot and/or Rhizoctonia can be identified before sowing using PREDICTA®B soil testing. This project identified two high-risk paddocks through PREDICTA®B testing for management research.
Aim
To demonstrate the effects of different management strategies on the severity and impacts of crown rot and Rhizoctonia in identified high-risk paddocks.
Paddock Details
Trial 1. Crown rot management
Location: Birchip
Crop year rainfall (Nov–Oct): 2020–2021: 256mm, 2021–2022: 521mm
GSR (Apr–Oct): 2021: 196mm, 2022: 407mm
Soil type: Clay loam
Paddock history: 2019 vetch hay, 2020 canola
Trial 2. Rhizoctonia management
Location: Manangatang
Crop year rainfall (Nov–Oct): 2020–2021: 173mm, 2021–2022: 533mm
GSR (Apr–Oct): 2021: 148mm, 2022: 462mm
Soil type: Sand
Paddock history: 2018 vetch (brown manure), 2019 barley, 2020 barley
Trial Details
Trial 1. Crown rot management
Crop type/s: 2021 Refer to Table 1, 2022 Scepter wheat
Treatments: Refer to Table 1
Target plant density: Cereals 130 plants/m², pulses 40 plants/m2
Seeding equipment: Knife points, press wheels, 30cm row spacing
Sowing date: 17 May 2021, 24 May 2022
Replicates: Four
Harvest date: 2 December 2021, No harvest 2022
Trial 2. Rhizoctonia management
Crop type/s: 2021 Refer to Table 2, 2022 Spartacus CL barley
Treatments: Refer to Table 2
Target plant density: Cereals 130 plants/m², pulses 40 plants/m2
Seeding equipment: Knife points, press wheels, 30cm row spacing
Sowing date: 13 May 2021, 17 May 2022
Replicates: Four
Harvest date: 2 December 2021, 2 December 2022
Trial Inputs
Fertiliser: Crown rot trial managed as per best practice in 2021 and as per Table 1 in 2022. Rhizoctonia trial managed as per best practice to keep nutrients a non-limiting factor across the trial period.
Herbicide/insecticide: Trials managed to be kept weed and pest-free.
Fungicide: Refer to Table 1 and Table 2.
Seed treatment: Refer to Table 1 and Table 2.
Method
Sites were chosen using PREDICTA®B testing to assess risk levels of crown rot and Rhizoctonia. At the relevant site, only crown rot or Rhizoctonia in isolation were considered a crop risk to reduce the chance of confounding results with other disease issues.
Two replicated trials were established in 2021 in randomised complete block designs. In 2022, these sites were oversown with additional treatments, resulting in two split plot trials.
Trials were assessed each season for disease presence, inoculum levels and yield.
Results & Interpretation
2021
PREDICTA®B sampling in 2021 before sowing identified two paddocks appropriate for trial work with risky levels of crown rot and Rhizoctonia at their relevant sites and lower levels of other pathogens (Table 3).
The 2021 season started with moist conditions for sowing and establishment, producing strong early growth. Dry conditions during August saw some crop stress. however late spring rainfall allowed for a soft finish (Figure 1).
In 2021, disease symptoms were observable but insignificant at both the Birchip and Manangatang sites from visual scoring (P>0.05) (data not shown). Root scoring of plants removed from plots at the Manangatang site on 2 August 2021 found increased symptoms of Rhizoctonia root rot in Spartacus CL plots without any seed treatment or management applied compared to all other treatments. However, disease severity was still low (Figure 2).
Yield results from 2021 at both sites found significant yield differences only due to variety and crop type, but none of the applied treatments (data not shown).
After the first year of treatments, PREDICTA®B sampling on a treatment basis found differences in inoculum levels, despite low levels of disease expression through the season (Table 4a and 4b).
After one year of treatments at the crown rot site, inoculum levels increased for all treatments with the exception of Morava vetch, the only non-host break crop included in the treatments (Table 4a).
Results from the Rhizoctonia site at Manangatang suggest a general reduction in inoculum levels across treatments, however all results returned a high risk for infection. PBA Hurricane lentil treatments had one of the lowest inoculum measurements. As a non-host break crop, this was expected (Table 4b). Overall inoculum level reduction at the site may have been due to seasonal conditions.
2022
The 2022 season brought decile 10 rainfall for Birchip and highest on record growing season rainfall at Manangatang. While most GSR fell in September and October, the season started with a good break, allowing trials to be sown into moisture and establish well (Figure 3).
Favourable conditions for growth, moist soils and adequate nutrition resulted in no above ground Rhizoctonia symptoms at the Manangatang trial site in the 2022 season. When roots were assessed, very minimal discolouration, pruning or spear tipping was present across treatments, with no significant difference found between treatments (data not shown). This was not surprising with adequate soil moisture and highest on record rainfall for the growing season at the site allowing plants to grow through Rhizoctonia disease pressure.
