Blackleg – new seed treatment, stubble management and fungicide resistance

Take home messages

  • Blackleg crown canker results from infection 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.
  • New succinate dehydrogenase inhibitor (SDHI) seed treatment fungicides have higher efficacy, increased longevity and improved seed safety.
  • The improved efficacy of SDHI fungicide may result in a reduced need for early foliar application of fungicide (4-10 leaf applications).
  • Modern farming systems that enable earlier sowing/germination may result in reduced damage from blackleg crown cankers.
  • Blackleg pathogen populations with resistance to the triazole fungicides fluquinconazole, flutriafol and a tebuconazole + prothioconazole mixture have been detected. No resistance has been detected for new SDHI and quinine-outside inhibitor (QoI) chemistries.
  • Blackleg upper canopy infection (UCI) is the collective term for flower, peduncle, pod, main stem and branch infection, but does not include crown canker.
  • UCI can cause yield losses of up to 30%. Yield loss is reduced by selecting cultivars with effective major gene resistance and using crop management strategies to delay the commencement of flowering to later in the growing season, especially in high disease risk areas.
  • Fungicide applications at 30% bloom often controls UCI but does not always result in yield gains. Thirty per cent bloom fungicide application is unlikely to control pod infection.

Blackleg crown canker - seed treatment

Do you need a seed treatment?

Severe crown canker is most likely to develop when plants are infected during the early seedling stage (cotyledon to 4th leaf). The fungus grows from the cotyledons and leaves asymptomatically through the vascular tissues to the crown, where it causes necrosis resulting in a crown canker at the base of the plant. Cankers at harvest are due to infection at the seedling stage. Yield loss results from restricted water and nutrient uptake by the plant.

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, is very variable from season to season. For infection to occur blackleg fruiting bodies on the canola stubble must be ripe and ready to release spores. Fruiting bodies typically become ripe approximately three weeks after the break of the season when the stubble has stayed consistently moist. Spore are then released with each rainfall event. Temperature also has a large influence as it will determine the length of time that the plant remains in the vulnerable seedling stage. Once plants progress to the 4th leaf stage they are significantly less vulnerable. That is, older plants will still get 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 whereas, plants sown later (mid-May) will progress slowly and remain in the vulnerable seedling stage for an extended period.

Plants sown early often have reduced crown canker severity due to rapid growth through the vulnerable seedling stage and the seedlings are likely to avoid blackleg spores as fruiting bodies are less likely to be mature and able to release spores early in the growing season. Consequently, modern farming systems that enable early sowing will reduce crown canker susceptibility. However, early sowing will likely result in earlier flowering times, which increases the risk of UCI (see following sections within this paper).

Seed treatments

Fungicide seed treatments are extremely effective control against blackleg for crown cankers. As previously mentioned, plants are susceptible at the early seedling stage and this is when seed treatments are most effective. However, seed treatments will not provide complete control so they should be used in conjunction with genetic resistance, for instance moderately susceptible (MS) to moderately resistant (MR) cultivars protected with Jockey® (fluquinconazole) when gown under high blackleg severity conditions are likely to get a yield response from a seed treatment. Cultivars with inadequate resistance, for example; MS-S will get a response but may still have significant damage while cultivars rated very highly for resistance such as MR-R to R will generally not respond to a seed treatment. The BlacklegCM app will predict responses from seed treatments based on the crop parameters that you enter.

New SDHI seed treatments

In 2020 new seed treatments from the SDHI fungicide class will be commercially available to growers. These new fungicides will be adopted very quickly and extensively for two reasons; firstly, they do not have the seed safety issues that may be associated with some other seed treatments.  Secondly, the SDHI fungicides have a higher efficacy and provide a longer period of protection compared to the demethylation inhibitors (DMI) fungicide. Further research is required, but it is likely that in some situations an early foliar fungicide may no longer be required if cultivars are protected with a SDHI fungicide rather than the current DMI fungicide seed treatment.

