Fungicide sensitivity monitoring of Alternaria spp. causing leaf spot of sugarbeet (Beta vulgaris) in the Upper Great Lakes

Authors: ROSENZWEIG, NOAH - Michigan State University; Hanson, Linda; MAMBETOVA, SALTANAT - Michigan State University; JIANG, QIANWEI - Michigan State University; GUZA, COREY - Michigan Sugar Company; STEWART, JAMES - Michigan Sugar Company; SOMOHANO, PAULA - Michigan State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2019
Publication Date: 4/2/2019
Citation: Rosenzweig, N., Hanson, L.E., Mambetova, S., Jiang, Q., Guza, C., Stewart, J., Somohano, P. 2019. Fungicide sensitivity monitoring of Alternaria spp. causing leaf spot of sugarbeet (Beta vulgaris) in the Upper Great Lakes. Plant Disease. https://doi.org/10.1094/PDIS-12-18-2282-RE.
DOI: https://doi.org/10.1094/PDIS-12-18-2282-RE

Interpretive Summary: Alternaria leaf spot (ALS), a foliar disease caused by species in the fungal genus Alternaria, can occur wherever sugarbeet is grown. ALS historically has been considered a minor issue for sugarbeet in the US. Recently, increased incidence and severity of ALS, with leaf damage high enough to cause yield loss, has been observed in Michigan. With a new need to consider potential management of this disease, we tested the response of Alternaria spp. from sugarbeet to three classes of fungicides currently labelled on sugarbeet. Leaves with symptoms of ALS were collected in east-central Michigan and Western Ontario and used to obtain isolates. These were used to determine sensitivity to seven fungicides from the three different classes. A spiral gradient dilution method was used to estimate the fungicide effective concentration which caused a 50% inhibition of fungal growth (EC50) for all isolates. There were significant differences in isolate responses, and shifts were detected in the frequencies of sensitivity types to two of the fungal classes between 2016 and 2017. There also were significant differences in the frequencies of resistance among Alternaria spp. isolates depending on the location where the original samples were collected. As well as the first report of tin tolerance in a fungus other than Cercospora beticola, isolates of Alternaria spp. were found that had resistance to two or even all three of the fungicide classes tested. These results indicate that an integrated pest management system will be needed and there is a need for developing fungicide resistance management recommendations as well as effective disease management strategies and alternative practices are likely to be needed for this emerging disease issue in the region.

Technical Abstract: Alternaria leaf spot (ALS), caused by Alternaria spp., can occur wherever sugarbeet is grown. Infection by Alternaria spp. and disease management has historically been considered a minor issue in sugarbeet production in the US. Recently, increased incidence and severity of ALS high enough to cause yield loss has been observed in Michigan. With a new need to consider potential management of this disease, the sensitivity was determined for populations of Alternaria spp. to three classes of fungicides currently labelled on sugarbeet, including: demethylase inhibitor (DMI), quinone outside inhibitor (QoI), and organo-tin. Leaves with symptoms of ALS were sampled from sugarbeet fields in east-central Michigan and Western Ontario. Mono-conidial isolates were obtained to determine sensitivity to each fungicide class above. A spiral gradient dilution method was used to determine and estimate the fungicide effective concentration (mg L-1) which caused a 50% inhibition of fungal growth (EC50) in vitro for all isolates. Isolates of Alternaria spp. were categorized phenotypically into 5 sensitivity categories based on mean EC50 values. Significant temporal shifts were detected in the frequencies of sensitivity phenotypes to DMI and QoIs but not organo-tin fungicides from 2016 through 2017. Additionally, there was significant spatial variability in the frequencies of sensitivity phenotypes for recovered Alternaria spp. to DMI, QoIs and organo-tin fungicides from 2016 through 2017. Individual isolates of Alternaria spp. were recovered with cross resistance to DMI, QoIs, and triphenyltin OH fungicides. To our knowledge, this is the first report of a fungus other than Cercospora beticola with resistance to organotin fungicides. Fungicide sensitivity monitoring suggests that an integrated approach, combining fungicide efficacy trials and pathogen biology is essential in developing fungicide resistance management recommendations as well as effective disease management strategies.