Control effect of fungicide pyraclostrobin alternately applied with Bordeaux mixture against apple Glomerella leaf spot and its residue after preharvest application in China

Apple Glomerella leaf spot (GLS) is caused by Colletotrichum fructicola, which causes severe leaf spot symptoms and leads to early defoliation, resulting in considerable economic losses. Since few efficient chemical control strategies were yet available in China, the aim of this work was to establish the basis of a control strategy for GLS. In the present study, we determined in vitro toxicity of three commercial fungicides prochloraz, pyraclostrobin and tebuconazole against mycelial growth and conidia germination of C. fructicola. The EC50 values of the three fungicides on mycelial growth ranged from 0.02 to 0.55 mu g ai mL(-1), and that on conidia germination ranged from 2.01 to 3.02 mu g ai mL-1. Two consecutive years of field experiment was conducted in 2017 and 2018 in four apple orchards of Shandong province in China for further verifying the control effect of these three agents. The four apple orchards that planted with susceptible cultivar 'Gala' use two training systems, one is the slender pyramid training system (SP), and the other is the slender spindle training system (SS). Besides the target agent treatment, Bordeaux mixture was also sprayed for one or two times in a growing season. Synthetic fungicide pyraclostrobin sprayed at the dose 810.00, 405.00, and 202.50 g ai ha(-1) in 2017, and 810.00, 405.00, and 303.75 g ai ha(-1) in 2018. Tebuconazole was applied at the dose 580.50 g ai ha(-1) in 2017, and prochloraz was applied at the dose 911.25 g ai ha(-1) in 2018. Results showed that the treatment of pyraclostrobin at 810.00 and 405.00 g ai ha(-1) combined with Bordeaux mixture have good control effect. The application frequency of fungicides in SS training system orchard is one to two times less than that in SP training system orchard, but the control effect of chemical agents in the former is greater than or equal to the latter. The final residues of all pesticides were below the Chinese national standards for pesticide residues. In conclusion, the application strategy of spraying pyraclostrobin combined with Bordeaux mixture can effectively prevent and control GLS caused by C. fructicola, and reasonable preharvest application of the fungicide will not cause excessive pesticide residues.

Automated summary

This study evaluated the toxicity of three commercial fungicides against Colletotrichum fructicola and conducted field experiments in apple orchards in China. The results showed that spraying pyraclostrobin combined with Bordeaux mixture effectively controlled Glomerella leaf spot, with no excessive pesticide residues.