Will increased canola density change blackleg and/or sclerotinia management decisions?

Key messages

• Increased canola area will result in increased canola stubble in subsequent years and therefore increased blackleg spore density.
• Spore density can (but does not always) result in increased disease severity.
• Increased canola stubble and canola crop area will reduce the ability of growers to maintain a 500m buffer between one-year-old stubble and current crops.
• Stubble quantity rather than stubble management has the largest effect on blackleg disease.
• Seasonal conditions will influence the relative significance of crown canker or upper canopy infection and whether control is warranted. It will be rare to have severe forms of both versions of blackleg in the same year.
• Crown canker years occur from late sowings, which results in plants remaining as seedlings during the winter infection period.
• Upper canopy infection years will likely result from early sowing times, which results in plants commencing flowering in late July/early August. Early flowering will result in increased infection and will provide the fungus with more time to cause damage prior to harvest.
• The canola industry is likely to become more reliant on fungicides due to increasing canola production (and the subsequent inability to avoid canola stubble).
• The decision to use a fungicide is not clear cut. The first step in the decision-making process is to understand the disease risk profile of your crop.
• Before sowing, use the BlacklegCM decision support tool to identify high risk paddocks and explore management strategies available to reduce yield loss.
• Fungicide application for upper canopy infection is a separate decision-making process from crown canker control. Fungicide control of upper canopy infection can result in very variable yield returns. It is important to understand the disease risk before applying a fungicide.

Increased canola density will increase blackleg inoculum (spore density). However, it is important to remember that inoculum is just one factor driving disease and that disease does not always result in yield loss.

Every hectare of canola grown in 2021 is now canola stubble. The blackleg fungus survives and reproduces on canola stubble, therefore more stubble results in more blackleg spores. Blackleg is a sexually reproducing pathogen, individual isolates of blackleg that have attacked the same plant will survive within the plant and then on the subsequent canola stubble. In the following season, these isolates will mate using the stubble as an energy and structural source. All blackleg isolates are one of two mating types and can only mate with a different mating type. Following mating, sexual fruiting bodies will appear on the external surface of the canola stubble. As Blackleg is a fungus it requires moist conditions for mating and subsequent reproduction, so this process will occur after the break of the season when the stubble stays moist for a considerable period. Typically, spores will be released from the sexual fruiting bodies once the stubble has stayed moist for three weeks. If looking at stubble with a magnifying glass the fruiting bodies that are still developing look like mountains and fruiting bodies that are releasing spores look like volcanoes. Once the fruiting bodies are developed, they will release spores every time it rains for the rest of the growing season.

During the growing season stubble will release blackleg spores for as long as it stays intact. Our research has measured spore release in canola stubble for up to four years. However, blackleg will release fewer spores as the stubble and the fruiting bodies age, so one-year-old stubble produces more spores than two- year-old stubble and so on. However, the main driver of spore production is stubble quantity. The number of spores available to infect a crop can be calculated by multiplying spore release per piece of stubble by stubble quantity (t/ha). Research has shown that about 99% of spores originate from the previous year’s crop, which means older stubble produces fewer spores and has less area to harbour the disease.

What about stubble conservation?

Previous work measured the effect of raking and burning stubble on spore release and we have also measured spore release after bush fires. We found that deliberate stubble destruction could reduce spore production by 50% but we were unable to measure any reduction in disease severity. This means that there were still enough spores in the raked and burned stubble to drive disease and that spore number was not the limiting factor. However, it should be noted that the raked and burned stubble was still producing vastly more spores per hectare than unimpacted two-year-old stubble.

Do modern practices of interrow sowing and full stubble conservation change management decisions?

Prior to the practice of inter-row sowing, canola stubble was knocked down each year via various tillage practices. The stubble lying in contact with the soil stayed moist during the growing season and released blackleg spores with each rainfall event. Stubble that was two or three years old produced very few spores and was highly unlikely to add to the annual disease severity. However, recent work has shown that stubble that remains standing following modern farming practices stays dry and does not develop sexual fruiting bodies at the same rate as stubble lying down, and therefore releases fewer spores later in the growing season.

But how does stubble conservation and interrow sowing affect disease?

During 2021 we compared disease severity in low or high quantities of one-year-old stubble and two-year-old stubble that was either lying or standing (Figure 1). For each stubble treatment, two times of sowing allowed for crown canker development (late sown) or upper canopy infection (early sown).

Impact of stubble treatment on crown canker infection

As expected, the earlier sown plants had significantly less crown canker (internal infection) than the later sown plants, irrespective of stubble treatment (Table 1). For the later sown plants, neither stubble load nor stubble type had any significant impact on disease level with average internal infection ranging between 75.9 – 92.5%.

For the earlier sown plants, crown canker severity differed significantly across the stubble treatments (Table 1). Significantly more disease was observed in the high load one-year-old stubble treatments (both standing and lying) than all other treatments. Within the same stubble load, no significant differences were seen between the standing and lying treatments. For the high stubble load treatments, the one-year-old stubble caused significantly higher levels of disease than the two-year-old stubble.

Taken together, the data suggest that later sowing leads to more significant crown canker compared to early sowing and that stubble load, rather than stubble orientation, has a more impact on crown canker disease severity. Stubble age, irrespective of stubble orientation, does have a significant impact on disease severity but only at high stubble density.

Impact of stubble treatment on upper canopy blackleg infection

As expected, the earlier sown plants had significantly higher upper canopy infection severity compared to the later sown plants. Very low levels of upper canopy infection were detected in the late sown plants, with no significant differences between any treatments.

