Resistance to Boscalid, Fluopyram and Fluxapyroxad in Blumeriella jaapii from Michigan (U.S.A.): Molecular Characterization and Assessment of Practical Resistance in Commercial Cherry Orchards.

Jacqueline Gleason,Cory A Outwater,Suzanne M Slack,Tyre J Proffer,Jingyu Peng,Nikki L Rothwell,George W Sundin

doi:10.3390/microorganisms9112198

Jacqueline Gleason, Cory A Outwater + Show 5 more

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https://doi.org/10.3390/microorganisms9112198

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Journal: Microorganisms	Publication Date: Oct 21, 2021
Citations: 6	License type: CC BY 4.0

Affiliation: Michigan State University

Abstract

Management of cherry leaf spot disease, caused by the fungus Blumeriella jaapii, with succinate dehydrogenase inhibitor (SDHI) fungicides has been ongoing in Michigan tart cherry orchards for the past 17 years. After boscalid-resistant B. jaapii were first isolated from commercial orchards in 2010, premixes of SDHI fungicides fluopyram or fluxapyroxad with a quinone outside inhibitor were registered in 2012. Here, we report widespread resistance to fluopyram (FluoR), fluxapyroxad (FluxR), and boscalid (BoscR) in commercial orchard populations of B. jaapii in Michigan from surveys conducted between 2016 and 2019. A total of 26% of 1610 isolates from the 2016–2017 surveys exhibited the fully-resistant BoscR FluoR FluxR phenotype and only 7% were sensitive to all three SDHIs. Practical resistance to fluopyram and fluxapyroxad was detected in 29 of 35 and 14 of 35 commercial tart cherry orchards, respectively, in surveys conducted in 2018 and 2019. Sequencing of the SdhB, SdhC, and SdhD target genes from 22 isolates with varying resistance phenotypes showed that BoscS FluoR FluxS isolates harbored either an I262V substitution in SdhB or an S84L substitution in SdhC. BoscR FluoR FluxR isolates harbored an N86S substitution in SdhC, or contained the N86S substitution with the additional I262V substitution in SdhB. One BoscR FluoR FluxR isolate contained both the I262V substitution in SdhB and the S84L substitution in SdhC. These mutational analyses suggest that BoscR FluoR FluxR isolates evolved from fully sensitive BoscS, FluoS, FluxS isolates in the population and not from boscalid-resistant isolates that were prevalent in the 2010–2012 time period.

Highlights

Cherry leaf spot (CLS), caused by the fungus Blumeriella jaapii (Rehm) Arx, is a significant disease problem on sour/tart cherry (Prunus cerasus L.)
We found that the boscalid-resistant B. jaapii strains isolated in Michigan were still susceptible to other succinate dehydrogenase inhibitor (SDHI) fungicides, including fluopyram and fluxapyroxad [4], and so Pristine was quickly replaced in CLS disease control programs by the fungicides Luna Sensation
Our results show that resistance to boscalid, fluopyram, and fluxapyroxad in B. jaapii is widespread in commercial tart cherry orchards in the major growing regions of Michigan

Summary

Introduction

Cherry leaf spot (CLS), caused by the fungus Blumeriella jaapii (Rehm) Arx, is a significant disease problem on sour/tart cherry (Prunus cerasus L.). Ascospores of B. jaapii are produced in apothecia on overwintered leaves on the orchard floor. The ascospores are typically released near petal fall and infect leaves via the stomata. In years with moderately warm and wet weather during bloom, initial infections can occur on bract leaves adjacent to flowers; this situation commonly results in CLS epidemics that can have a major negative impact on fruit ripening. As leaves accumulate CLS lesions, they become chlorotic and abscise; CLS-mediated defoliation typically follows initial leaf infection by about two to four weeks [1]. With CLS disease in Michigan, in situations where

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