A decision support tool, BlacklegCM, is available and should be used to assess the risk for blackleg crown canker prior to cultivar selection and sowing. BlacklegCM is available for iPad or android tablets. BlacklegCM does not work on iPhones. The tool is interactive, allowing growers and advisers to determine the blackleg risk for each paddock and consider the possible economic return of different management strategies. The tool also provides in-season support for the application of foliar fungicides.

Fungicide resistance

With the high use of fungicides comes the risk of fungicide resistance developing. In 2018 and 2019, 300+ Leptosphaeria maculans populations have been screened for resistance to all commercially available and soon to be released fungicides (Table 1). The 2019 screens showed similar results to 2018 whereby 25% and 20% of populations have a high frequency of isolates resistant to the DMI fungicides, flutriafol and fluquinconazole, while only 7% of populations have a high frequency of resistance to the tebuconazole + prothioconazole mixture. No resistance was detected to any of the SDHI or QoI fungicides. Screening of populations in 2020 will continue, to monitor changes in the frequency of resistance to both the old DMI chemistries and the new SDHI and QoI chemistries.

Although these screens have detected fungicide resistance within Australian populations, it is currently unknown what proportion of the isolates within a population have resistance. Therefore, it remains unclear whether these resistance isolates are impacting on the efficacy of fungicide use or not. Further work is underway to try and determine the impact of these fungicide resistant isolates to on-farm practices.

The development of fungicide resistance in blackleg pathogen populations in Australia highlights the importance of fungicide-use stewardship. Oversees experience informs us that the new SDHI fungicides are more likely than the current DMI fungicides to develop resistance. To reduce the potential risk of fungicide resistance evolving, it is recommended that a maximum of two chemical applications from a single fungicide class be used within a growing season.

Table 1. The percentage of populations with high, moderate and low levels of resistance to all currently used and upcoming fungicides.

Fungicide

Fungicide class

Percentage of populations with high, moderate and low levels of resistance

2019 results

2018 results

High

Moderate

Low

High

Moderate

Low

Flutriafol®

DMI

25.1

22.0

52.9

28.6

31.6

39.8

Jockey®

DMI

20.4

24.6

55.0

22.4

22.4

45.9

Prosaro®

DMI

7.3

13.1

79.6

7.1

7.1

75.5

Saltro®

SDHI

0

0

100.0

0

0

100.0

Veritas®

QoI + DMI

0

3.1

96.9

0

1.0

99.0

Aviator®

SDHI + DMI

0

0

100.0

0

0

100.0

ILeVo®

SDHI

0

0

100.0

0

0

100.0

Miravis®

SDHI

0

0

100.0

0

0

100.0

Fungicide resistance screening sample submission

If you would like to screen your blackleg populations for fungicide resistance in 2020, 30 pieces of canola stubble from your 2019 paddock is required. Please email Angela Van de Wouw at angela@grainspathology.com.au for  stubbles collection protocol. The fungicide resistance results for the current DMI blackleg fungicides and the new SDHIs will be provided to you. The cost is free to growers/advisers. Costs are covered by an Australian Research Council (ARC)/private industry investment.

Blackleg spore release has changed with modern farming systems

Prior to 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 which was two or three years old produced very few spores that were highly unlikely to add to annual disease severity. Research work undertaken in the mid-1990s led to the recommendations to maintain a 500m buffer between your current canola crop and the previous year’s stubble and to not be so concerned with rotation length as was the prior recommendation. However, recent work has shown that stubble that remains standing in modern farming practices stays dry, is not developing sexual fruiting bodies at the same rate as the lying down stubble, and therefore, releases fewer spores and the release is later in the growing season (Figure 1). It is hypothesised that delayed spore release in the growing season may result in increased UCI as the reproductive parts of the plant are directly infected rather than seedlings and leaves.

Figure 1. Proportion of total spores from specific stubble types and sections produced over a growing season for four sites in 2018.

However, what happened to the standing stubble when it is eventually knocked down in the second year? This is particularly pertinent as it is the second year that is often sown back to a canola crop.

