While lower Eyre Peninsula canola growers have gained new fungicides for the control of blackleg, simply replacing DMIs with repeated or high frequency use of SDHIs could encourage new resistant strains of the pathogen.

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

Blackleg management on the lower Eyre Peninsula

Case study

The Lower Eyre Peninsula region of South Australia enjoys an annual rainfall of 450-600 mm and regularly achieves canola yields in the range of 3 tonnes per hectare.

These factors encourage growers to plant canola in close rotations, including as a canola-wheat-canola program. Canola crops are planted adjacent to the previous seasons’ canola stubble.

The farming practices and high rainfall leads to considerable disease pressure from blackleg, which is managed with the selection of resistant cultivars and the use of treated seed, in-furrow and foliar fungicide treatments.

KEY POINTS

REGION: Lower Eyre Peninsula, South Australia
CROP: Canola
DISEASE: Blackleg of canola (Leptosphaeria maculans)
RESISTANCE: Group 3 DMIs
Varying degrees of resistance or reduced sensitivity to:
• Jockey® (fluqinconazole DMI)
• Impact® (flutriafol DMI)
• Prosaro® (prothioconazole + tebuconazole DMIs)
DISEASE PRESSURES: High rainfall, high yield potential
Tight rotations
Sowing adjacent to canola stubble
RESISTANCE PRESSURES: Above average foliar fungicide use

Cultivar choice

Owing to the consistently high disease pressure, growers in the region plant Resistant to Moderate Resistant (R-MR) canola cultivars far more frequently than growers nationally and Moderately Susceptible (MS) cultivars far less frequently [Table 1.]

Blackleg rating
Region R R-MR MR MR-MS MS MS-S
Eyre Peninsula, SA 4.3 52.2 27.5 10.1 2.9 2.9
Average (all responses) 8.4 16.3 27.5 12.0 32.6 3.1
Table 1. Canola selection for blackleg resistance from 2015 to 2020 as a percentage of responses, Lower Eyre Peninsula (69 responses) versus national (509 responses).

Perhaps because of this tendency, most major gene resistance to blackleg has been overcome in the region. Lower Eyre Peninsula growers now rely on a cultivar’s quantitative resistance (blackleg rating) rather than its absolute resistance.

Group B cultivars dominate in the area compared to Group A and ABD cultivars nationally. [Table 2.]

Blackleg R group
Region A AB ABD ABDF ABF AC AD AS B BC BF C H
Eyre Peninsula, SA 15.9 1.4 1.4 0.0 7.2 0.0 0.0 0.0 58.0 0.0 4.3 11.6 0.0
Average (all responses) 46.0 1.4 12.4 0.8 3.7 0.2 0.6 1.0 15.9 1.4 5.5 10.6 0.6
Table 2. Blackleg Resistance Group selection from 2015 to 2020, Lower Eyre Peninsula (69 responses) versus national (509 responses).

Fungicide use

These combined environmental and agronomic pressures mean fungicide use is seen as essential for blackleg control in the region.

Lower Eyre Peninsula canola growers have reported higher use of seed and in-furrow fungicides, more regular use of foliar fungicides, and a generally higher number of fungicide applications per season [Tables 3 and 4.]

Seedling fungicide use
Foliar fungicide application use
Region Jockey® Impact® in-furrow Jockey® + Impact® in-furrow None 4-6 leaf 20-50% bloom Both foliar applications None
Eyre Peninsula, SA 31.4 7.1 58.6 2.9 7.1 35.7 2.9 54.3
Average (all responses) 24.5 22.5 40.5 12.6 4.9 19.6 1.2 74.3
Table 3. Fungicide use as a percentage of all respondents from 2015 and 2020, Lower Eyre Peninsula (70 responses) versus national (494 responses).
Number of applications
Region 0 1 2 3 4
Eyre Peninsula, SA 2.9 12.9 61.4 22.9 0.0
Average (all responses) 10.1 35.8 43.3 10.5 0.2
Table 4. Number of fungicide applications per season as a percentage of all respondents from 2015 and 2020, Lower Eyre Peninsula (69 responses) versus national (494 responses).

Until 2020, the only fungicides available for blackleg control on the Lower Eyre Peninsula were the Group 3 demethylase inhibitors (DMIs) or “azoles”.

Group 7 Succinate dehydrogenase inhibitors (SDHIs) have since been registered for seed treatments, early foliar applications and late foliar applications. DMI+SDHI mixtures were also registered for early and late foliar applications from 2020.

This increased range of fungicide Modes of Action will be essential for the continued chemical control of blackleg by local growers.

 

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.

Canola samples submitted from the Lower Eyre Peninsula exhibited a significantly higher percentage of resistance detected to Jockey® but average resistance detected to the DMI compounds in Impact and Prosaro®. [Table 5.]

No samples exhibited resistance to Group 7 SDHI or Group 11 Strobilurin/quinone outside inhibitors (QoIs).

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

Region Detected
(%)
Not detected (%) Samples screened
Eyre Peninsula 54 46 70
Average (all regions) 43 57 489

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

Region Detected
(%)
Not detected (%) Samples screened
Eyre Peninsula 46 54 52
Average (all regions) 52 48 289

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

Region Detected
(%)
Not detected
(%)
Samples screened
Eyre Peninsula 21 79 52
Average (all regions) 22 78 289
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

While Lower Eyre Peninsula canola growers have gained new fungicides for the control of blackleg, simply replacing DMIs with repeated or high frequency use of SDHIs could encourage new resistant strains of the pathogen.

Local growers and agronomists will be able to protect the long-term effectiveness of their new fungicides by strategically using these fungicides 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 from different resistance groups 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 crops are sown early enabling the seedlings to 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 in combination should not both be needed 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. For 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 should also protect the long-term effectiveness of their new fungicides by reducing their dependence on chemical controls and embracing an Integrated Disease Management (IDM) strategy to reduce disease pressure. Where possible, this IDM strategy will begin with adding more rotations to 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