Sensitivity of Phytophthora nicotianae From Tobacco to Fluopicolide, Mandipropamid, and Oxathiapiprolin.

Black shank incited by Phytophthora nicotianae is a devastating disease in the production of tobacco. Fungicides have been commonly used for managing the disease; however, there is only a narrow pool of effective fungicides. A few new fungicides became available in recent years, including fluopicolide, mandipropamid, and oxathiapiprolin, which reduced diseases incited by oomycetes under field conditions. Limited information is available regarding sensitivity of P. nicotianae isolates to these new fungicides. Research was conducted to determine effects of the three new fungicides on P. nicotianae isolates from tobacco in Georgia. Studies with 106 isolates indicated that they did not grow when agar medium was amended with the fungicides at the rate of 1 μg/ml. Twenty isolates were used for in vitro studies to determine sensitivity to the fungicides. Fluopicolide, mandipropamid, and oxathiapiprolin inhibited mycelial growth of the isolates with mean EC values (effective concentrations that provide 50% growth reduction) of 0.09, 0.04, and 0.001 μg/ml, respectively. EC values of fluopicolide, mandipropamid, and oxathiapiprolin for inhibiting sporangial formation were 0.15, 0.03, and 0.0002 μg/ml, respectively. EC values for suppressing zoospore germination averaged 0.16, 0.04, and 0.002 μg/ml for fluopicolide, mandipropamid, and oxathiapiprolin, respectively. Results from the study indicated that P. nicotianae isolates from tobacco in Georgia were sensitive to the fungicides, with lower EC for oxathiapiprolin than for fluopicolide and mandipropamid. The information on effectiveness and baseline sensitivity of fungicides on P. nicotianae will facilitate monitoring of resistance development in the pathogen population.