Will I get an economic response from applying fungicide to canola for the control of blackleg?

Take home messages

• The canola industry has become more reliant on fungicides to control blackleg, in some regions there is reduced emphasis on cultural practices to reduce disease.
• The decision to use a fungicide is not clear cut and should be based on the disease risk profile of the crop.
• Severe blackleg crown canker occurs when plants are infected during early seedling growth. Prior to sowing, use the BlacklegCM decision support tool to identify high risk paddocks and explore management strategies to reduce yield loss.
• Early vegetative (4-10 leaf) foliar fungicide application should be based on the risk profile of the crop, cultivar blackleg rating and estimation of the potential yield after scouting for leaf lesions.
• Fungicide application decision-making for upper canopy infection is separate to the decision process for crown canker. Fungicide applications to control upper canopy infection can result in variable yield responses. It is important to understand the disease risk before applying a fungicide.
• Knowledge on upper canopy infection is improving and it is likely that decision making will become more reliable. A decision support tool is expected to be released via GRDC investment when there is sufficient confidence on recommendations to aid decision making.

Will I get an economic return from applying a fungicide to my canola crop?

Recently, new fungicide actives and timing recommendations have produced large yield responses. However, these are variable ranging from nil to 20% yield increases in on-farm strip trials and nil to 40% yield increases in small-plot research trials. So how do you determine where your crop will sit in 2021 (i.e., within the nil to 40% response range)?

Predicting a yield response would be very accurate if you knew exactly how much disease will occur, but the level of crop damage caused by disease is determined by numerous interconnected factors. Additionally, other diseases such as Sclerotinia stem rot, white leaf spot, powdery mildew and alternaria can also influence economic returns.

The key is to identify the blackleg risk for each individual crop and then determine the cost of application compared to that of potential yield loss. In most years, this is relatively easy. For example, a low rainfall year is low risk and in a high rainfall year with high yield potential, it is very easy to gain an economic advantage from fungicide application. But in the decile 4 to 7 years there is lots to be gained or lost from fungicide decisions.

Blackleg crown canker

Do I need a seed treatment and/or fungicide amended fertiliser?

Risk factors:

1. Canola growing region – high canola intensity and high rainfall = high risk. One in four-year rotations and 500m isolation between this year’s crop and last year’s stubble reduces risk.
2. Cultivar resistance – cultivars rated resistant (R) to moderately resistant (MR) or above have very low risk of developing crown cankers. Moderately resistant will develop cankers but only if grown under high disease pressure for example, canola/wheat/canola in high rainfall.
3. Blackleg population – if you’ve grown the same cultivar for a number of years and crown canker severity is increasing, you will be at a higher risk of crown cankers if you then sow a cultivar from the same resistance group.
4. Timing of crop emergence - severe crown canker is most likely to develop when plants are infected during the early seedling stage (cotyledon to 4th leaf). The driving factors for seedling infection are the growth stage at which first infection occurs and the length of time that the plant is exposed to blackleg infection while in the vulnerable seedling stage. Therefore, the risk of seedling infection that leads to crown canker varies from season to season. For infection to occur, blackleg spores must be mature and ready to release from stubble, a process reliant on a combination of moisture and suitable temperatures. Fruiting bodies typically become ripe approximately three weeks after the break of the season when the stubble has stayed consistently moist. Once mature, spores are then released with each rainfall event. Temperature also determines the length of time that plants remain in the vulnerable seedling stage. Plants are significantly less vulnerable to crown canker after the 4th leaf stage. Older plants will still get leaf lesions, but the pathogen is less likely to cause damaging crown canker as it cannot grow fast enough to get into the crown. Typically, plants sown earlier in the growing season (April) will develop quickly under warmer conditions and progress rapidly past the vulnerable seedling stage compared to plants sown later (mid-May) which progress slowly and remain in the vulnerable seedling stage for an extended period.
5. Farming system - inter-row sowing with full stubble retention influences the timing and quantity of ascospores from stubble, which are a primary source of inoculum. Standing stubble stays drier between rainfall events compared to stubble that is lying down and in contact with the soil. Standing stubble delays spore maturation and reduces the release of spores early in the season at the time when fungicide applied to seed and fertiliser are most effective. Standing stubble produces more spores later in the season, however these spores are unlikely to produce severe crown canker but may increase severity of upper canopy blackleg. However, standing stubble that is knocked down 12 months later can then produce spores early the following growing season.

