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Management of plant disease in agro-ecosystems ideally relies on a combination of host genetic resistance, chemical control, and cultural practices. Growers increasingly rely on chemical and genetic options, but their relative benefits in disease control, yield, and economic outcomes are rarely quantified. We explore this relationship for blackleg crown canker disease (caused by Leptosphaeria maculans ), a major biotic constraint limiting canola production globally. Data from 20 field trials conducted from 2013 to 2015 in canola-growing regions of Australia were used to assess the effects of host resistance and fungicide treatment on blackleg severity, grain yield, and gross margin. In the absence of fungicide, blackleg disease was 88% lower in the most resistant compared with the most susceptible blackleg resistance category. In the most susceptible resistance category, the most effective fungicide treatment significantly reduced blackleg severity (from 50 to 6%) and increased grain yield (478 kg/ha, 41%) and gross margin (AU$120/ha, 17%). However, the mean benefits of fungicide tended to decrease with increasing levels of genetic resistance, to the point that yield, disease, and gross margin benefits were close to zero in the most resistant cultivars. Overall, these findings suggest that fungicides can reduce blackleg severity, but the benefits of application strongly depend on associated levels of genetic resistance. Canola cultivars with higher genetic resistance reliably reduced blackleg disease and maintained grain yield without the associated cost of fungicide application. The intensification of canola production to meet increasing global demand will require strategies to sustainably manage and protect finite genetic resistance resources to control blackleg disease., Competing Interests: The author(s) declare no conflict of interest.

Continued use of fungicides provides a strong selection pressure towards strains with mutations to render these chemicals less effective. Previous research has shown that resistance to the demethylation inhibitor (DMI) fungicides, which target ergosterol synthesis, in the canola pathogen Leptosphaeria maculans has emerged in Australia and Europe. The change in fungicide sensitivity of individual isolates was found to be due to DNA insertions into the promoter of the erg11/CYP51 DMI target gene. Whether or not these were the only types of mutations and how prevalent they were in Australian populations was explored in the current study. New isolates with reduced DMI sensitivity were obtained from screens on DMI-treated plants, revealing eight independent insertions in the erg11 promoter. A novel deep amplicon sequencing approach applied to populations of ascospores fired from stubble identified an additional undetected insertion allele and quantified the frequencies of all known insertions, suggesting that, at least in the samples processed, the combined frequency of resistant alleles is between 0.0376% and 32.6%. Combined insertion allele frequencies positively correlated with population-level measures of in planta resistance to four different DMI treatments. Additionally, there was no evidence for erg11 coding mutations playing a role in conferring resistance in Australian populations. This research provides a key method for assessing fungicide resistance frequency in stubble-borne populations of plant pathogens and a baseline from which additional surveillance can be conducted in L. maculans. Whether or not the observed resistance allele frequencies are associated with loss of effective disease control in the field remains to be established., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Protection of many crops is achieved through the use of genetic resistance. Leptosphaeria maculans, the causal agent of blackleg disease of Brassica napus, has emerged as a model for understanding gene-for-gene interactions that occur between plants and pathogens. Whilst many of the characterized avirulence effector genes interact with a single resistance gene in the host, the AvrLm4-7 avirulence gene is recognized by two resistance genes, Rlm4 and Rlm7. Here, we report the "breakdown" of the Rlm7 resistance gene in Australia, under two different field conditions. The first, and more typical, breakdown probably resulted from widescale use of Rlm7-containing cultivars whereby selection has led to an increase of individuals in the L. maculans population that have undergone repeat-induced point (RIP) mutations at the AvrLm4-7 locus. This has rendered the AvrLm4-7 gene ineffective and therefore these isolates have become virulent towards both Rlm4 and Rlm7. The second, more atypical, situation was the widescale use of Rlm4 cultivars. Whilst a single-nucleotide polymorphism is the more common mechanism of virulence towards Rlm4, in this field situation, RIP mutations have been selected leading to the breakdown of resistance for both Rlm4 and Rlm7. This is an example of a resistance gene being rendered ineffective without having grown cultivars with the corresponding resistance gene due to the dual specificity of the avirulence gene. These findings highlight the value of pathogen surveillance in the context of expanded knowledge about potential complexities for Avr-R interactions for the deployment of appropriate resistance gene strategies., (© 2022 The Authors. Molecular Plant Pathology published by British Society for Plant Pathology and John Wiley & Sons Ltd.)