Efficacy of Azoxystrobin Fungicide against Sore Shin of Shade Tobacco Caused by Rhizoctonia Solani

Azoxystrobin fungicide was evaluated for efficacy against Rhizoctonia solani, causal agent of tobacco sore shin, in greenhouse and in vitro tests. Azoxystrobin significantly increased root weights and reduced disease ratings compared with untreated controls in 2004 and 2005, and also reduced the percent of the stem girdled in 2005 (not measured in 2004). Some phytotoxicity (observed as flecks) was visible on 1 or 2 leaves of Quadris-treated transplants about 1 week after treatment. Leaves that expanded after treatment did not show symptoms. Activity of azoxystrobin fungicide in vitro was not as effective as might be expected from field observations and greenhouse experiments. Dose– response experiments of R. solani mycelial growth rate on azoxystrobin-amended half-strength Potato dextrose agar measured mycelial growth inhibition of only 30% by 1,000 mg/ ml ai. Salicylhydroxamic acid (SHAM), used to inhibit the alternative oxidase pathway in a number of fungi, was tested at 100 mg/ml SHAM in media to determine the efficacy of azoxystrobin against 2 isolates of R. solani in the absence of the alternative oxidase pathway. In the presence of SHAM, the inhibition of mycelial growth was over the LD50 for all concentrations tested, indicating that R. solani does use an alternative oxidation pathway in vitro. However, the observed inhibition decreased over time; inhibition averaged over all azoxystrobin concentrations was 63.3% after the first 24 hr, 52.0% after 48 hr and only 26.5% during the third–fourth day of exposure (P 5 0.001). It appears that an additional mechanism of alternative oxidation may become active over time in R. solani. Azoxystrobin (Quadris) is registered for management of tobacco blue mold and should be valuable as a transplant band treatment to protect shade-grown tobacco plants from sore shin caused by R. solani. Additional key words: alternative oxidase pathway, Nicotiana tabacum, Quadris, sore shin