In summary

An economic return is unlikely if sowing an R rated cultivar in a one in four-year rotation in mid-April with >500m from the previous year’s canola crop (and you don’t retain stubble). If sowing a MS rated cultivar in a canola / wheat / canola rotation at the end of May, you will likely get a large return from your fungicide application. The challenge with seed treatments and fungicide-amended fertiliser is that the decision to use these products is made a long time before sowing (or you don’t have any influence over it when you purchase commercial seed), and therefore, you will not know the emergence date, and therefore, the individual season risk. But you will know the risks associated with your canola region, cultivar blackleg rating and distance to last year’s stubble.

Do I need a vegetative foliar fungicide application?

As with fungicides applied at sowing, vegetative foliar fungicides applied during 4-10 leaf growth stage are also designed to protect plants from crown cankers. The main advantage with this fungicide timing is that the level of disease risk can be assessed at the time of application, considering the blackleg rating of the cultivar, whether a seed treatment and/or fungicide amended fertiliser has been used and the prevalence and severity of leaf lesions observed in the crop.

1. Cultivars with effective major gene resistance will have none or very few leaf lesions even under high disease situations and will therefore be protected from crown canker. Cultivars are classified into Blackleg resistance groups (A, B, C, etc) according to their complement of major genes. An abundance of lesions in cultivars which are expected to have effective major gene resistance indicates that the resistance is being overcome and application of a foliar fungicide may be prudent as the underlying level of quantitative resistance is uncertain. In cultivars lacking effective major genes, the blackleg rating gives an indication of the level of quantitative resistance to crown canker, i.e., the level of resistance to crown canker in the plant following leaf infection. All cultivars that are reliant on quantitative resistance may get a similar level of leaf infection but a cultivar with an Rblackleg rating will not develop crown cankers whereas an MR cultivar may develop some crown cankers and an MS-S cultivar may have severe cankering and lodged plants.
2. Fungicides applied at sowing will reduce crown canker even on crops with quite severe leaf lesions. In most cases, if a cultivar with adequate resistance is sown with a seed or fertiliser fungicide treatment then a vegetative foliar fungicide is unlikely to be necessary. Monitor your crop and make an in-season decision.
3. Leaf lesions are most damaging on the cotyledons and early leaves, and therefore, a foliar fungicide is most likely to give an economic benefit to protect this vulnerable stage.

Analysis of the fungicide trials clearly showed that fungicides only provided a yield benefit in high disease situations, such as:

1. You may have chosen to grow a cultivar with a lower blackleg rating because the cultivar is the highest yielding or you have chosen to retain seed, etc. For example, it is common practice to grow older cultivars with reduced blackleg resistance and then protect these cultivars with fungicide applications.
2. The pathogen population has changed to render major genes ineffective.
3. The season is very conducive for blackleg with spore maturity coinciding with emergence and the vulnerable stage of crop growth.

Use of the BlacklegCM App is recommended to help make blackleg management decisions. BlacklegCM is an interactive tool allowing users to compare scenarios and determine the likely yield response from altering various disease management strategies.

Upper canopy blackleg fungicide application

Blackleg Upper Canopy Infection (UCI) refers to blackleg infection of the upper stem, branches, flowers and pods. Although research is improving the understanding of these symptoms, there is still a lack of knowledge on how individual cultivars react to UCI in terms of yield loss. Furthermore, our research shows that similar symptoms of UCI can cause severe yield loss in one season and no yield loss in another. As such, our recommendations for managing blackleg UCI are constantly improving.

Should I apply a fungicide for UCI protection?

The question of whether to apply a fungicide for UCI protection is a real dilemma. Get it wrong and it will cost your crop a lot of money, but currently there is no way to accurately predict economic return from fungicide application. GRDC investment is working on improving knowledge, including determining the timing of infection leading to yield loss, weather parameters associated with yield loss and strategies for screening for genetic resistance.

Some factors however that are driving disease risk:

