Canola disease update

 

In general, disease levels were low across southern NSW despite the wet conditions. Widespread use of foliar fungicides in high disease risk districts was effective in managing the disease. 

How does the disease 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:

• Softening and germination of soil borne sclerotia. 
• Apothecia development and release of ascospores.
• Infection of petals by air-borne ascospores. 
• Senescence of infected petals in the presence of moisture and subsequent stem infection.

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.

 

Research findings in 2016

Commercial canola crops and trial sites were monitored for the development of sclerotinia stem rot in high sclerotinia risk districts in 2016. These crops were located in southern NSW and northern Victoria where the disease is an annual problem. Consistent with the previous year’s results, observations within these crops found a very strong relationship between prolonged periods of leaf wetness and stem rot development. 

In addition, a small scale petal survey was conducted across southern NSW and northern Victoria in 2016. The aim of this survey was to investigate the relationship between petal infestation with the sclerotinia fungus and stem rot development. 

Stem infection

Infection levels at disease monitoring sites were generally low (<10%). Despite above average rainfall, foliar fungicides applied by growers were effective at keeping potential disease levels low. However, some reports were received of higher levels of stem infection in some commercial crops, depending on where rainfall events occurred and crop growth stage. Consistent with previous seasons, results showed that extended periods of leaf wetness of at least 48 hours were most effective for the development of stem infection within crops.

Petal testing

For the second year, a petal survey was conducted in southern NSW and northern Victoria. The highest levels of petal infestation (>90%) were detected in crops grown in higher rainfall districts with a high frequency of canola. Crops further west had reduced levels of infestation in general (<60%), with levels of infestation fluctuating with environmental conditions. These results are consistent with the previous year’s findings. 

Every crop that was tested in central and southern NSW and northern Victoria returned a positive result at some stage during the growing season, but not every crop developed symptoms of stem rot. This confirms the wide distribution of the sclerotinia fungus, but also the importance of environmental factors on the development of the disease.

Once again, the results confirm findings that were observed in research from 10 years ago which found no direct correlation between the numbers of canola petals infested with the sclerotinia pathogen and stem rot development within the crop. This confirms the importance of leaf wetness within the crop canopy as the driving factor behind development of stem rot.

Where did the disease occur in 2016?

Traditionally, sclerotinia outbreaks are sporadic in southern NSW and northern Victoria and usually restricted to those districts with a history of sclerotinia, high intensity of canola and reliable spring rainfall. Due to above average spring rainfall in 2016, outbreaks of the disease were widespread and in districts that rarely see the disease develop. Damaging levels of sclerotinia were largely restricted to the ‘traditional’ districts that frequently see the disease, while in lower rainfall districts the disease was observed but did not significantly affect canola yields in most cases.

What are the indicators that sclerotinia stem rot could be a problem in 2017?

• Spring rainfall. Epidemics of sclerotinia stem rot generally occur in districts with reliable spring rainfall and long flowering periods for canola.
• Frequency of sclerotinia outbreaks. Use the past frequency of sclerotinia stem rot outbreaks in the district as a guide to the likelihood of a sclerotinia outbreak. Paddocks with a recent history of sclerotinia are a good indicator of potential risk, as well as those paddocks that are adjacent. Also consider the frequency of canola in the paddock. Canola is a very good host 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 which flower earlier in winter (late June-July) are more prone to disease development and exposure to multiple infection events. 

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

There are a number of steps that can be taken to reduce the risk of sclerotinia:

• 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.
• 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 break crops such as cereals, field peas or faba beans.
• Follow recommended sowing dates and rates for your district. Be aware of the maturity rating of the variety and time of sowing. Early flowering crops are more prone to developing sclerotinia stem rot by increasing opportunities for infected petals to lodge in a wet crop canopy. Wider row spacings can also help by increasing air flow through the crop canopy to some degree and delaying the onset of canopy closure.
• 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.
• Monitor crops for disease development and identify the type of stem infection. Main stem infections cause the most yield loss and indicate infection events early in the growing season. Lateral branch infections cause lower levels of yield loss and indicate infection events later in the growing season.

Use of foliar fungicides

At this time, there are no commercial canola cultivars available on the Australian market with resistance to sclerotinia stem rot. Management of the disease relies on the use of cultural and chemical methods of control. Foliar fungicides should be considered in those districts which are at a high risk of disease development, such as districts where the disease frequently occurs, long flowering period and reliable spring rainfall. There are several foliar fungicides currently registered for use in Australia to manage sclerotinia stem rot including Prosaro^® and products containing procymidone or iprodione. 

 

To maximise the economic benefits from foliar fungicides, consider the factors that lead to disease development. All three factors (host, pathogen and environment) need to coincide for the disease to develop. Figure 1 shows those factors that lead to stem rot development.

Figure 1. Host, pathogen and environment factors that lead to stem rot disease development.

Points to consider when using a foliar fungicide to manage sclerotinia stem rot

• The most yield loss from sclerotinia occurs from early infection events. Early infection is likely to result in premature ripening of plants that produce little or no yield.
• Plants become highly susceptible to infection once flowering commences. Research in Australia and Canada has shown that an application of foliar fungicide around the 20%-30% bloom stage (20% bloom is 14-16 flowers on the main stem, 30% bloom is approx. 20 flowers on the main stem) can be effective in significantly reducing the level of sclerotinia stem infection. Most registered products can be applied up to the 50% bloom (full bloom) stage.
• 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 lower leaves, leaf axils and stems). Timing of fungicide application is critical. 
• A foliar fungicide application is most effective when applied before an infection event, such as before a rain event during flowering. These fungicides are best applied as protectants and have no curative activity.
• In general, foliar fungicides offer a period of protection of up to three weeks. After this time, the protectant activity of the fungicide is compromised. In some crops, development of lateral branch infections later in the season is not uncommon if conditions favourable for the disease continue. The greatest yield loss occurs when the main stem becomes infected, especially early. Lateral branch infection does cause yield loss, but at a much reduced level.
• Use high water rates and fine droplet sizes for good canopy penetration and coverage. Weigh up the benefits of ground rig application versus aircraft. 
• Fungicide choice is often secondary to timing of application.
• Be aware that the maximum number of applications of Prosaro^® in a season is two.

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

 

Useful resources

NSW DPI Winter Crop Variety Sowing Guide (disease updates, variety resistance, fungicide products).
NSW DPI Southern NSW Research Results 2015 
NSW DPI Twitter account

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the author would like to thank them for their continued support.
The authors also wish to thank NSW DPI for investment into this research.

Contact details

Kurt Lindbeck (Pulse and Oilseed Pathologist)
NSW Department of Primary Industries, Wagga Wagga Agricultural Institute 
02 69 381 608
kurt.lindbeck@dpi.nsw.gov.au