Unfortunately, the crown rot trial was heavily grazed before harvest and scoring for white heads and yield data could not be collected.
Post-harvest PREDICTA®B sampling was performed at both sites, despite the lack of in-season results, to help understand the effects of treatments following a second season.
Results indicate a general decline in rhizoctonia inoculum levels across the trial with no consistent results in relation to the in furrow fungicide treatment applied in the 2022 season and 2021 treatments with average site results changing from 1.9 log(pg DNA/g soil) following year one of treatments to 1.5 log(pg DNA/g soil). The lower risk of rhizoctonia following the 2022 is likely to be linked with the high rainfall received and moist soil conditions.
Crown rot risk remained moderate to high following the second year of treatments with the exception of the oversown Morava treatment (high N) returning a low risk result (1.2 log(pg DNA/g soil)). All other treatments returned a moderate to high risk of yield loss. While this highlights the importance of rotation in controlling crown rot risk, variability in results (comparing the Morava low N treatment that returned 3.1 log(pg DNA/g soil)). Similarly to the Rhizoctonia site, overall crown rot inoculum levels reduced from 2021 to 2022 however, not by enough to reduce yield loss risk significantly. This confirms that reducing crown rot inoculum level is a long term strategy that requires more than one season of break to be effective.
Commercial Practice and On Farm Profitability
Controlling crown rot and Rhizoctonia requires an integrated approach to reduce inoculum levels and control infection. Understanding what drives the risk of crop damage and yield loss are key factors in identifying useful management tools.
Soil testing
- PREDICTA®B soil testing is the initial step to identify the presence of pathogens in soil and the potential level of risk of yield loss in a susceptible crop.
Soil type
- Rhizoctonia is more prominent in low fertility sandy soils and is often isolated to sandy rises within paddocks. Rhizoctonia is more severe in compacted and dry soils (GRDC Huberli, 2016).
- Crown rot can be an issue in most soil types.
Summer weed control
- Host plants of Rhizoctonia and crown rot can build inoculum over the summer (Agriculture Victoria, 2020). Good grass weed control is essential to inhibit inoculum levels developing in a break crop rotation and over the summer fallow period.
Crop rotation
- Crown rot can infect multiple host crops, however yield damage is not seen across all these crops. Severity in damage by crop type is as follows:
Durum wheat > bread wheat > barley > oats - All cereals are susceptible to infection by Rhizoctonia, with the severity of damage across crop types as follows:
Barley > wheat > oats - Break crops (pulses and oilseeds) are known to reduce inoculum levels in the soil for the following crop and should be part of the rotation for paddocks with a high risk of crown rot and Rhizoctonia
Variety selection
- There is some variability in the resistance of wheat varieties to crown rot (GRDC Brown, 2022). Resistance is only effective in healthy plants — once the crop becomes stressed or begins to senesce at the end of the season, crown rot infection will take hold regardless of resistance level.
- There is no genetic resistance to Rhizoctonia solani (GRDC Gupta, 2016).
Seed source
- Crown rot can infect grain, which is known as fusarium head blight (FHB). This occurs either from airborne spores or infection that has made its way up the stem.
- FHB was evident in grain across the Wimmera and Mallee in 2022. This grain should not be used as a seed source to limit further infection and spread of crown rot (Agriculture Victoria, 2022).
Stubble retention
- Increased stubble retention and low disturbance systems result in stubble being broken down over a longer duration (Scott et al., 2010). As a stubble-borne disease, crown rot remains an issue in stubble-retained paddocks where infected stubble has not yet broken down (Agriculture Victoria, 2020).
- Low disturbance systems reduce the physical breakage of fungal hyphae in the soil allowing Rhizoctonia to spread more easily through the top 5cm of soil (GRDC Huberli, 2016).
Seeding equipment and sowing
- Rhizoctonia fungal hyphae grow in the soil and are normally found within the top 5cm of soil. Placing seed below this point increases the chance of seedlings establishing roots below the high-risk infection zone (Agriculture Victoria, 2020).
- Seeding equipment with low soil disturbance, such as a disc seeder, reduces how much fungal hyphae is broken up during sowing, allowing for better survival and growth of the pathogen in the top soil.
- In a known crown rot paddock, inter-row sowing may help reduce infection by placing the seed further from the inoculum source (GRDC Hollaway, 2019).
Chemical options
- There are no in-season foliar fungicides to control Rhizoctonia root rot or crown rot (Agriculture Victoria, 2020). Some fungicide options registered for suppression or control can be applied either in-furrow or as seed treatments.
- A new option for control of crown rot in seed treatments is Victrato®, which was included in the second year of treatments at the Birchip site.
Seasonal conditions
- Yield loss from crown rot is likely to be worse in seasons with a good start and a dry finish.
