Distribution and stability of QoI fungicide resistance in populations of potato pathogenic Alternaria spp. of Wisconsin

Authors: Halterman, Dennis; DING, SHUNPING - University Of Wisconsin; GEVENS, AMANDA - University Of Wisconsin

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2019
Publication Date: 6/20/2019
Citation: Halterman, D.A., Ding, S., Gevens, A. 2019. Distribution and stability of QoI fungicide resistance in populations of potato pathogenic Alternaria spp. of Wisconsin. Plant Disease. 103(8):2033-2040. https://doi.org/10.1094/PDIS-11-18-1978-RE.
DOI: https://doi.org/10.1094/PDIS-11-18-1978-RE

Interpretive Summary: In this manuscript, we describe the characterization of Alternaria solani and A. alternata (causal agents of potato early blight and brown spot, respectively) strains collected from Wisconsin. The ability of these strains to resist a commonly used fungicide was studied, as well as the genetic changes that may facilitate this ability. Stains were also subjected to fungicide in cultures in the laboratory and DNA changes in the microbes were studied over time. DNA changes within the cytochrome b gene were linked to fungicide resistance. The results of this work will impact the ability of scientists to accurately predict the ability of Alternaria strains to resist common fungicides which will, in turn, assist in the development of accurate fungicide application protocols for growers.

Technical Abstract: Quinone outside inhibitor (QoI) fungicides have been an important class in managing potato early blight caused by Alternaria solani and brown spot caused by A. alternata. Due to the single site mode of action character of QoI fungicides which were relied upon for management of diseases in Wisconsin, and the abundant asexual conidia production of the Alternaria species, pathogen isolates with QoI resistance were selected quickly after just a few years of QoI fungicide usage in commercial production fields. Resistance to QoIs in A. solani has been attributed to the F129L mutation in the cytochrome b gene, and resistance to QoIs in A. alternata has been attributed to the G143A mutation in the cytochrome b gene. A TaqMan SNP genotyping assay was designed based on the sequences of the cytochrome b gene (partial) from the isolates collected in Wisconsin. This assay successfully identified the mutations conferring QoI resistance in Alternaria isolates collected from 4 locations each year from 2015 to 2017. During the course of this study, the frequency of A. solani isolates which carried the F129L mutation was consistently high and showed primarily the TTA type of mutation; the frequency of A. alternata isolates which carried the G143A mutation started low and increased towards the end of the season in each year (P = 0.0109, P = 0.2083, P = 0.0159). The organic potato field in Madison showed a significantly lower (P < 0.05) frequency of A. alternata isolates carrying G143A than the commercial potato fields in Arena, WI. The overall frequency of A. alternata isolates carrying G143A in the four locations was similar over the three years (P = 0.2971). The difference in the frequency of isolates carrying QoI resistance mutations in the two Alternaria species may be related to their different host range. However, QoI resistant isolates could be selected rapidly in both Alternaria species, as evidenced by an elevated EC50 when grown on media amended with QoI for just a single subculture transfer. The QoI resistance characteristics of the isolates were stable even when QoI selection pressure was removed for several subculture transfers, and the type of codon 129 and 143 in the cytochrome b gene in A. solani and A. alternata, respectively, remained the same. This work provided important pathogen fungicide resistance characterization that was useful in aiding growers in adjusting their fungicide programs to most effectively manage diseases caused by Alternaria species.