Sensitivity assessment of Phytophthora nicotianae to fungicides of fluazinam and its mixture with carboxylic acid amide

Tobacco black shank caused by Phytophthora parasitica var. nicotianae (Breda de Hean) is one of the most serious diseases in tobacco growing, and few effective fungicides are available currently. The purpose of this study was to assess the feasibility of using fluazinam to treat tobacco black shank disease. In vitro toxicities of fluazinam to 62 isolates of P. nicotianae were determined by mycelial growth rate method. The EC₅₀ values ranged from 0.294 6 to 2.389 9 μg/mL with a mean of 1.084 0 μg/mL. The sensitivity distribution was unimodal, and there was no resistant isolate in P. nicotianae population. The carboxylic acid amide (CAA) fungicides were single-site in action and activity to oomycetes. The synergistic interactions were tested by Wadley method between fluazinam and dimethomorph, flumorph, mandipropamid, respectively. The mixture containing dimethomoph:fluazinam at 1:2 ratio showed additive actions, with the synergy ratio of 1.2. The mixture containing flumorph and fluazinam at 1:5 ratio had the highest synergy ratio (SR 2.07). Mandipropamid and fluazinam had an additive interaction and the proportion of 2:1 had the lowest EC₅₀ value (0.020 7 μg/mL). This study demonstrated the sensitivity of P. nicotianae to fluazinam, and provided the experimental foundation to use fluazinam for tobacco black shank management.