Efficacy and timing of application of oxathiapiprolin against black shank of flue-cured tobacco

Phytophthora nicotianae, the causal agent of black shank, is one of the most important soilborne pathogens of tobacco (Nicotiana tabacum L.). The use of soil applied fungicides has been a significant component of effective black shank management for tobacco growers. Oxathiapiprolin is a new fungicide with a novel mode of action that has shown efficacy against many oomycetes, including P. nicotianae. Studies were conducted in 2012 in fields naturally infested with P. nicotianae to determine the efficacy of different rates, methods, and timing of applications of oxathiapiprolin against black shank. Field trials were conducted in 2013 and 2014 to examine the efficacy of alternating applications of oxathiapiprolin and mefenoxam against black shank. Fungicide treatments were applied to the soil on the day of transplanting tobacco to the field, on the day of first cultivation (14-25 days after transplant), and during last cultivation (46-65 days after transplant). A single application of oxathiapiprolin in the transplant water at 0.07, 0.14, or 0.28 kg a.i./ha significantly decreased area under the disease progress curve (AUDPC) compared to the non-treated control in the 2012 field trials. In addition, treatments with three applications of oxathiapiprolin at all tested rates significantly reduced AUDPC compared to non-treated control. In field trials from 2013 and 2014, alternating applications of oxathiapiprolin at transplant, mefenoxam at first cultivation, and oxathiapiprolin at last cultivation significantly lowered AUDPC compared to the non-treated control. In contrast, the AUDPC after three applications of mefenoxam at transplant, first cultivation, and last cultivation did not differ significantly from the non-treated control. Overall, our results indicate that soil-directed applications of oxathiapiprolin, alone or in alternation with mefenoxam are efficacious for the reduction of black shank of tobacco.