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ARS Home » Midwest Area » East Lansing, Michigan » Sugarbeet and Bean Research » Research » Publications at this Location » Publication #342617
Title: First report of QoI resistance in Alternaria spp. infecting sugar beet (Beta vulgaris) in Michigan, USA

Author
item ROSENZWEIG, NOAH - Michigan State University
item Hanson, Linda
item PRATT, DAVID - Michigan Sugar Company
item STEWART, JAMES - Michigan Sugar Company
item SOMOHANO, PAULA - Michigan State University

Submitted to: New Disease Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2017
Publication Date: 8/2/2017
Citation: Rosenzweig, N., Hanson, L.E., Pratt, D., Stewart, J., Somohano, P. 2017. First report of QoI resistance in Alternaria spp. infecting sugar beet (Beta vulgaris) in Michigan, USA. New Disease Reports. 36:5.

Interpretive Summary: Alternaria leaf spot (ALS) of sugar beet (Beta vulgaris), is caused by species for the fungus Alternaria. It is found wherever sugar beet is grown, but has been a minor issue for sugar beet production in the USA. It has been more important in some sugar beet production areas outside the USA. Since 2010, increased ALS has been observed in Michigan, USA at levels with the potential to cause yield loss. To investigate Alternaria spp. in the region, in 2015 and 2016 Alternaria spp. were tested for sensitivity to the major fungicide classes used for managing other leaf spot diseases in the Michigan growing region. Isolates with insensitivity to the quinone outside inhibitor (QoI) class of fungicides were recovered from fields in Michigan. Based on the percent of germinated conidia at varying fungicide concentrations, effective concentration for 50% inhibition in germination (EC50) was determined. The EC50 value for a sensitive, intermediate-resistant and resistant isolate was 0.38, 5.32 and 22.19 mg/L, respectively with similar results using two different assays. Resistant isolates contained a mutation known to confer QoI resistance in Alternaria species from other crops. The fungicide resistance may be a factor in the increased Alternaria leaf spot on sugar beet in the region.

Technical Abstract: Alternaria leaf spot (ALS) of sugar beet is caused by Alternaria spp. in the A. alternata species complex. ALS is common wherever sugar beet is grown, but historically has been a minor issue for sugar beet production in the USA with damage usually not affecting crop yield significantly. Occurrence of ALS is a concern in some sugar beet production areas outside the USA including parts of Europe, when loss of photosynthetic area results in significantly reduced sugar yields. Recently, increased incidence and severity of ALS has been observed in Michigan, at levels with the potential to cause yield loss. In 2015 and 2016 Alternaria spp. with reduced sensitivity to quinone outside inhibitor (QoI) fungicides were recovered from fields in Michigan. Mono-conidial isolates of Alternaria spp. from individual ALS lesions were obtained to determine sensitivity to the QoI pyraclostrobin. Isolate sensitivity was determined by the effective control of 50% of germinating conidia (EC50) on water agar amended with pyraclostrobin. The EC50 value for a sensitive, intermediate-resistant and resistant isolate was 0.38, 5.32 and 22.19 mg/L, respectively. Isolates also showed reduced fungicides sensitivity in a growth assay, with similar response based on a spiral gradient dilution method and a relative growth assay. To examine the genetics of resistance, genomic DNA was extracted from sensitive, intermediate-resistant and resistant isolates. Partial sequences of the cytb (227 bp) gene (GenBank: MF001498-MF001504) shared 99% identity to QoI-sensitive (GenBank: AY263408.1) and -resistant (GenBank: AY263409.1) reference isolates of Alternaria spp. Sequence analysis revealed that all intermediate and resistant isolates contained a substitution of G143A, which confers QoI resistance in Alternaria spp. The QoI resistant populations of Alternaria spp. may be one factor in recent disease control issues.