Pathogen Identification and Fungicide Selection for Leaf Spot Disease on Plectranthus scutellarioides in Fujian

  Objective  Identification of the leaf spot pathogen on Plectranthus scutellarioides in Fujian and screening fungicides for effective disease prevention and control were conducted.

  Method  A causative pathogen was isolated from the diseased tissues, and the species verified by Koch’s rule, morphological observations under a microscope, and molecular ITS and LSU sequence analyses. An in vitro toxicity test on 6 commercially available fungicides was performed on the pathogen to determine their efficacy.

  Result  The coleus leaves inoculated with the conidia of the isolated ZZCYC1706 produced same symptoms as those observed on the diseased plants in the field. The ITS and LSU sequences of ZZCYC1706 suggested it be Alternaria alternata. The toxicity test showed, among the 6 fungicides, 96% pyrisoxazole to be most effective in inhibiting the pathogenic growth with the lowest EC₅₀ of 0.621 μg·mL⁻¹. It was followed by 98% iprodione with an EC₅₀ at 1.155 μg·mL⁻¹ , 95% pyrimethanil at 5.258 μg·mL⁻¹, and 96.4% captan, which was the least effective, at 89.010 μg·mL⁻¹.

  Conclusion  The pathogen of the leaf spot disease on P. scutellarioides in Fujian province was identified as A. alternata, which was the first reported in China. The fungicides, pyrisoxazole, iprodione, and pyrimethanil, were shown in the laboratory effective in inhibiting the growth of A. alternata. And, being of different types, these fungicides could potentially be applicable for the coleus leaf spot disease control with a reduced concern of the development of drug resistance by the pathogen.