- Crown rot infects the crown tissue, restricting movement of water throughout the plant. Moist conditions favour the growth of Fusarium graminearum early in the season through the soil from infected stubble (GRDC Hollaway, 2019).
- Rhizoctonia fungus is competitive in dry conditions but is easily outcompeted by other microbes when conditions are moist.
- Research has found 20mm of rainfall will effectively reduce rhizoctonia levels in the soil within a week (GRDC Gupta). Soils with larger background populations of suppressive microbes will reduce this level more effectively than those without.
Nutrition management
- Rhizoctonia can thrive in low nutrient conditions where other microbes in the soil cannot (GRDC Huberli, 2016). Managing in-season nutrition adequately can keep crops healthy and competitive, reducing the yield impacts of infection.
- For crown rot, where nitrogen is applied to a susceptible crop and high biomass is the result, inoculum levels will increase significantly, producing higher stubble loads following harvest.
- If high biomass produced early in the season is followed by a dry spring, plants may become prematurely moisture stressed and be more severely affected by crown rot, whereas a crop with smaller biomass may still have remaining subsoil moisture (GRDC Hollaway, 2019).
- Zinc deficiency in plants is known to increase the expression of crown rot and severity of damage. However, increasing zinc levels beyond plant requirements will not result in greater protection (Agriculture Victoria 2020).
Conclusion
While limited data was collected due to lack of disease expression and unplanned grazing across the two years of trial work, PREDICTA®B testing showed a change in inoculum levels at both sitesduring this time. Removing the host of these pathogens clearly has the greatest impact on reducing inoculum levels, highlighting the importance of well-managed crop rotations. Identifying high-risk paddocks is the first step towards successful management. While there have been some advancements in chemistry, controlling these diseases requires an integrated approach to reduce inoculum levels and crop risk.
References
Agriculture Victoria (AgVic), 2020, Field Crop Diseases Victoria, ‘Crown rot’, Available online at <https://extensionaus.com.au/FieldCropDiseasesVic/docs/identification-management-of-field-crop- diseases-in-victoria/soil-borne-diseases/crown-rot/>.
Agriculture Victoria (AgVic), 2020, Field Crop Diseases Victoria, ‘Rhizoctonia Root Rot’, Available online at <https://extensionaus.com.au/FieldCropDiseasesVic/docs/identification-management-of-field-crop- diseases-in-victoria/soil-borne-diseases/rhizoctonia-root-rot/>.
Agriculture Victoria (AgVic), 2022, ‘Disease carryover between seasons – Managing seed and hay affected by fungi’, Available online at <https://extensionaus.com.au/FieldCropDiseasesVic/disease- carryover-between-seasons/>.
Fischer R. A., 2009, ‘Farming systems of Australia: Exploiting the synergy between genetic improvement and agronomy’, In: Sadras, V. and Calderini, D. (Eds), editor/s. Applied Crop Physiology; Boundaries with genetic improvement and agronomy, pp. 23–54, Available online at <https://publications.csiro.au/rpr/ pub?list=BRO&pid=changeme:3597>.
GRDC Brown S., 2022, ‘Victorian Crop Sowing Guide’, pp. 7–27, Available online at <https://grdc.com. au/resources-and-publications/all-publications/nvt-crop-sowing-guides/2021/2022-vic-crop-sowing- guide.pdf>.
GRDC Gupta V., 2016, ‘Tips and Tactics – Rhizoctonia Southern Region’, Accessible online at <https://grdc.com.au/__data/assets/pdf_file/0025/170386/grdc_tips_and_tactics_rhizoctonia_ southern_web.pdf.pdf>.
GRDC Hollaway G., 2019, ‘Crown Rot – Southern Fact Sheet’, available online at <https://grdc.com. au/resources-and-publications/all-publications/factsheets/2019/crown-rot-southern/GRDC_FS_ CrownRotSouth_1902_13-002.pdf?utm_source=website&utm_medium=download_link&utm_ campaign=pdf_download&utm_term=South&utm_content=Crown%20Rot%20-%20Southern>.
GRDC Huberli D., 2016, ‘Tips and Tactics – Rhizoctonia Western Region’, Accessible online at <https://grdc.com.au/__data/assets/pdf_file/0018/170343/grdc_tips_and_tactics_rhizoctonia_ western_web.pdf.pdf>.
Scott B., Eberback P., Evans J. and Wade L., 2010, Stubble Retention in Cropping Systems in Southern Australia: Benefits and Challenges, pp. 1–51, Available online at <https://www.csu.edu.au/__data/ assets/pdf_file/0007/922723/stubble-retention.pdf>.
Acknowledgements
This research was funded by the GRDC project ‘Soilborne Pathogen Identification and Management Strategies for Winter Cereals’ (FLR1912-003RTX).
