Paddock Practices: SDHI fungicide resistance detected in the blackleg fungus

Paddock Practices: SDHI fungicide resistance detected in the blackleg fungus

Date: 13 Dec 2024

Melbourne University researchers
Melbourne University researcher Dr Angela Van de Wouw (rt) and PhD student Alec McCallum (lt) are collecting and testing blackleg fungus samples to track any emerging fungicide resistance. Photo: Anvil Media

Canola growers should take steps to reduce fungicide resistance developing following a detection of SDHI fungicide resistance in the blackleg fungus from samples taken from the Eyre Peninsula, South Australia.

Key points

  • Resistance to SDHI (Group 7) fungicides has been detected in the blackleg fungus for the first time in Australia from samples taken on the Eyre Peninsula, South Australia. No field failure has been reported yet.
  • Growers should use a range of strategies to manage canola to minimise disease pressure. For example, select cultivars with a high blackleg rating, reduce fungicide applications, rotate chemical groups, limit SDHI fungicide use to twice per season. Refer to the Blackleg Management Guide for more recommendations.
  • Monitor crops to ensure fungicides are working efficiently and consider leaving unsprayed strips for comparison.

How and where the resistance was detected

The University of Melbourne has detected SDHI resistance in a small number of blackleg samples collected from the Eyre Peninsula, South Australia. No field failure has been reported at this stage.

The resistance was detected as part of routine monitoring and surveillance for the emergence of fungicide resistance. Routine monitoring has been conducted in other canola growing regions. Resistance to SDHI fungicides has not been detected outside of the Eyre Peninsula.

Recommendations for growers and industry

  • Where possible, use management strategies to minimise disease pressure such as selecting cultivars with high blackleg rating and isolating this year’s canola crop at least 500m away from last year’s canola stubble.
  • Select cultivars with an adequate blackleg rating for your region to reduce reliance on fungicides for controlling blackleg disease.
  • In areas where resistant isolates have been detected, if using a SDHI as a seed treatment or foliar application, use an effective fertiliser/in-furrow treatment combination, tank mix or co-formulation with an alternate mode of action that includes a DMI (Group 3) or QoI (Group 11) fungicide.
  • If a SDHI seed treatment has been used with a foliar application to control blackleg, the seedling fungicide application at 4-6 leaf stage targeting blackleg should not contain a SDHI.
  • Do not apply more than 2 applications containing SDHI fungicides per growing season.
  • If fungicides are required, minimise the number of applications. For example, if sowing early, avoid using a 4-6 leaf foliar spray for crown canker. If sowing late, you may require 4-8 leaf foliar application for crown canker but could avoid upper canopy infection and not require a spray at 30% bloom.
  • If putting on multiple applications in a season, rotate chemical groups as well as specific actives, where possible.
  • If applying fungicides for Sclerotinia, be aware that these sprays will also put selection pressure on the blackleg pathogen even if you aren’t targeting to control blackleg.
  • Monitor crops to ensure fungicides are working efficiently. Potentially leave unsprayed strips for comparison. Report any potential field failures to Dr Angela Van de Wouw.

The actives tested and the results

Two different kinds of isolates (an isolate is an individual in a population of blackleg spores) different types of the blackleg fungi – showed resistance to the SDHI actives pydiflumetofen, fluopyram, fluxapyroxad and bixafen.

One type of isolate is completely resistant to all SDHI actives tested. The other type has increased tolerance to the SDHI actives pydiflumetofen and fluxapyroxad, but not to fluopyram.

Neither of these isolate types showed any resistance towards the DMI or QoI actives, however there was reduced efficacy towards fludioxonil (Group 12).

Isolates with increased tolerance to the DMI fungicides have continued to be detected from most regions across Australia. These isolates have increased tolerance, meaning the fungicides have reduced efficacy, rather than complete loss, towards these isolates.

The effect of different fungicides

Isolates susceptible to fungicide (susceptible isolate) cannot cause disease on the DMI-treated or SDHI-treated canola seedlings. The fungicides remain effective.

In comparison, the SDHI-resistant isolate can cause complete disease on the SDHI-treated seedlings. However, these SDHI-resistant isolates are still completely sensitive to the DMI fungicides.

The DMI-resistant isolate can cause partial disease on the DMI-treated seedlings but no disease on the SDHI-treated seedlings.

This work is a co-investment of the Australian Research Council Industry Fellowship, Syngenta and the Grains Research and Development Corporation (GRDC).

Resistant panel
The effects of different fungicides on different fungicide-resistant blackleg isolates. Photo: University of Melbourne

Useful resources

More information

Dr Angela Van de Wouw
University of Melbourne
apvdw2@unimelb.edu.au

GRDC Project Code: MGP2307-001RTX,