Influencing factors of dipping seedling dip with thiophanate-methyl and chlorothalonil against the black rot of sweet potato

In this work, the effects of different factors on the infection of black rot of sweet potato seedlings were investigated and the necessity of optimizing seedling dip technology according to the types of fungicides was studied. Thiophanate-methyl with internal absorption activity and chlorothalonil without internal absorption activity were used in our experiments. Firstly, the control efficiencies on black rot of sweet potato seedlings using different dipping time and pesticide concentrations were determined, respectively. Secondly, the orthogonal experiments were conducted to envaluate the effects of dipping time, treatment concentration and spore concentration by measuring the disease index. Thirdly, the relationship between the dipping conditions and the control efficiencies was explored by the combination experiments of the dipping time and the treatment concentration. Finally, the control efficiencies of dipping seedlings were verified by the experiments in the disease nursery. When the dipping time was less than 6 h for the thiophanate-methyl group and less than 3 h for the chlorothalonil group, the control efficiencies against black rot of sweet potato seedlings increased with the dipping time. Similarly, When the treatment concentrations were less than 600 mg/L for the thiophanate-methyl group and less than 700 mg/L for the chlorothalonil group, their control efficiencies increased with the treatment concentrations. For the thiophanate-methyl group, the effects of spore concentration, dipping time and the pesticide concentration on the occurrence of disease were weakened successively. Whereas, for the chlorothalonil group, the effects of spore concentration, pesticide concentration and dipping time on the occurrence of disease were weakened successively. All three factors had significant effects on the occurrence of disease. The gap of control efficiencies between 700 mg/L and 400 mg/L of thiophanate-methyl widened with dipping time, and the maximum value was 26.8%. The gap of control efficiencies between 360 min and 30 min widened with concentration, and the maximum value was 42.8%. However, similar trends were not observed in test results of the chlorothalonil group. In the disease nursery, the control efficiencies of thiophanate-methyl and chlorothalonil treatment for 6 h were 83.6% and 85.2%, respectively. And the corresponding fresh weights were 60.1 g and 58.8 g, respectively. The performance of those two treatments were significantly better than others. However, when the dipping time was decreased to 2 h or 10 min, the control efficiencies of the chlorothalonil group were significantly higher than those of the thiophanate-methyl group, which were consistent with the pot experiments. Dipping time, pesticide concentration and spore concentration are the key factors which affect the control efficiency. The influences of each factor on the occurrence of disease vary with the type of the fungicide. The required dipping time of the thiophanate-methyl group is significantly higher than that of the chlorothalonil group. Different types of fungicides need to be matched with the corresponding dipping technology to ensure the control efficiency.