Blackleg: developing a prioritised management strategy

Kurt Lindbeck¹, Steve Marcroft², Angela van de Wouw^2,3 and Vicki Elliott²

¹NSW-Department of Primary Industries, Wagga Wagga Agricultural Institute, Pine Gully Road, Wagga Wagga, NSW 2650;
²Marcroft Grains Pathology P/L, Grains Innovation Park, Horsham, Vic. 3400, Australia;
³The University of Melbourne

Why is blackleg difficult to manage?

Blackleg, caused by the pathogen Leptosphaeria maculans, is the most damaging disease of canola and juncea-canola in Australia. The disease is difficult to control and many growers accept yield loss from blackleg as a normal part of canola production.

Blackleg is challenging to control as the pathogen:

• Survives on stubble resulting in higher levels of spore release in districts with intensive canola production,
• Spreads via windborne and rain-splashed spores resulting in inoculum being spread extensively and quickly,
• Grows systemically within the plant resulting in limited efficacy of foliar applied fungicides,
• Reproduces sexually resulting in diverse pathogen populations that can overcome resistance genes within a few years,
• Individual blackleg isolates that are virulent against an individual canola cultivar will increase in frequency quickly if that particular cultivar is grown for a number of years in a row, resulting in resistance being overcome. In this scenario other canola cultivars that have different resistance genes will maintain their resistance.

What do we know about the interaction between the blackleg pathogen and canola resistance?

1. When individual canola cultivars (with specific resistance sources) are grown on a large scale and over a number of years, the blackleg isolates that are virulent (can infect) against that particular resistance gene dramatically increase in frequency, resulting in increased disease development.
2. When a cultivar based on a different source of resistance is sown in areas where another source of resistance has been extensively sown, this cultivar is likely to have fewer blackleg symptoms, as the majority of blackleg isolates are virulent towards the previously-grown source of resistance.
3. Newly deployed novel sources of resistance remain effective for a number of years before virulent blackleg isolates increase in frequency to a level where significant disease results.
4. Resistance sources that have become ineffective may regain some resistance after a number of years, as blackleg isolates virulent to that particular resistance source decrease to a lower frequency.

How do we best manage blackleg?

The most effective approach to reduce the impact of blackleg is to use an integrated strategy that utilises cultivar resistance, cultural control and the strategic use of fungicides.  The most effective management practices that can reduce the impact of blackleg include:

1. Sowing canola cultivars with appropriate levels of blackleg resistance. This is particularly important in districts with a high intensity of canola production where cultivars should be sown with high levels of blackleg resistance.  Plant resistance is the first line of defence against blackleg.
2. Avoid canola stubble, especially from the previous season’s crop. The distance from last season’s canola stubble will largely determine the severity of blackleg in this season’s canola crop.  Where possible a distance of at least 500m will significantly reduce the disease pressure from blackleg on this season’s crop.  Spores of the blackleg pathogen are released from old canola stubble onto emerging canola crops.  The greater the distance from this inoculum source the better.
3. Apply seed dressing or fungicide-amended fertiliser. Application of a fungicide seed dressing or use of fungicide amended fertiliser will provide extra protection from blackleg in the critical early stages of crop emergence and establishment.  In high blackleg pressure situations this is very important.

Other management options

In addition to the above practices, several other management practices to consider include:

1. Canola stubble conservation practices.  Since the blackleg pathogen survives on canola stubble, the management of old canola stubble can be important.  Care should be taken where canola is inter-row sown into old stubbles.  In tight canola/wheat rotations make sure this season’s canola crop is not being sown into 2 year old canola stubble, which can still harbour the blackleg pathogen.  Burning or burying old canola stubble will not eliminate the blackleg pathogen completely or reduce blackleg infection.
2. Time of sowing.  Canola is most vulnerable to blackleg as a seedling.  Follow the recommended sowing window for your district and avoid late sowing of canola crops.  The emergence of late sown crops often coincides with the release of spores from old canola stubble, which can significantly increase the risk of early infection by the blackleg pathogen.

New tools in blackleg management

In 2012 several new tools became available for the management of blackleg. These include the option of a foliar fungicide, the release of ‘resistance’ groups and the Blackleg Management Guide publication.

1.  Foliar fungicides.  In certain situations it may be economical to apply a foliar fungicide to extend the length of protection from blackleg, such as if disease severity is very high, if genetic resistance is inadequate or has been overcome by the fungus.  There is now a foliar fungicide registered for use on canola to manage blackleg in 2013.  Results of field experiments indicated that use of a fungicide seed dressing in combination with the application of a foliar fungicide gives good levels of protection. Timing is crucial, with an application at the 4-6 leaf growth stage found to be significant in decreasing blackleg infection. However, the benefits are only found in those canola cultivars with a low level of resistance to blackleg and in situations of high disease pressure. 

The use of foliar fungicides for blackleg management should not become a routine practice for most canola producers, but viewed as a post emergent tool to be used under high disease pressure conditions where preventative measures to decrease blackleg cannot be undertaken. Reliance on fungicides to control blackleg poses a high risk of fungicide resistance.

2.  Consult the Blackleg Management Guide. The Blackleg Management Guide is available from the GRDC website. The purpose of the guide is to enable growers and advisors to determine their own risk of blackleg development by assessing all known factors that can influence blackleg severity and ultimately determine the overall risk of their individual paddock and this year’s canola crop. Growers can then alter individual factors to reduce risk; for example, sow cultivars with a different source of resistance, use a seed dressing or change paddock selection.  In addition, the guide now provides the latest canola cultivar resistance ratings and resistance group information, which was previously available in a separate guide.

