Effects of Several Fungicides on the Spore Growth Period of Alternaria dauci, a Carrot Black Leaf Blight Fungus, Using a Rezasulin-based Spore Survival Assay

Abstract

The effects of five fungicides on the spore growth phase of Alternaria dauci, which causes carrot leaf blight, were tested using the spore viability assay (SVA) and agar dilution method (ADM). The average EC50 values for chlorothalonil against seven isolates of A. dauci examined by SVA and ADM were 14.21 μg/ml and more than 100 μg/ml. Dithianon and folpet also had lower EC50 values in SVA than in ADM, while iminoctadine tris-albesilate had lower EC50 value in ADM. For fluazinam, the EC50 values of SVA and ADM were 1.63 and 2.40 μg/ml, respectively. As EC50 values of five fungicides according to the spore growth phase of A. dauci KACC 42997, the efficacy of each fungicide as chlorothalonil, dithianon, and folpet decreased when treated after spore germination rather than when treated with spores before germination. However, iminoctadine tris-albesilate was more effective when treated after spores germinated than when treated before treatment. The excellent effect of fluazinam on the pathogen was maintained until A. dauci KACC 42997 was cultured in potato dextrose broth for 6 hr and the germ tube grew beyond the size of the spore. However, when treated with iminoctadine tris-albesilate and fluazinam after culturing for 12 hr, as the EC50 values of the two fungicides increased to 8.87 and 20.65 μg/ml, their efficacies decreased. The results of this study show that the treatment time of the fungicide should be determined by considering the effect of the fungicide on the spore growth phase of pathogens.