Dynamics of genetic mutations conferring azoxystrobin resistance in two populations of the frogeye leaf spot pathogen, Cercospora nicotianae, under different spray schedules

Frogeye leaf spot in tobacco is initiated by the fungal pathogen, Cercospora nicotianae. Management approaches that reduce frogeye severity are crop rotation and application of azoxystrobin fungicides. In most years and for most tobacco growers, frogeye is not a major concern, especially when the disease occurs late in the season on the oldest tobacco leaves. However, in 2016, several crops in South Central KY were affected by frogeye unseasonably early and to a much greater extent. Frogeye spots appeared in the red? and tip?grade leaves in addition to very high severity in the flyings grade, which affected quality and compromised yields. The affected growers had used azoxystrobin fungicides in their production systems for 8?10 years, since their initial labeling, and some questioned the possibility of resistance development. After sampling and testing a small number of isolates, nearly all were found to grow on fungicide concentrations 10 or more times greater than the concentration that inhibited a Cercospora isolate not recently exposed to azoxystrobin. Rotation with fungicides with different modes of action is a common strategy to manage fungicide resistance within a population. However, it is unknown how quickly tobacco frogeye pathogen populations may respond to fungicide rotations. In two separate but related proposals, we have proposed to test four spray programs in two field trials to evaluate different combinations of fungicides for efficacy in managing frogeye leaf spot. One of these trials will be conducted in a grower?collaborator field where poorly controlled frogeye occurred in 2016, while the other will be conducted on a research farm with presumed fungicide?sensitive pathogen populations. Another proposal supports testing the frogeye isolates using typical microbiological methods to determine how sensitivity in the populations changes given the various fungicide programs. The project proposed here will further evaluate same? season changes in fungicide sensitivity in Cercospora isolates from a molecular perspective. This will be accomplished by screening for genetic mutations that confer resistance to azoxystrobin fungicides. The starting point for the genetic screens will be primers developed in other Cercospora pathosystems where resistance to azoxystrobins has been documented, including soybean and beet.