Experiments undertaken in Horsham in 2019 (Table 2) found that stubble which is standing in year 1 and lying in year 2 released fewer spores in the first half of the growing season but increased in proportion of released spores in the second half of the growing season. The data missing from this experiment is the tonnes/ha of stubble that is available to produce blackleg spores. In the 1990s experiments found that few canola stalks survive lying/lying for two years (stalks are either buried or decompose). Therefore, it is now known that standing stubble in year 1 releases few spores but it will release spores in the second year if it is knocked down and becomes lying stubble in year 2. The key driver in this situation is that the stubble has been preserved in the inter-row sowing system and has therefore not been buried or decomposed. The other very intriguing part of this story is that if stubble is maintained standing in the second year it will produce very few spores (Table 2).

Further investigation is required to determine what impact standing stubble has on disease pressure, and therefore, yield losses associated with blackleg.

Table 2. Percentage of total blackleg spore released from two year old canola stubble that is either lying or standing.

Stubble standing or lying

Month

Season spore release

May

June

July

August

Sept

Oct

Nov

Lying yr 1 /
lying yr 2

64

70

69

44

40

69

4

58

Standing yr 1 /
lying yr 2

31

29

29

55

42

18

38

36

Standing yr 1 /
standing yr 2

6

2

2

1

17

12

58

6

Blackleg upper canopy infection (UCI)

Blackleg can infect all parts of the canola plant. UCI is a collective term that describes infection of flowers, peduncles, pods, upper main stem and branches (Figure 2). UCI has become increasingly prevalent over recent years and may be associated with earlier flowering crops because of the earlier sowing of cultivars and more rapid phenological development during warmer autumns and winters. There is also evidence of delayed and prolonged release of blackleg spore release in stubble-retained systems and increased intensity of canola production. While crown canker blackleg is well understood, the factors contributing to UCI and possible control strategies are currently under investigation. An outline of findings to date are presented:

Figure 2. Upper canopy infection includes blackleg infection of flowers, peduncles, pods, main stems and branches

Blackleg upper canopy infection research results

In field experiments, UCI has caused up to 30% yield loss. The impact on yield varies depending on the timing of infection and the plant part infected. Flower loss from infection of flowers or peduncles is unlikely to directly reduce yield as the plant can compensate by producing more flowers. However, the fungus can grow into the associated branch which can then affect seed set and grain filling in surrounding pods. Infection of pods or peduncles after pod formation can result in significant yield loss. Infected branches and upper main stems can affect all developing flowers and pods above the point of infection causing a reduction in pod and seed set as well as smaller seed. Severe infection can cause stems and branches to break off, premature ripening leading to shattering or difficulty in ascertaining correct windrow timing due to maturity differences between seed affected or unaffected by blackleg.

New knowledge from 2019

Entry of UCI blackleg into the plant is via the stomatal openings and/or physical damage to the plant by insects, hail or frost. Up until 2018 it was thought that the damage UCI caused was the physical lesion or death of the flower. However, it is now evident that UCI infections are also systemic, causing damage to the plant’s vascular tissue similar to traditional blackleg crown infections. The issue for growers is that the external symptoms may appear insignificant, but internal vascular damage may cause significant yield losses. Preliminary results indicate that this may be why fungicide applications on crops with few symptoms can still result in economic yield returns. Interestingly, researchers have noted that symptoms of internal vascular damage result in blackened stems post the windrowing growth stage; post 100% seed colour change (Figure 3).

Figure 3. Blackened branches caused by internal vascular damage; symptoms become visible post 100% seed colour change. These symptoms may not occur in crops that received the Sclerotinia 30% bloom fungicide application.

During 2019 two experiments were managed to develop new techniques for artificially inoculating plants to enable specific experiments to be undertaken. A laboratory/controlled environment glasshouse experiment (Table 3) showed that on average the external lesions from the artificial inoculation were 38mm long but when the plants were individually cut open the blackleg pith inside was 134mm. Polymerase chain reaction (PCR) and microscopy are currently being done to determine if symptomless infection has also occurred. This data shows clearly that blackleg is invading the vascular tissue of the plant, and therefore, a small external lesion may reduce moisture and nutrient supply to the entire branch because of vascular tissue damage.

