Growers should monitor the risk factors (outlined in the table below) for each paddock shortly after first flower (when 75% of canola plants have at least 3 open flowers). Moisture is the key factor in Sclerotinia stem rot risk. Risk will be greater in high rainfall areas and in low lying parts of the landscape. Extended wet periods in mid-late winter combined with humid/mild (20-25°C) conditions during petal drop will increase the Sclerotinia risk. If crops have high yield potential the economic payback from fungicide application is more likely.
RISK FACTORS CHECKLIST – Sclerotinia stem rot in canola
- Number of years since last canola crop.
- Disease incidence in last host crop (including sunflower, soybean, chickpeas).
- Crop density. Rain in last two weeks before first flower.
- Weather forecast (rain or high humidity) during and after flowering.
- Regional risk for apothecia development (presence of fruiting bodies that release spores in late winter).
The Sclerotinia threat tends to be location specific so blanket recommendations may result in inappropriate management. Sclerotinia apothecia (fruiting body) are quite small and they can be hard to find. Those spores that land on canola petals many result in stem infection but the reliability of petal tests alone is in question. Sclerotinia stem rot had emerged as a disease of concern in wetter seasons in parts of Australia. Regions that have experienced seasons with high levels of Sclerotinia stem rot affecting yields include northern NSW, south-eastern NSW, north-eastern Victoria, the Victorian western district around Geelong and parts of Western Australia around Esperance and Geraldton.
MANAGING SCLEROTINIA STEM ROT
- Clean seed – Sow only good quality seed that is free of Sclerotinia.
- Crop isolation – Avoid sowing canola into or next to paddocks that were heavily infected with Sclerotinia in the previous 3 years. The spores are airborne and it is preferable that crops be sown on the western side or ‘up wind’ from old canola stubbles or infected crops.
- Crop rotation – Close rotation of susceptible crops, such as lupins, may increase fungal inoculum build-up.
- Plant populations – The use of wider row spacing and lower plant population targets for your region can affect the canopy microclimate required for spore infection by Sclerotinia. Also avoid high fertiliser rates that may induce crop lodging.
- Varieties – Current Australian canola cultivars are not known to have resistance to Sclerotinia stem rot. Pioneer is the only seed company with Sclerotinia resistant canola hybrids although commercialisation of this proprietary native trait in Australia under the Pioneer Protector® brand is still pending.
- Assess yield loss – A common ‘rule of thumb’ used to estimate yield loss due to Sclerotinia is – Yield loss = half the percentage of the number of plants infected. For example, 10% yield loss = 20% plants collected are infected with Sclerotinia.
- Foliar fungicide use Fungicides are currently registered in Australia to manage the level of Sclerotinia stem rot. Due to the sporadic nature of the disease it is not economical to apply fungicides routinely. If conditions indicate risk (use checklist). Foliar fungicides should be applied between 20% and 50% flowering.
Key reference: “Managing Sclerotinia stem rot in canola”, GRDC August 2008