Sclerotinia stem rot - Managing the disease in 2013

Kurt Lindbeck
NSW - Department of Primary Industries, Wagga Wagga Agricultural Institute, Wagga Wagga, NSW 2650

Take home messages

• Canola crops in high rainfall areas should be monitored closely for sclerotinia stem rot.  Good spring growing conditions for canola usually means good conditions for sclerotinia development. 
• Paddocks with a recent history of sclerotinia (in crops other than canola) should be avoided from growing canola in 2013.
• Consult the Managing Sclerotinia Stem Rot in Canola guide for further information.  This publication is available from the GRDC website.

Why don’t I see sclerotinia stem rot every year?

The disease cycle of sclerotinia stem rot is complex compared to other plant diseases which is why the disease tends to be sporadic in appearance between years and districts.  For sclerotinia stem rot to develop soil moisture and temperature conditions have to be favourable for fruiting bodies to develop and release ascospores. This has to occur in the presence of a flowering canola crop for petal infection to occur, and leaf wetness is essential for the ascospores to colonise flower petals.  Moisture also has to be present for fallen flower petals to become lodged and infect canola stems. 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 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.

How does the disease develop?

The fungal pathogen that causes sclerotinia stem rot is called Sclerotinia sclerotiorum.  This fungus can infect over 300 plant species, mostly broadleaf plants, including many crop, pasture and weed species. This includes plants like canola, lupin, pulses, sunflower, lucerne, cape weed, and shepherds purse. The main feature of the sclerotinia stem rot pathogen is the production of hard, black, survival bodies on infected plant tissue called sclerotia which enable the fungus to survive for up to 8 years in the field.

The main features of the disease are:

1. Airborne spores of the fungus that are released from apothecia (small, golf tee shaped structures, 5 – 10 mm in diameter) which germinate from sclerotia in the soil. For this to occur, prolonged moist soil conditions in combination with moderate temperatures of 15°C to 25°C are considered ideal. Only sclerotia in the top few centimetres of the soil will germinate and produce apothecia. Sclerotia that are buried deeper will remain dormant for extended periods and may germinate if moved closer to the soil surface. Most sclerotia will remain viable for up to 3 – 4 years then survival slowly declines.
2. Spores of the sclerotinia pathogen cannot infect canola leaves and stems directly. They require petals as a food source for spores to germinate, grow and colonise the petal. When the petal eventually drops, it may become lodged in a leaf axil or at branch junctions along the stem. If conditions are moist the fungus grows out of the petal and invades healthy plant stem tissue which will result in a stem lesion and production of further sclerotia within the stem which will be returned to the soil after harvest.
3. Sclerotia also have the ability to germinate in the soil, produce mycelium and directly infect canola plants in close proximity, causing a basal infection. These plants are identified in the field by having infection at the stem base and often infection extending into the upper taproot. In the past this type of infection was generally rare. In 2012, higher levels of this type of infection were observed, suggesting that some paddocks have a high level of viable sclerotia present.

Where did the disease occur in 2012?

In 2012 epidemics of sclerotinia in southern NSW were confined to high rainfall districts which received good rainfalls in spring. These included districts east of Cootamundra, south of Henty and around Corowa and Howlong. Infection levels observed in some crops were as high as 30 – 60%.  In other districts, crop infection levels were generally low with most infections developing in late spring with later rainfall events.

If I had sclerotinia in my canola crop last year, what should I do this season?

One of the biggest challenges in managing sclerotinia stem rot is deciding whether or not there is a risk of disease development. Research in Australia and Canada has shown that the relationship between the presence of the pathogen (as infected petals) and development of sclerotinia stem rot is not very clear due to the strong reliance on moisture for infection and disease development.

Epidemics of sclerotinia stem rot generally occur in districts with reliable spring rainfall and long flowering periods for canola.  Paddocks with a recent history of sclerotinia are a good indicator of potential risk, as well as those paddocks that are adjacent.

Important management options include:

1. Sowing canola seed that is free of sclerotia. This applies to growers retaining seed on farm for sowing.  Consider grading seed to remove sclerotia that would otherwise be sown with the seed and infect this season’s crop.
2. Separate this season’s paddock away from last year’s canola stubbles. Not only does this work for other diseases such as blackleg, but also for sclerotinia.
3. Rotate canola crops. Continual wheat/canola rotations are excellent for building up levels of viable sclerotia in the soil. A 12 month break from canola is not effective at reducing sclerotial survival. Consider other low risk crops such as cereals, field pea or faba bean.
4. Follow recommended sowing dates and rates for your district.  Canola crops which flower early, with a bulky crop canopy are more prone to developing sclerotinia stem rot.  Bulky crop canopies retain moisture and increase the likelihood of infection.  Wider row spacings can also help by increasing air flow through the canopy to some degree until the canopy closes.
5. Consider the use of a foliar fungicide.  Weigh up yield potential, disease risk and costs of fungicide application when deciding to apply a foliar fungicide.

When is the best time to apply a foliar fungicide?

Research in Australia and Canada has shown that an application of foliar fungicide around the 20% - 30% flowering stage can be effective in reducing the level of sclerotinia infection. The objective of the fungicide application is to prevent early infection of petals while ensuring that fungicide also penetrates into the lower crop canopy to protect potential infection sites (such as leaf axils and stems). The chemical is only active on that plant tissue that is present at the time of spraying. The fungicide will not protect petals that emerge after spraying, but some fungicide coverage within the canopy will help to restrict infection. Timing of fungicide application is critical.  Often the fungicides are applied too late, resulting in poor sclerotinia control. Growers and advisers should also be aware that foliar fungicides will have no affect on managing basal infection by sclerotinia, as this infection occurs under the soil surface and beyond the activity of foliar fungicides.

Consult the Managing Sclerotinia Stem Rot in Canola guide for further information.  This publication is available from the GRDC website.

Contact

Kurt Lindbeck, NSW Department of Primary Industries, Wagga Wagga Agricultural Institute
02 69 381 608
kurt.lindbeck@dpi.nsw.gov.au