The other meaningful finding from 2019 is that the plant development stage at infection must also be considered with the seasonal timing of infection. For instance, June inoculation at 30% bloom appears to cause more damage than identical inoculation in August or September. The data from 2019 suggests that the fungus requires sufficient time to colonise the vascular tissue and then cause yield reducing damage. Early sown/flowering plants mature slower under cooler conditions compared to later sown/flowering plants that mature quickly under warmer spring conditions. This is a major finding and is likely to provide knowledge on why yield responses to fungicides can vary so much across regions. If a plant is infected earlier in the growing season the vascular damage will be greater than an identical plant infected at the same growth stage but infected later in the season.

The above new knowledge appears to correlate with 2019 field results in Victoria; wet conditions in late August triggered severe leaf and flower infections. In some cases, these infections resulted in yield responses from fungicide applications whereas, in other situations the same blackleg severity in late August did not result in yield gains from fungicide. It may have been that the blackleg had not caused enough damage to the vascular tissue by windrowing.

Table 3. Artificial infection of canola plants for upper canopy blackleg, effect of internal infection and timing of infection.

Experiment location

Time of sowing

Inoculated at
30% bloom

External lesion length (mm) Average

Internal pith colonisation (mm) Average

Glasshouse lab inoculation

21-Mar

10-Jun

38

134

Glasshouse lab inoculation

3-Jun

21-Aug

12

5

Spore shower from stubble

21-Mar

29-Jun

183

NA

Spore shower from stubble

24-May

6-Sep

43

NA

Blackleg upper canopy infection control strategies

Genetic resistance

Effective major gene resistance prevents infection of all canola plant parts (cotyledons, leaves, stems, branches, flowers, pods). Effective major genes can thereby prevent both crown canker and blackleg UCIs. Unfortunately, most major genes present in current cultivars have been overcome by the blackleg pathogen across many canola producing regions. It is therefore crucial to know if major genes are effective or have been overcome in your growing region. A network of 34 blackleg monitoring sites are established across Australia each year, sown with cultivars representing each resistance group. These sites are used to provide regional information on the effectiveness of resistance genes (Table 4). Blackleg Management Guide - GRDC provides information that is relevant for control of blackleg crown canker.

Table 4. Regional effectiveness of major gene resistance across 34 monitoring sites across Australia. Cultivars representing each of the resistance groups were sown adjacent to 34 canola trials across Australia and monitored for levels of blackleg. These data indicate which resistance groups have high levels of disease compared to the other groups at a particular site.

Key:

Low (L) blackleg severity compared to other groups at that site suggesting major gene resistance still effective - Continue with current management strategy.

  

Moderate (M) blackleg severity compared to other groups at that site – monitor crops for disease, see the Blackleg Management Guide for management options.

  

High (H) blackleg severity compared to other groups at that site – suggests major gene is ineffective and therefore disease control relies on quantitative resistance. If growing cultivars from this resistance group, select cultivar with appropriate blackleg rating for your region and consider a fungicide control for upper canopy infection if seasonal conditions are conducive – see the Blackleg Management Guide for management options.

 

No data (blank)

Site

Resistance Group

Victoria

A

B

C

ABD

ABDF

BF

BC

H

Charlton

H

M

H

L

L

H

M

 

Diggora

H

M

H

L

L

M

M

L

Hamilton

H

H

H

M

L

H

H

L

Kaniva

H

H

M

L

L

H

M

L

Lake Bolac

H

H

H

M

M

H

H

L

Minyip

H

H

H

L

L

H

M

 

Wunghnu

H

H

H

L

L

H

M

L

Yarrawonga

H

M

H

L

L

H

M

 

Site

Resistance Group

SA

A

B

C

ABD

ABDF

BF

BC

H

Arthurton

H

H

M

L

L

H

M

 

Bordertown

H

H

H

L

L

H

M

L

Cummins

H

M

H

M

L

H

M

L

Riverton

M

M

H

L

L

H

M

 

