The Wimmera has many cereal and pulse options, which facilitates the use of good agronomic practices such as allowing three or four years between canola rotations and separating new crops from previous seasons’ stubble. These practices should help keep disease pressure at moderate levels. However, blackleg can still be an issue in above-average rainfall years and for individual crops.

Wimmera growers work to manage blackleg fungicide resistance

Case study

Many grain growers in the Wimmera region of Western Victoria include a canola rotation in their cropping program.

A medium rainfall range of 400 to 500 mm per annum offers good yield potential, with 1.8 to 2.0 tonnes per hectare considered average. The decision to plant canola will depend on prices for competing grains as well as retained soil moisture and the date of opening rains.

These factors and a suite of cereal and pulse options help limit the intensity of canola rotations in the region. Canola stubble, which provides the host for blackleg is retained with only windrows burnt. However, three- and four-year rotations are typical, and growers practice good separation between previous years’ canola stubble and new plantings.

KEY POINTS

REGION: SA / Victoria Bordertown Wimmera
CROP: Canola
DISEASE: Blackleg of canola (Leptosphaeria maculans)
RESISTANCE: Group 3 DMIs
High level of resistance to:
• Jockey® (fluqinconazole DMI)
• Impact® (flutriafol DMI)
REDUCED SENSITIVITY Prosaro® (prothioconazole + tebuconazole DMIs)
DISEASE PRESSURES: Heightened risk in high rainfall years
RESISTANCE PRESSURES:

Loss of gene resistance in cultivars
Historically limited to DMI fungicides

Cultivar choice

With generally low rotation frequencies and good stubble separation, blackleg pressure in the Wimmera is usually moderate but canola growers still select cultivars with good levels of resistance as blackleg can be severe in higher rainfall years.

Surveys shows that many growers in the region will select Moderate Susceptible (MS) cultivars more frequently than growers nationally [Table 1.]. Fewer growers are using Resistant-Moderately Resistant (r-MR) varieties compared to the national average.  

Blackleg rating
Agroecological zone R R-MR MR MR-MS MS MS-S
SA Vic Bordertown Wimmera 12.5 3.4 18.2 11.4 52.3 2.3
Average (all responses) 10.9 17.3 16.3 10.9 42.7 1.8
Table 1. Canola selection for blackleg resistance from 2018 to 2020 as a percentage of responses, SA Vic Bordertown Wimmera (88 responses) versus national (394 responses).

The most popular major gene resistance groups planted in the region are Group A and Group C.

These dominate almost 60% of total plantings with Wimmera growers selecting Group C cultivars at a far higher rate than the national average. Group ABD, Group BF and Group B cultivars make up most of the balance [Table 2.]

Blackleg R group
Agroecological zone A AB ABD ABDF ABF AC AD B BC BF C H
SA Vic Bordertown Wimmera 46.6 0 8.0 1.1 3.4 0 2.3 9.1 3.4 9.1 13.6 0
Average (all responses) 43.9 0.3 9.1 1.3 4.6 0.3 2.5 19.3 2.0 8.1 7.9 0.8
Table 2. Blackleg Resistance Group selection from 2018 to 2020, SA Vic Bordertown Wimmera (88 responses) versus national (394 responses).

Fungicide use

With medium rainfall conditions and agronomic practices that reduce blackleg pressure, Wimmera growers have favoured DMI chemistries, with Jockey® applications being above average while the combination of Jockey®+ Impact® in-Furrow is significantly lower. The percentage of growers applying no foliar fungicide treatment is also above average. [Table 3.]

Wimmera growers use fewer fungicide applications overall in managing blackleg. The majority need just one application per season, while a significant proportion use none [Table 4.]

Seedling fungicide use
Foliar fungicide application use
Agroecological zone Jockey® Impact® in-furrow Jockey® + Impact® in-furrow None 4-6 leaf 20-50% bloom Both foliar applications None
SA Vic Bordertown Wimmera 38.6 12.9 25.7 12.9 2.9 8.6 2.9 85.7
Average (all responses) 26.1 22.8 43.6 7.6 3.3 26.1 1.3 69.3
Table 3. Fungicide use as a percentage of all respondents from 2018 to 2020, SA Vic Bordertown Wimmera (88 responses) versus national (394 responses).
Number of applications
Agroecological zone 0 1 2 3 4
SA Vic Bordertown Wimmera 11.4 51.4 27.1 8.6 1.4
Average (all responses) 7.6 30 50.2 11.6 0.7
Table 4. Number of fungicide applications per season as a percentage of all respondents from 2018 to 2020, SA Vic Bordertown Wimmera (88 responses) versus national (394 responses).

Fungicide resistance

National screenings for blackleg resistance to the DMI fungicide Jockey were conducted in 2015, and to all current fungicide chemistries in 2018 to 2020. The screening system used identifies mutated blackleg spores that can cause infection on fungicide treated seedlings. The screen is a glasshouse screen that results in severe disease pressure, plant death will often occur in untreated seedlings.