• Timing of flowering.
  Earlier flowering crops are at a higher risk than later flowering crops as they flower in conditions more conducive for blackleg infection. Earlier flowering crops also have a longer period until harvest which allows the fungus to proliferate within the plant, thereby reducing yield potential.
• Spring rainfall and temperature.
  Our preliminary data suggests that UCI, given enough time, will cause damage to the vascular tissue in the stems and branches, reducing yield potential by restricting water and nutrient flow to developing flowers, pods and seed. However, similar levels of disease can cause different amounts of yield loss depending on the weather during pod fill. Plants without moisture/heat stress can tolerate a higher disease load before it impacts on yield.
• Genetic resistance.
  Genetic resistance is the missing piece of the puzzle. As with crown canker, effective major gene resistance protects against UCI. If it is ineffective or has been overcome, the crop may be completely susceptible to UCI, however, this should have become evident by the prevalence and severity of leaf lesions observed during the seedling stage. The effect of quantitative resistance for crown canker on UCI is currently under investigation. It is clear that cultivars with good quantitative resistance do get UCI symptoms, but we are unsure whether these cultivars have less damage to the vascular tissue than more susceptible cultivars. This could be similar to the way cultivars react at the seedling stage, whereby varieties with the same level of leaf infection develop different levels of crown canker.
• Fungicides.
  Our work has shown a wide window of response times with good results (if you have a damaging level of disease) from fungicide application from first flower to 50% bloom. However, for several reasons, it is suggested that 30% bloom is aimed for. Firstly, the 30% bloom stage is as late as you can go and still get good penetration into the canopy; your main aim is to protect the main stem as this will have a greater impact on yield compared to individual branches. Secondly, this timing may provide some control of any initial infections that have already occurred. Thirdly, the 30% bloom timing will provide protection for a few weeks into the future by which stage any later infections are less likely to result in significant yield loss. Pod infection is unlikely to be controlled through fungicide application. However, there was some control of pod infection at some sites in 2020 by spraying at 30% bloom but this has not been observed in previous seasons. Pod infection occurs when there are rainfall events during podding and the fungal spores land directly on the pods to cause disease, this results in an additional yield loss of up to 20%. Unfortunately, there are no fungicides registered for application during podding due to maximum residue limit (MRL) regulations. Effective major gene resistance will control pod infection.

What are the steps to determining a UCI spray decision?

1. Yield potential – yield potential is an economic driver. A 1% return on a 3t/ha crop is worth more money than a 1% return on a 1t/ha crop.
2. Leaf lesions – presence of leaf lesions indicates that blackleg is present, and that the cultivar does not have effective major gene resistance. No leaf lesions = no reason to spray.
3. New leaf lesions on upper leaves as the plants are elongating – this observation is not critical, but it does give an indication that blackleg is active as the crop is coming into the susceptible window. However, numerous wet days at early flowering stage will still be high risk even if there were no lesions on new leaves up to that point. Remember it will take two to three weeks after rainfall to observe leaf lesions. More lesions = higher blackleg severity.
4. Date of first flower - the earlier in the season that flowering occurs = higher risk. This date will vary for different regions. Generally, shorter season regions can, more safely, commence flowering at an earlier date compared to longer season regions. Earlier harvest date results in less time for the fungus to invade the vascular tissue and cause yield loss. Consequently, if you’re in a long growing season rainfall region and your crop flowers in early August and is harvested in December, you are in a very high-risk situation.

How can I determine if I should have sprayed for UCI?

1. UCI symptoms are most readily observed at windrowing or even later as the plants mature. They can progress very quickly during this time.
2. Check for external lesions and ensure correct identification.
3. Where lesions are present, slice open the branch/stem and check for blackened pith which is indicative of vascular damage and likely yield loss.
4. Observe darkened branches; these branches go dark after vascular damage and are indicative of yield loss.
5. Pod infection will cause yield loss, unfortunately there is nothing that can be done to prevent pod infection.
6. Leave unsprayed strips to check for yield returns.

Which fungicide active should I use?

There are two parts to the question of which fungicide active should I use? Firstly, in terms of which active will give better control, there are few side by side comparisons that have been undertaken for blackleg control. However, the GRDC blackleg rating project has undertaken comparisons for the seed treatment fungicides which indicate the succinate dehydrogenase inhibitors (SDHI) fungicides provide longer protection compared to the demethylation inhibitors (DMI) fungicides. Ultimately, crop development stage, determining your risk, and therefore, potential economic return are more important factors when choosing a fungicide.

The second aspect of choosing a fungicide active is in regards to managing the risk of fungicide resistance. Resistance towards the DMI fungicides has been detected in approximately 30% of Australian blackleg populations over the past three years whilst no resistance has been detected for the SDHI fungicides. However, excessive use of the SDHI fungicides has the potential to select for fungicide resistance more quickly than DMIs. Therefore, limitations on the number of applications for each fungicide active within a growing season have been developed and can be found at the CropLife website

If you use a SDHI seed treatment you cannot use a SDHI early foliar (4-8 leaf) application. At this point, SDHI seed treatment and SDHI 30% bloom spray is considered safe. Research will be testing these different scenarios to provide accurate data for modelling fungicide management.

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 authors would like to thank them for their continued support. The authors would also like to thank Elizabeth Sheedy, Alistair Smith and Buffy Harrison.

Useful resources and references

BlacklegCM App for iPad and android tablets

GRDC Publication – Blackleg Management Guide 

GRDC Groundcover - Canola: The Ute Guide 

Marcroft Grains Pathology website

GRDC National Variety Trials Website

Contact details

Steve Marcroft
Marcroft Grains Pathology
Grains Innovation Park
Natimuk Rd, Horsham, VIC 3400
0409 978 941
Steve@grainspathology.com.au