Assessing the resistance potential of Phytophthora nicotianae, the causal agent of black shank of tobacco, to oxathiopropalin with laboratory mutants

Black shank, caused by Phytophthora nicotianae van Breda de Haan, is a devastating disease of tobacco (Nicotiana tabacum L.) throughout the world. The newly-discovered fungicide, oxathiapiprolin has shown efficacy against P. nicotianae, but the potential for fungicide resistance is unknown. Production of oxathiapiprolin resistant P. nicotianae isolates was attempted through mass selection of zoospores, UV light mutagenesis, and mycelial adaptation through repeated culturing on oxathiapiprolin-amended medium. No resistant isolates were detected from mass zoospore selection. UV light mutagenesis of mycelium generated two isolates, EdgeB7-M1 and YadA28-M1, with stable mycelial and sporangial resistance to oxathiapiprolin. Isolate EdgeB7-M1 remained pathogenic to tobacco, while YadA28-M1 was nonpathogenic. For mycelial adaptation, 48 colonies, 12 replicate colonies from four isolates, were transferred 15 times on oxathiapiprolin-amended medium. Twelve of the 48 colonies had a significant increase in mycelial insensitivity to oxathiapiprolin; however, mycelial adaptation was not stable. Mycelial growth and sporangia production of the adapted isolates was significantly less compared to the wild-type whereas only one remained pathogenic to tobacco. In summary, the generation of oxathiapiprolin resistant P. nicotianae isolates was feasible. Constant exposure to oxathiapiprolin increased insensitivity, however fitness costs associated with insensitivity reduced the probability of isolate reproduction and survival.