  • Detected score indicates that fungicide treated seedlings have infection.
  • Not detected score indicates that very few / no seedlings were infected.

The fungicide resistance screens indicate that fungicide resistant mutants are present in the blackleg population, however we do not know what frequency they occur and do not think they result in fungicide field failure. In fact, no field DMI fungicide failures have been verified in Australia and therefore these fungicides can still be used.

Blackleg samples submitted by growers in the SA VIC Bordertown-Wimmera agroecological zone exhibited slightly lower levels of resistance compared to the national averages for all DMI fungicides. This is likely due to lower disease pressure resulting from the long cropping rotations [Table 5].

Group 7 Succinate dehydrogenase inhibitors (SDHI) and DMI+SDHI chemistries were registered for use against blackleg of canola in 2020. These alternatives will give Wimmera growers more opportunity to rotate their fungicide to reduce resistance pressure.

No resistance to Group 7 SDHI or Group 11 Strobilurin/quinone outside inhibitors (QoIs) has been detected in the national screenings.

Detection of pathogen mutants with resistance to Jockey® (DMI)

Agroecological zone Detected
(%)
Not detected (%) Samples screened
SA VIC Bordertown Wimmera 58 42 81
Average (all regions) 63 37 369

Detection of pathogen mutants with resistance to Impact® in-furrow (DMI)

Agroecological zone Detected
(%)
Not detected (%) Samples screened
SA VIC Bordertown Wimmera 63 37 81
Average (all regions) 66 34 369

Detection of pathogen mutants with resistance to Prosaro® (DMI)

Agroecological zone Detected
(%)
Not detected
(%)
Samples screened
SA VIC Bordertown Wimmera 26 74 81
Average (all regions) 37 63 369
Table 5a, b & c. Blackleg fungicide resistance levels as a percentage of total samples screened.

Fungicide strategies

  1. Only use fungicides if there is a high probability of yield return i.e. grow a cultivar with a high blackleg rating and scout for disease before making a decision to apply a foliar fungicide.
  2. Do not use two applications of the same mode of action (MOA) for crown canker control ie in-furrow, seed treatment, four-to-eight leaf foliar.
  3. The same MOA may be used once for crown canker control (seedling) and then again for UCI. Avoid using the same MOA more than twice in a season.
  4. If you use Group 7 SDHI or Group 11 Strobilurin/quinone for multiple applications for crown control you may not have the technology for very long.

Agronomic strategies

The Wimmera has many cereal and pulse options, which facilitates the use of good agronomic practices such as allowing three or four years between canola rotations and separating new crops from previous seasons’ stubble. These practices should help keep disease pressure at moderate levels. However, blackleg can still be an issue in above-average rainfall years and for individual crops.

Consistent use of Group 3 DMI fungicides has led to some DMI resistance and this should be seen as a warning that resistance will occur. Although SDHIs and Qols are currently completely effective, sustained use could promote new resistant strains of the pathogen to these fungicide groups.

Local growers and agronomists will be able to protect the long-term effectiveness of their new fungicides by strategically using chemical controls only when they are most likely to result in an economic yield gain.

The following agronomic considerations will help support fungicide longevity:

  1. Selection of cultivars with appropriate resistance and careful monitoring of each season’s blackleg risk will help reduce the need to apply fungicides. Planting the most resistant cultivar is generally best although rotating cultivars will limit the risk of blackleg overcoming the main genetic resistance.
  2. Seed treatments and flutriafol on fertiliser is less likely to be needed when seed is sown early because the seedlings can develop before any major winter spore showers occur. Later sowings should use a SDHI treatment in preference to a DMI.
  3. Seed treatment and fertiliser fungicide should not both be needed in combination, unless a susceptible cultivar is being planted.
  4. Ideally, growers will only need to resort to chemical controls when disease pressure demands it. For example, if severe blackleg lesions are found on plants before the fifth leaf stage a foliar fungicide from a different MoA Group to the seed treatment can be applied.
  5. When considering Upper Canopy Infection fungicide applications, crops that commence flowering early (July to early August) will have a greater probability of yield loss compared to crops that flower from mid-August onwards.
  6. Finally, growers can use the BlacklegCM app to model their potential costs and yield losses under different fungicide scenarios. If a foliar fungicide is applied, leaving an untreated strip will provide a valuable comparison to inform fungicide strategies for future seasons.

 Growers can also protect the long-term effectiveness of fungicides by embracing an Integrated Disease Management (IDM) strategy to reduce disease pressure and the need for chemical controls. This IDM strategy should include planting a variety of rotations in cropping programs and ensuring canola crops are planted more than 500 metres from a previous season’s canola stubble. 

For specific fungicide mode of action rotation guidelines visit the CropLife Australia website.

MORE INFORMATION:

Dr Steve Marcroft

Dr Steve Marcroft

Marcroft Grains Pathology

Dr Angela Van de Wouw

Dr Angela Van de Wouw

School of BioSciences, University of Melbourne