Forty percent prothioconazole-tebuconazole suspension concentrate(SC) has been registered to control multiple crop diseases in China. This study aims to evaluate its efficacy for the control of Alternaria leaf spot of ginseng, so as to lay the foundation for the management of resistance of Alternaria spp. causing Alternaria leaf spot of ginseng to fungicides. The study first detected its antifungal activity against three species(A. alternata, A. panax, and A. tenuissima) and biochemical activity against A. alternata and then determined its efficacy against Alternaria leaf spot of ginseng in vitro. Field experiments lasting for one year at three locations were conducted to evaluate its field efficacy against Alternaria leaf spot of ginseng. Spearman's correlation analysis was used to determine the cross-resistance of 40% prothioconazole-tebuconazole SC with eight commonly used fungicides. The results showed that prothioconazole-tebuconazole had a good inhibitory effect on mycelial growth and spore germination of the three Alternaria species, and the inhibitory effect on mycelial growth was stronger than that on spore germination. The treatment with prothioconazole-tebuconazole at the concentrations of 0.1, 1.0, and 10.0 μg·mL~(-1) resulted in deformity of mycelia, shortened germ tubes of conidia, reduced content of DNA and soluble proteins, and decreased permeability of cell membrane of A. alternata. Under in vitro conditions, the protective and curative effects of 40% prothioconazole-tebuconazole SC in the concentration range of 50-150 μg·mL~(-1) on Alternaria leaf spot of ginseng were 65.45%-100.00% and 65.58%-96.36%, respectively, which were significantly better than those of tebuconazole at the same concentration. There was no cross-resistance between prothioconazole-tebuconazole and the eight fungicides(difenoconazole, propiconazole, tebuconazole, iprodione, pyraclostrobin, azoxystrobin, kresoxim-methyl, and mancozeb). These eight fungicides were widely used to prevent and control Alternaria leaf spot of ginseng, and A. panax causing Alternaria leaf spot of ginseng had resistance to them. The results of the field experiments at three locations showed that 40% prothioconazole-tebuconazole SC was more effective than difenoconazole, propiconazole, tebuconazole, and iprodione 7, 15, and 30 d after its last application and had long effective duration. In conclusion, 40% prothioconazole-tebuconazole SC had high antifungal activity and biochemical activity against Alternaria spp. and better efficacy for the prevention and control of Alternaria leaf spot of ginseng in the field than the commonly used fungicides to which Alternaria spp. were resistant, and no cross-resistance was found between 40% prothioconazole-tebuconazole SC and the fungicides. It can be used for resistance management of Alternaria spp.
Keywords: Alternaria spp.; Panax ginseng; antifungal activity; biochemical activity; cross-resistance; field efficacy; prothioconazole-tebuconazole.