Assessment of fungicide resistance in fungal pathogens of grape in Oklahoma

Grapes are an important crop around the world, consumed as fresh fruit, raisins, and juices. As well, they are the main element for wine production. Historically, grape cultivation came to North America with the arrival of the first colonizers who brought French varieties, which were susceptible to diseases, while wild grapes grown in the territory were resistant. This fact made possible the exchange of plant material between Europe and America but also crosses of different species with the grape wild types of America to improve the establishment and disease resistance of this crop. Currently, America has the highest diversity of Vitis vinifera. Oklahoma was at the beginning of the 20th century one of the states with the highest wine production, which was almost completely closed by political changes. It wasn’t until the late 1990s that the wine industry began to rise again and kept growing until today. Grapes are susceptible to several fungal pathogens and the principal tool to control them are the fungicides. Constant application of fungicides can promote adaptations of the fungal pathogens to acquire resistance to fungicides. In this project, 331 samplings were performed between August 2019 and July 2020, from 5 different vineyards located in Oklahoma. A total of 331 fungal isolates were obtained and underwent morphological identification. Molecular identification of the isolates was performed using the internal transcribed spacer region sequence. A representative sample of 60 isolates that were recognized as potentially pathogenic was selected to perform fungicide resistance tests. Four commonly used fungicides in Oklahoma were used: Iprodione, Thiophanate methyl, Azoxystrobin, and Myclobutanil, were used in three different concentrations. 27 isolates from the collection showed greater radial growth at low doses than the control. 9 isolates displayed greater radial growth in comparison with the control at the second dose of the fungicides and only 1 isolate display stimulation at the high dose. Only one isolate presented stimulation to two different fungicides being the only cross-resistant example we obtained. The results showed possible hormetic activity due to exposure of the isolates to low doses of fungicides and the development of resistance to the highest concentrations of fungicides.