Roseworthy

H

M

H

L

L

M

M

 

Spalding

H

M

H

L

L

M

M

 

Wangary

H

H

H

H

M

H

H

 

Yeelanna

H

H

H

M

M

H

M

M

Site

Resistance Group

NSW

A

B

C

ABD

ABDF

BF

BC

H

Beckom

Insufficient data due to drought

Condobolin

Insufficient data due to drought

Cootamundra

H

H

H

L

L

M

M

L

Cudal

H

H

H

L

L

H

M

L

Gerogery

H

H

H

L

L

H

M

 

Grenfell

Insufficient data due to drought

Lockhart

Insufficient data due to drought

Parkes

Insufficient data due to drought

Wagga Wagga

H

H

H

L

L

H

M

L

Wellington

Insufficient data due to drought

Site

Resistance Group

WA

A

B

C

ABD

ABDF

BF

BC

H

Bolgart

H

H

H

L

L

H

M

 

Gibson

H

H

H

L

L

L

M

L

Katanning

H

H

M

L

L

M

M

L

Kendenup

No data

Kojonup

H

H

H

L

L

L

M

L

Stirlings South

H

H

H

L

L

M

M

L

Williams

H

H

H

L

L

H

H

L

Yealering

H

M

H

L

L

M

M

 

Commencement of flowering

There is a strong relationship between the earlier onset of flowering and yield loss caused by UCI.

Plants commencing flowering early in the growing season are more likely to be infected as they will flower under cooler wetter conditions which are conducive for lesion development. However, it is now also known that plants infected earlier in the growing season have more time for the fungus to damage the vascular tissue prior to plant maturity and harvest.

Canola plants are particularly susceptible to stress during the early stages of flowering (Kirkegaard et al. 2018). Evidence from controlled environment and field experiments indicates that plants infected by blackleg on the upper main stems and branches during the early flowering period results in the greatest reduction of grain yield compared to crops that flower later or are infected at later growth stages. Yield loss can be due to a reduction in seed size, seeds/pod and/or pods per m2. Oil content can also be reduced. By delaying the commencement of canola flowering, growers may be able to avoid severe UCI infections.

Fungicides

If UCI occurs, it has been shown that fungicides that are used to control Sclerotinia will also reduce UCI severity and yield losses. Application of Prosaro®/Aviator® Xpro for Sclerotinia control around 30% bloom can also provide protection from blackleg infection during early flowering. The 30% bloom spray may control flower, peduncle, stem and branch infections but is unlikely to provide pod protection. There are currently no control strategies for pod infection. High levels of pod infection tend to occur in seasons with frequent late rainfall events (such as 2016) or where there is physical damage to the pods from hail (such as 2018). In 2019, fungicide applications gave excellent control of UCI but did not control pod lesions. Although UCI was controlled it did not always result in yield returns from fungicides.

Acknowledgements

The research undertaken as part of this project has been made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC and ARC-linkage, the authors would like to thank them for their continued support.

Useful resources and references

BlacklegCM App for iPad and android tablets

Blackleg Management Guide - GRDC

Canola: the ute guide (https://grdc.com.au/resources-and-publications/groundcover/ground-cover-issue-27/canola-the-ute-guide)

Van de Wouw et al. (2016) Australasian Plant Pathology 45: 415-423

Marcroft Grains Pathology website: www.marcroftgrainspathology.com.au

Kirkegaard et al. (2018) Ten Tactics for Early-Sown Canola - GRDC

www.nvt.com.au

Contact details

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

Angela Van de Wouw
University of Melbourne
School of BioSciences,
University of Melbourne, VIC 3010
0439900919
apvdw2@unimelb.edu.au

Susie Sprague
CSIRO Agriculture and Food
Clunies Ross Street, Canberra, ACT 2600
02 62465387, 0466 643 227
Susan.Sprague@csiro.au

GRDC Project Code: UOM1904-004RTX, UM00051, CSP00187, MGP1905-001SAX,