Fungicide sensitivity and resistance in the blackleg fungus, Leptosphaeria maculans, across canola growing regions in Australia
A. P. Van de Wouw
A * , J. L. Scanlan A , S. J. Marcroft B , A. J. Smith B , E. M. Sheedy B , N. W. Perndt B , C. E. Harrison B , L. M. Forsyth C and A. Idnurm A
+ Author Affiliations
- Author Affiliations
A School of BioSciences, The University of Melbourne, Parkville, Vic. 3010, Australia.
B Marcroft Grains Pathology, Grains Innovation Park, Horsham, Vic. 3400, Australia.
C Syngenta Crop Protection, Macquarie Park, NSW 2113, Australia.
Crop & Pasture Science 72(12) 994-1007 https://doi.org/10.1071/CP21369
Submitted: 27 May 2021 Accepted: 20 September 2021 Published: 29 November 2021
© 2021 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)
Abstract
Fungicide use has become a fundamental part of many crop protection systems around the world, including to control blackleg disease on canola (Brassica napus L.). In Australia, most canola growers routinely apply at least one fungicide, and potentially multiple fungicides with different modes of action, in a single growing season. There is evidence for the emergence of fungicide resistance in Leptosphaeria maculans, the causal agent of blackleg disease, to the demethylation inhibitor (DMI) class of fungicides in Australia. However, it is not known whether resistance exists towards other chemical classes such as the succinate dehydrogenase inhibitors (SDHI). In this work, 397 samples were screened for resistance towards seven fungicide treatments in stubble-borne L. maculans populations collected from eight canola-growing agro-ecological regions of Australia from 2018 to 2020, a time frame that bridges the introduction of new chemicals for blackleg control. We confirmed that DMI resistance in L. maculans is pervasive across all of the sampled canola-growing regions, with 15% of fungal populations displaying high levels (resistance scores >0.5) of resistance towards the DMI fungicides. Although resistance to newly introduced SDHI fungicides was low, we found evidence of positive cross-resistance between established DMI-only fungicides and a newly introduced combined DMI and quinone outside inhibitor fungicide, suggesting that the efficacy of the latter may be limited by widespread DMI resistance. Proactive surveillance, as performed here, may provide a means to avoid the rapid loss of fungicide efficacy in the field.
Keywords: blackleg disease, DMI fungicides, fungicide resistance, fungicide sensitivity, in planta assays, population surveys, QOI fungicides, SDHI fungicides.
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