Wimmera growers work to manage blackleg fungicide resistance

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.]

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.]

Table 3. Fungicide use as a percentage of all respondents from 2018 to 2020, SA Vic Bordertown Wimmera (88 responses) versus national (394 responses).

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.

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.