As with crown canker severity, stubble load had a significant effect on the severity of upper canopy blackleg infection in early sown plants across all stubble treatments (Table 1). The impact of stubble orientation caused some significant differences, but only at the low stubble density. Standing one-year-old stubble caused significantly larger stem lesions (34.5cm) than lying one-year-old stubble (8.3cm) at low density, however no differences were detected in branch infection. Similarly, the standing two-year-old stubble caused significantly larger stem lesions (20.4cm) than the lying two-year-old stubble (9.6cm).  Again, no differences in branch infection were observed.

These data support previous findings that earlier sowing times lead to significantly higher levels of upper canopy infection than later sowing times. Furthermore, stubble load has a significant impact on disease compared to stubble age or orientation. Stubble orientation impacted the level of stem lesion infection only when stubble loads were low.

What does all this mean for stubble management?

1. Avoid one-year-old stubble by keeping new crops 500m from the previous year’s crop. In our research high stubble quantity generated significantly higher disease severity for both crown canker and upper canopy infection.
2. Two-year-old stubble still produced inoculum and disease but the difference between treatments were minor, making the management of two-year-old stubble probably not warranted.
3. The largest effect on disease severity is time of sowing. Early sown crops get upper canopy infection while late sown crops remain safe. However, late sown crops are vulnerable to crown canker while early sown crops remain safe.

Canola crops are unlikely to get both crown canker and upper canopy infection in the same year.

Research over the past few years indicates that most years will be defined as either a crown canker or upper canopy infection year. It will be rare to have severe forms of both blackleg diseases in the same year. In most regions, 2021 was a crown canker year. The relative risk of each disease is determined by the timing of the opening rains each year, the subsequent  germination date and then 1^st flower date. It will be rare to warrant applying fungicides to control both crown canker and upper canopy infection in the same year.

1. Crown canker — severe crown canker is most likely to develop when plants are infected during the early seedling stage (cotyledon to 4^th leaf stage). The driving factor for seedling infection is the length of time that the plant is exposed to blackleg infection while in the seedling stage. Therefore, the risk of seedling infection, which leads to crown cankers, varies considerably from season to season. Once plants progress to the 4^th leaf stage they are significantly less vulnerable to crown canker. Older plants can still develop leaf lesions, but the pathogen is less likely to cause damaging crown cankers as the fungus cannot grow fast enough to get into the crown. Typically, plants sown early in the growing season (April) will develop quickly under warmer conditions and progress rapidly past the vulnerable seedling stage while plants sown later (mid-May) will progress more slowly and remain in the vulnerable seedling stage for an extended period.
2. Upper canopy infection (UCI) — UCI occurs when the plants become reproductive early in the growing season, typically when crops commence flowering in late July to early August. This can coincide with cool, moist conditions conducive to infection and allows enough time for the pathogen to cause tissue necrosis prior to harvest. UCI flower and branch infection can occur at any time but only results in yield loss when it occurs early in the season because the pathogen must have enough time to grow from the infection point to within the vascular tissue to cause necrosis and yield loss. In 2021, crops that commenced flowering in September in many cases developed UCI infection but the infection did not progress to the vascular tissue, so no yield losses occurred.

Management for increased canola intensity

If canola production has increased it is almost certain that blackleg severity will also increase, but it is important to determine if the increased blackleg severity will result in increased yield loss. If the current farming system has low levels of disease, a small to moderate increase in disease will not be yield limiting. However, if the canola crops have already suffered some yield loss, the increased disease severity will lead to an increased yield loss.

1. Know your region – high canola intensity and high rainfall = high risk. Blackleg disease risk will be reduced by using a one in four-year rotation and imposing a 500m isolation between the current year’s crop and last year’s stubble. Monitor crops for both UCI and crown canker to determine if practices should be retained or changed.
2. Distance to canola stubble – crops sown adjacent to one-year-old stubble will have the highest amount of disease, so maintain a 500m buffer if possible.
3. Cultivar resistance – cultivars rated R-MR or above have very low risk of developing crown cankers. MR cultivars will develop cankers but only if grown under high disease severity, for example canola/wheat/canola in high rainfall. www.grdc.com.au/resources-and-publications/all-publications/publications/2020/blackleg-management-guide
4. Pathogen population – if the same cultivar has been grown for several years and disease severity is increasing, sowing a cultivar from the same resistance group will place the crop at a higher risk of crown cankers.
5. Understand the seasonal risk and management needs of developing crown canker or UCI infections based on sowing/germination timing.
6. Fungicides – use the BlacklegCM App to determine potential economic returns for fungicides for crown canker.
7. Fungicides for UCI – if the canola crop has blackleg symptoms and commenced flowering in late July/early August it is more likely to benefit from a fungicide application. Later flowering crops are unlikely to have yield losses. Cultivar resistance to UCI can be effective but we do not yet have a reliable cultivar screening system. If a cultivar has had significant yield increases from 30% bloom fungicide applications in previous years, it is likely to be susceptible and benefit from fungicide application.

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 also thank Elizabeth Sheedy, Alistair Smith and Buffy Harrison.

Useful resources and references

• BlacklegCM App for iPad and android tablets
• www.grdc.com.au/resources-and-publications/all-publications/publications/2020/blackleg-management-guide
• Canola: the ute guide (https://grdc.com.au/resources-and-publications/groundcover/ground-cover-issue-27/canola-the-ute-guide)
• Marcroft Grains Pathology website: www.marcroftgrainspathology.com.au
• www.nvt.com.au

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