First Report of SDHI Resistant Strains of Venturia inaequalis From Commercial Orchards in Northern Italy

Venturia inaequalis (Cooke) G. Winter, the causal agent of apple scab, is a disease of great economic importance especially in regions such as northern Italy where relatively wet and cool weather conditions are commonplace in spring and early summer. Disease control on the susceptible apple cultivars primarily relies on the frequent application of single-site fungicides, leading to a high selective pressure for the development of resistant strains. While strains of V. inaequalis with resistance to several fungicide classes, such as dodine, myclobutanil (DMI), trifloxystrobin (QoI), and cyprodinil (AP) (Beresford et al. 2012; Fiaccadori et al. 2011; Köller et al. 2004; Larsen et al. 2013) have been observed, little is known about resistance to the SDHI fungicide boscalid in V. inaequalis. The increasing number of fungicide chemistries in the SDHI class and increasing use of this chemistry for disease management highlights the need for monitoring. During a fungicide resistance monitoring program carried out between 2011 and 2013 in 19 orchards located in northern Italy (Lombardy), four isolates characterized by reduced sensitivity to boscalid were identified. Infected leaves were randomly collected from the field and V. inaequalis strains were isolated from single germinating conidia placed on 1.5% water agar incubated at 20°C overnight. Fifty-six isolates were characterized for their sensitivity levels toward boscalid by measuring the mycelium growth on potato dextrose agar (PDA) medium at 4 to 6 weeks after incubation at 20°C. PDA was amended with technical grade boscalid at the following concentrations: 0, 0.01, 0.1, 1, and 10 mg/liter. The fungicide was dissolved in acetone at 10 g/liter and diluted to 1 g/liter in water before adding it to the medium. Three mycelial plugs derived from actively growing colonies were placed in individual petri plates (6 cm diameter) for each fungicide concentration. The assay was repeated three times. The EC50 values of the isolates were calculated by probit analysis (SPSS v. 21) and resistance factors (RF) were obtained by dividing the individual EC50 value by the average EC50 values of three sensitive strains collected from orchards never treated with fungicides. The majority of the strains were designated as sensitive to boscalid, having EC50 values of 0.85 ± 0.3 mg/liter and RF < 10. Four strains were designated as resistant strains, with one having an RF of 12, whereas the others had RF > 90, with EC50 values close to or higher than field rates for boscalid (138.6 mg/liter). Further investigations should be carried out to characterize the molecular mechanism of resistance. Italy is one of the top apple producers in the world, with about 2,000,000 MT production in 2012 (http://faostat.fao.org/site/339/default.aspx). It is important to monitor the presence of resistant V. inaequalis strains and to carefully follow antiresistance strategies in order to avoid the development of practical resistance.