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2022 Blackleg Update - Should I Be Applying an Early Flowering Fungicide?

It’s already that time of the year again where 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 conditions set in the pressure increased with Hybrid and Open Pollinated cultivars both showing a medium level of blackleg pressure.

 

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 successful 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

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 the canola plant.

 

It is now becoming more common place for us to control this later season infection (Through the use of fungicides at flowering) off the back of the work carried out by Steve Marcroft (Marcroft Grains Pathology)

Picture 1. Photo above shows the resultant blackleg disease from spore shower into the upper canopy in canola. As can be seen the spores will enter any part of the host plant that they land on with the help of some moisture.

Control Options Later in the Season

This work done by Steve Marcroft (Head of the national Canola Pathology Program) has proven that under the right scenarios, growers can see up to 20% yield improvements from well-timed early flowering fungicide application. The most effective time for application is 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 nil treatment strip or two through the paddock to show justification for the spray.

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What Scenarios must occur to give a Return on investment from a flowering fungicide?

All of these Factors below must occur in the same crop for it to be worth spraying:

  • There are leaf lesions present in the crop Canopy at the start of flowering (This indicates that you will be susceptible to Upper Canopy Infection and that the major resistance genes your variety has are not effective at preventing blackleg infection in the Upper canopy)

  • Flowering is occurring relatively early when there is plenty of moisture about to enable infection (Varieties that begin to flower later in the season when rainfall is less likely are not going to be worth the effort)

  • The crop has good biomass and economically the potential yield warrants fungicide investment (The higher the canola price is, the less yield potential the crop must have in order for the fungicide to make you a return on investment)

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Management strategies

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 higher levels of blackleg spore shower.

 

Blackleg should be controlled by using a range of management strategies and not just fungicides. These include growing the crop 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 and Agronomists should monitor the crops to determine whether there has been 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.

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App to help Manage Black leg in Canola

This app is a great industry initiative and a very handy tool for helping work out if you need to spray a bloom fungicide or not.

Currently the Blackleg CM App assumes all canola varieties have a Moderately resistant (MR) blackleg status. Steve and his team are currently working on getting each Canola variety currently available put into a Blackleg infection resistance Rating system that is specific just for Upper Canopy infection. They are still a few years away from completing this work.

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With a promising canola season at hand and strong canola prices, the management of Upper Canopy Blackleg infection in your canola has never been more relevant. Please contact your 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

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