Characterization of Quinone Outside Inhibitor Fungicide Resistance in Cercospora sojina and Development of Diagnostic Tools for its Identification

Abstract

Frogeye leaf spot of soybean, caused by the fungus Cercospora sojina, reduces soybean yields in most of the top-producing countries around the world. Control strategies for frogeye leaf spot can rely heavily on quinone outside inhibitor (QoI) fungicides. In 2010, QoI fungicide-resistant C. sojina isolates were identified in Tennessee for the first time. As the target of QoI fungicides, the cytochrome b gene present in fungal mitochondria has played a key role in the development of resistance to this fungicide class. The cytochrome b genes from three QoI-sensitive and three QoI-resistant C. sojina isolates were cloned and sequenced. The complete coding sequence of the cytochrome b gene was identified and found to encode 396 amino acids. The QoI-resistant C. sojina isolates contained the G143A mutation in the cytochrome b gene, a guanidine to cytosine transversion at the second position in codon 143 that causes an amino acid substitution of alanine for glycine. C. sojina-specific polymerase chain reaction primer sets and TaqMan probes were developed to efficiently discriminate QoI-resistant and -sensitive isolates. The molecular basis of QoI fungicide resistance in field isolates of C. sojina was identified as the G143A mutation, and specific molecular approaches were developed to discriminate and to track QoI-resistant and -sensitive isolates of C. sojina.

Widespread Occurrence of Quinone Outside Inhibitor Fungicide-Resistant Isolates of Cercospora sojina, Causal Agent of Frogeye Leaf Spot of Soybean, in the United States

“…Isolates of C. sojina highly resistant to QoI fungicides were collected from a soybean field in Tennessee in 2010, for which QoI fungicides were ineffective in managing frogeye leaf spot (Zhang et al 2012a). This was the first report of QoI fungicide resistance in C. sojina, and these isolates were later confirmed to possess the G143A mutation, which confers resistance to QoI fungicides (Zeng et al 2015). Fungi with the G143A mutation have an amino acid substitution that occurs in the cytochrome b gene, where glycine is substituted with alanine at position 143 (Fungicide Resistance Action Committee 2014).…”

“…Fungi with the G143A mutation have an amino acid substitution that occurs in the cytochrome b gene, where glycine is substituted with alanine at position 143 (Fungicide Resistance Action Committee 2014). Since the initial observation of QoI fungicide-resistant C. sojina isolates in Tennessee in 2010, QoI fungicide-resistant C. sojina isolates also have been reported in Arkansas, Illinois, Kentucky, and Mississippi (Standish et al 2015;Zeng et al 2015;Zhang 2012). According to a survey conducted by the U.S. Department of Agriculture-National Agricultural Statistics Service, 11% of the 2015 soybean hectares planted in the United States received a fungicide application, and of those hectares, approximately 9% (approximately 2.9 million hectares) received a fungicide application of a product that contained a QoI fungicide as one of the active ingredients (https://www.nass.usda.gov/).…”

Cercospora zeinafrom Maize in South Africa Exhibits High Genetic Diversity and Lack of Regional Population Differentiation

“…Third, the genome sequence revealed the problem of additional copies of MAT gene exons found in some Dothidiomycetes (Bolton et al 2014), and allowed the design of a MAT idiomorph specific multiplex PCR assay. Finally, in the event that fungicide resistance is observed in C. zeina in future (challenge (iv)), draft genome sequence information can be used to design PCR primers to likely targets for mutations in resistant isolates, such as cytochrome B genes (Zeng et al 2015).…”

“…strobilurins) are popular foliar fungicides due to perceived additional plant health benefits (Mallowa et al 2015), however development of resistance by Cercospora species has been documented. Isolates of C. sojina from soybean have exhibited reduced sensitivity to QoI fungicides in several USA states (Standish et al 2015;Zeng et al 2015;Zhang et al 2012), and QoI resistance in C. beticola from sugarbeet has been reported from USA and Italy Bolton et al 2013). QoI resistance has not been reported from Cercospora pathogens of maize, however baseline sensitivities have been established for monitoring purposes (Bradley and Pedersen 2011).…”

“…The availability of genome sequence information for C. zeina in future would allow the design of diagnostic assays for common mutations that lead to QoI resistance, such as the G143A mutation in the cytochrome B gene reported in other Cercospora spp. (Zeng et al 2015).…”