Sclerotinia update

Key messages

• Outbreaks of sclerotinia stem rot are sporadic and depend on growing season conditions.
• Saturated canopy conditions for more than 48 hours during flowering favour the development of the disease.
• Current and adjacent paddocks with histories of sclerotinia disease in broadleaf crops over the past four years are a good indicator of potential risk for the current season’s crop.
• The frequency of canola or lupin in a paddock is very important in determining the risk of a sclerotinia outbreak, as these crops are very good hosts for the disease and can quickly build up levels of soil-borne sclerotia.
• For main stem protection foliar fungicides for management of the disease are best applied at 20–30% bloom (15-20 flowers off the main stem).

How does sclerotinia develop?

The complexity of the disease cycle of sclerotinia stem rot results in disease outbreaks being sporadic compared to other diseases. There are several key stages that must be synchronised and completed in order for plant infection to occur.  Weather conditions must be suitable for the pathogen at each stage. These stages of development include:

1. Softening and germination of soil borne sclerotia
2. Apothecia development and release of ascospores
3. Infection of petals by air-borne ascospores
4. Senescence of infected petals in the presence of moisture and subsequent stem infection
5. Weather conditions during flowering play a major role in determining the development of the disease. The presence of moisture during flowering and petal fall will determine if sclerotinia stem rot develops. Dry conditions during this time can quickly prevent development of the disease, hence even if flower petals are infected, dry conditions during petal fall will prevent stem infection development.

What drives the development of sclerotinia stem rot?

• Frequency of sclerotinia outbreaks. Past frequency of sclerotinia stem rot outbreaks in the district can guide the likelihood of sclerotinia developing this season. Paddocks with a recent history (past five years) of sclerotinia outbreaks are a good indicator of potential risk, as well as adjacent paddocks. The frequency of canola and lupin in the paddock can also increase disease risk. Canola and lupin are very good hosts for the disease and can quickly build up levels of soil-borne sclerotia.
• Commencement of flowering. The commencement of flowering can determine the severity of a sclerotinia outbreak.  Spore release, petal infection and stem infection have a better chance of occurring when conditions are wet for extended periods, especially for more than 48 hours.  Canola crops that flower earlier in winter (late June–July) are more prone to disease development and exposure to multiple infection events.
• Spring rainfall. Epidemics of sclerotinia stem rot occur in districts with reliable late winter and spring rainfall and long flowering periods for canola. These provide long periods of canopy wetness necessary for the disease to develop, at least 48 hours or more. Overnight dews do not generally trigger epidemics of the disease.

Key points for low-medium rainfall districts

• Compared to high rainfall districts, serious outbreaks of sclerotinia stem rot will be highly sporadic, once in very 5 – 7 years, often in years of above average rainfall.
• Background levels of sclerotia are likely to be much less, due to less frequent outbreaks of the disease, reducing disease pressure.
• Shorter flowering periods for canola reduces the opportunity for the disease to develop to damaging levels and crops canopies are less bulky.
• The intensity of canola in the system is often less, reducing high disease risk situations from high inoculum loads.

Pre-sowing sclerotinia management

Crop rotation

• Rotate canola once in every four to five years to reduce build-up of sclerotia.
• Incorporate lower-risk crops into the crop rotation e.g., cereals, field pea and faba bean.
• Separate the previous year’s canola stubble from new season crops by at least 500m.
• Ascospores spread within 100m to 400m of the apothecia.

Burning

• Burning of stubbles and windrows will kill some sclerotia but will not significantly reduce the risk of disease.

Clean seed

• Where possible always use seed free of sclerotia.
• If in doubt about retained seed grade before sowing to remove sclerotia.
• Grain receival standards allow a maximum of 0.5% sclerotes per sample.

Variety selection

• There are no Australian canola varieties with known resistance to sclerotinia. Some differences may be observed in the level of stem rot in some seasons. This is likely to be related to the timing of flowering and infection events.

Crop management

• Always follow the recommended sowing time and seeding rate for your region.
• Early maturing varieties sown early can be prone to developing stem infection due to flowering commencing earlier when conditions are wet for prolonged periods
• Once flowering starts, the crop becomes susceptible to infection and prolonged exposure to infested senescent petals means greater chance of stem infection.
• Bulky crop canopies can retain more moisture and are conducive for the development of stem infections.
• Wider row spacing or reduced seeding rates can increase airflow through the canopy, reducing moisture retention and potential for infection.

Use the SclerotiniaCM app (see useful resources) to determine the most appropriate management strategies for your district.

Post-sowing sclerotinia management – fungicide application

• Use foliar fungicides to prevent early stem infection via infested petals.
• Always use fungicide products that are currently registered in Australia.
• Timing of foliar fungicide application is more important than choice of fungicide product in reducing potential levels of stem infection.
• Foliar fungicide application is most effective before an infection event.
• Applying foliar fungicide at 20–30% bloom stage is most effective in reducing main stem infection and most yield loss as it protects early petals from infection and penetration of fungicide product into the crop canopy to protect potential infection sites from falling petals.
• Multiple foliar fungicide applications may be needed in high-risk-disease districts with a high yield potential. Applications at both 10–20% and 50% bloom provide critical early and follow up protection from multiple infection events.
• Use high water rates (at least 100 litres per hectare) to achieve good coverage and penetration into the canopy.
• Foliar fungicides generally have an active life of two to three weeks. The protection provided may wear off during the critical infection period or where crops have an extended flowering period. A single fungicide application applied too early may be ineffectual.
• Foliar fungicides will have no effect on managing basal infections, as these occur below the soil surface and beyond the activity of foliar fungicides.

Always

• Determine disease risk as the crop enters the flowering period.
• Assess bloom stage, seasonal conditions and weather forecasts to identify the potential risk to the crop.
• Identify how many consecutive wet days are forecast as the crop commences flowering and in the week ahead, especially consecutive wet days of 48 hours or more.
• Monitor crops for disease development and identify the types of infection. Basal and main stem infections cause the most yield loss.

Useful resources

NSW DPI Winter Crop Variety Sowing Guide (Disease updates, variety resistance, fungicide products).
SclerotiniaCM App for iPad and android tablets

Acknowledgements

The authors wish to thank NSW DPI and GRDC for investment into this research.

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