Sensitivity of Venturia inaequalis to dodine in Uruguay

The feasibility of resuming the use of dodine for post-infection spraying in Uruguay was evaluated. For this purpose, the current status of sensitivity of Venturia inaequalis populations in commercial apple orchards was determined. V. inaequalis populations collected from six commercial orchards, with different histories of dodine use in the last 10 years, were assessed by measuring mycelial relative growth (RG) at a single assessment concentration (SAC) of 0.2 mg a.i. L-1 of dodine. The SAC was previously determined by fitting linear regressions between log(10) EC50 and RG of a subset of isolates at seven increasing concentrations of dodine, from 0.05 to 5 mg a.i. L-1. In addition, the post-infection activity of dodine was evaluated in the field. For this, 2-year-old potted apple trees were exposed during pre-bloom stage to two natural rainfall events in an apple orchard with history of dodine use. Forty hours after the start of each rainfall, eight potted apple trees were hand sprayed with dodine at label concentration and eight were sprayed with water and used as controls. The in vitro assay indicates the presence of resistant isolates (RG > 90%) to dodine in all V. inaequalis populations assessed. Highly-resistant isolates (RG >= 95%) also were found in all populations, ranging from 3.4 to 28.6%, and showed statistical differences among orchards (P <= 0.10). In the field assay, dodine-sprays showed an incomplete control. Typical apple scab symptoms developed in dodine-treated (27.6% of the leaves) and control apple trees (60.6% of the leaves). These results show that dodine resistance is present in V. inaequalis populations of commercial apple orchards in Uruguay and confirms the occurrence of practical resistance in the field. Therefore, the resumption of dodine use should be cautious for apple scab management in Uruguay.