3.  Canola ‘resistance’ groups. The blackleg fungus, Leptosphaeria maculans, has a high propensity to overcome resistance in Brassica napus (canola) cultivars as it is sexually reproducing, resulting in enormously diverse populations. The blackleg fungus is a haploid species so it only carries 1 copy of each gene and can easily gain or lose these genes during sexual reproduction. Therefore, the fungal population evolves very rapidly and responds quickly to selection pressures such as wide-scale sowing of cultivars with specific resistance genes. This will lead to resistance being overcome when cultivars of the same resistance gene are sown for 3 or more years. However, we can use the fungal life traits to manipulate the fungal population. By changing cultivars with different sources of resistance, the selection pressure on the fungal population is constantly changing. This prevents the build up of isolates that are virulent (attacking). For example, if you sow a Group D cultivar, the frequency of blackleg fungal isolates able to attack Group D cultivars will increase each year that you sow the same cultivar. But at the same time, the frequency of blackleg fungal isolates able to attack the other resistance groups will decline.  Therefore if you sow the same cultivar every year you are likely to break the cultivar’s blackleg resistance. This process takes at least three years in regions of high rainfall and intensive canola production and is likely to take longer in other regions. The flip side is, if every three years you change your cultivar to one containing different resistance genes, you are likely to reduce yield losses and reduce the probability of resistance breakdown occurring.

How are cultivars allocated to a resistance group?

From 2011 onwards all canola cultivars and NVT lines have been classified for their type of blackleg resistance. There are two types of resistance to blackleg; seedling and adult plant resistance. Individual blackleg isolates are used to identify the seedling resistance while infected canola stubble is used to release sexual spores and screen plants for stem canker adult plant resistance. Using the combination of the seedling and adult data enables the classification of all cultivars into 7 different groups.

Why do some cultivars have multiple groups?

Some cultivars will fall into a single group and some will fall into multiple groups. The reason why they may end up in multiple groups is due to two factors. Firstly each cultivar is allocated a group based on its seedling resistance. Some cultivars contain single seedling resistance genes whilst others contain more than one. For example, if a cultivar contains resistance genes Rlm1 and Rlm4, the cultivar will be placed in Groups A (Rlm1) and B (Rlm4).

Secondly, the adult screen can result in cultivars being placed into singular or multiple groups. The stem canker adult plant resistance screen involves infecting each cultivar with fungal spores released from canola stubble of each of the 7 groups. If a cultivar is susceptible to only one stubble type, then it will be allocated the group to which it is susceptible, but if it is susceptible to multiple stubbles then it will end up in each of the groups to which it is susceptible. For instance, if a cultivar is susceptible to only Group A stubble and resistant to the other 6 stubbles it will classified as Group A. If a cultivar is susceptible to Group A, B and D stubbles but resistant to the other stubbles it will be classified as a Group A B D cultivar. 

Why are cultivars with a blackleg rating lower that MR not given a resistance group?

When susceptible cultivars are screened against the 7 different stubbles they are susceptible to many or even all of the stubbles, so they fall into many or all of the groups. This suggests that these cultivars can not be rotated with any other cultivars. However, cultivars with better resistance can be rotated with these less resistant cultivars. Therefore if you have a blackleg problem and are growing a susceptible cultivar your first action should be to change to a cultivar with higher levels of resistance.

Do you need to switch to a different resistance group each year?

No. Although every year you sow a cultivar from the same resistance group the number of virulent isolates that can attack your cultivar increases, the fastest cases of resistance breakdown have occurred over 3 years. In most regions it will take longer than 3 years.

Therefore, the best policy is to monitor the level of blackleg in your cultivar on your farm, if you observe the level of disease increasing then switch to a different group. If blackleg severity is not increasing, you can continue with your current cultivar.   

Can a cultivar change groups?

Yes, cultivars may change resistance groups over time. Since resistance groups are partially determined using stubble from the field this will be influenced by the fungal population present on that stubble. Since the fungal population is constantly changing it may be adapting to different resistance genes in different populations. However, the characterisation of seedling resistance will always be constant.  Where canola cultivars have continued to be sown after evidence of resistance breakdown, we have found those cultivars to gradually increase in susceptibility to more stubbles types and consequently be allocated to multiple Resistance Groups. AV-Garnet is an example of a cultivar that was highly resistant when released.  It has since been grown over a long period and has continued to be sown after resistance breakdown was observed.  Consequently it is currently allocated to 3 different resistance groups. 

Conclusion

The management of blackleg is integral to canola production and has to be considered at all points of crop management.  The best management strategy for blackleg should utilise a range of management options including cultivar resistance, paddock selection and the strategic use of fungicides.  Growers and advisors are encouraged to refer to the Blackleg Management Guide, available from the GRDC website (www.grdc.com.au/Resources/Factsheets/2012/06/Blackleg-Management-Guide-Fact-Sheet-Western-and-Southern-Region) for management information, blackleg risk assessment and canola cultivar information.

Contact details

Kurt Lindbeck
NSW – Department of Primary Industries, Wagga Wagga
Ph: 02 69 381 608
Email: kurt.lindbeck@dpi.nsw.gov.au

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