Objective Pathogen causing tomato leaf spot disease in Xinjian District, Nanchang City, Jiangxi Province was identified, and effective germicides investigated.

Method Possible pathogens were isolated and purified from the tomato leaves with typical symptoms of the leaf spot disease by tissue isolation method. Selected isolates were subjected to pathogenicity test, morphological observation, and a phylogenetic analysis based on rDNA-ITS, Alt a1, EF-1α, GAPDH, LSU, and RPB2 sequences. Toxicity of 7 fungicides against the identified isolates was determined in vitro by a mycelial growth rate method.

Result Twenty-two strains showing identical cultural characteristics were tested to be pathogenic to tomato plant with PDA cultural and morphological characteristics consistent with those of Alternaria genus described in literature. Perfectly matching sequences on the multiple genes, except EF-1α homogenous at 99.58%, applied for the phylogenetic analysis were observed between the strains and A. alternata as shown in GenBank. And at a 99% supporting rate of being in the same branch on the phylogenetic tree, the isolated strain, codenamed FQLGB1, was determined to be A. alternata, the pathogen that caused the tomato leaf spot disease. In the in vitro toxicity test, 40% difenoconazole suspension, 450 g·L⁻¹ prochloraz emulsion, and 500 g·L⁻¹ iprodione suspension with EC₅₀ of 0.1572 μg·mL⁻¹, 0.2688 μg·mL⁻¹, and 0.5310 μg·mL⁻¹, respectively, effectively inhibited the growth of FQLGB1.

Conclusion The causal pathogen of tomato leaf spot disease occurred in Xinjian District, Nanchang, Jiangxi was identified to be A. alternata. Either 40% difenoconazole suspension, 450 g·L⁻¹ prochloraz emulsion or 500 g·L⁻¹ iprodione suspension could be used for the disease control.