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Canola Blackleg Update & Control Options for Late Season Application

It’s already that time of the year again, when clients and their agronomists must make the decision on whether to put an early flowering fungicide over their canola. This year there was low seedling blackleg pressure in early sown crops and then as winter and heavy rainfall set in the pressure increased with heavy seedling infection in susceptible varieties. 

 

Blackleg is caused by the fungus ‘Leptosphaeria Maculans’ and is the most economically damaging disease for Canola in the Medium to high rainfall cropping areas throughout Australia. With the right management strategies this pathogen and resulting disease can be controlled. 
 

Periods of Infection

Part of the reason that blackleg is so prevalent is that it can infect any part of the canola plant at multiple times throughout the crop’s growth cycle. Lesions form on the leaves right through the growing season but severe crown canker is most likely to occur when plants are infected whilst in the young seedling stage. The fungus grows on the cotyledons and leaves asymptomatically through the vascular tissues to the crown of the plant where it causes necrosis resulting in crown canker at the base of the plant. Yield loss then results from restricted water and nutrient uptake by the plant. It is crucial to protect the crop during this seedling stage to lesson the severity of crown canker. Lesions can also develop on all other plant parts and these infections may go on to develop cankers as well.
 

Figure 1. Periods of infection by blackleg for different parts of the canola plant in relation to the period of blackleg spore release and start of flowering in medium and high rainfall zones. Solid lines show main periods of infection and dashed lines show reduced risk from infection. For start of flowering, solid line indicates the optimal period in which yield is maximised while reducing disease risk

 

Upper Canopy Infection (UCI)

Upper canopy blackleg infection occurs when mature blackleg spores from crop residue (triggered by rainfall) land on flowers, peduncles, pods, main stems and branches which then infect the crop. During late winter there are often long periods of crop wetness (this is common around the beginning of flowering in the Western district) which gives the perfect conditions for airborne spores to survive whilst they enter their host plant. Its only in recent years that we have been looking at control of this later season infection.

Photo 1. Upper canopy infection (UCI) causing pod abortion which can cause significant yield reduction

 

Control Options Later in the Season.

Recent work done by Steve Marcroft (Head of the national Canola Pathology Program) has proven that under the right scenarios, there are sizeable yield improvements from well-timed early flowering fungicide applications.

 

One of the more popular fungicides for use at early flowering is Aviator Xpro (dual modes of action) at rates between 550-650ml/ha to be applied around 20-30% flowering stage which is when there are between 14-20 open flowers on the main stem on average. It is always a good idea to leave a nill treatment strip or two through the paddock to compare any yield significance.

Table 1. Kaniva 2016 Crop Maturity blackleg and sclerotinia effect on 3 cultivars by 7 foliar fungicide application treatments.

 

Below are some factors and requirements that indicate that an early flowering fungicide application could give a strong return on investment:

  • There are leaf lesions present at the start of flowering (This indicates that you will be susceptible to UCI and that the major genes your variety has are not effective)

  • Flowering is occurring early when there is plenty of moisture about.

  • The crop has good biomass and economically the potential yield warrants fungicide investment. 

  • There was medium to high blackleg pressure earlier in the season.

 

A major factor in upper canopy infection is the time of flowering which makes cultivar choice and time of sowing a very powerful tool when it comes to lowering upper canopy infection risk. Shorter season varieties sown too early will reach flowering quite early and be exposed to high levels of blackleg spore shower.

 

Management strategies
Blackleg should be controlled by using a range of management strategies. These include growing this years crop at least 500m away from the previous years crop, growing cultivars with high levels of blackleg resistance, use of fungicide applications (foliar and Infurrow) and rotation of cultivars with different resistance groups. Each year growers should monitor their crop to determine whether there is any yield loss due to blackleg. Growers can then go through the GRDC blackleg management guide and follow the management steps to reduce the effect of blackleg. Another handy tool to help monitor blackleg persistence and aid in the decision making for fungicide application is the ‘BlacklegCM app’, see below. 

 

BlacklegCM app, developed with GRDC investment, allows the user to input information such as paddock selection, variety choice, seed dressing, and banded or sprayed fungicide, and takes into account costs, yield benefits and grain prices to give the best/worse-case scenario and likely estimated economic return.
 

Growers can change the parameters on the app to tailor the output to their own individual crop. It can be downloaded onto tablets (not Smartphones) from both the App Store and Google play, https://www.agric.wa.gov.au/apps/blacklegcm-blackleg-management-app

Please contact a Western Ag Agronomist if you would like to know more about blackleg management and how it can be implemented into your program.

Article produced by - Lachlan Bullen, Western AG Ballarat