Resistance to dodine in populations of Venturia inaequalis in Quebec, Canada.

Dodine was introduced in the USA and Canada in the early 1960s for the control of apple scab. Following control failure, growers stopped using dodine in the mid1970s. Despite the curtailment of dodine use more than 30 years ago, persistent resistance to the fungicide was suspected in V. inaequalis populations. The dodine sensitivity was determined for two populations that were not exposed to dodine for at least 30 years − a wild type population (25 monoconidial isolates) and a population constructed with isolates collected in orchards managed for apple scab (156 isolates). The sensitivity to dodine was determined by monitoring growth of these isolates on agar Petri dishes amended with 0, 0.01, 0.1, 1.0, or 10.0 μg/ml of dodine. Sensitivity to fungicide was evaluated based on ED values. Both populations showed a lognormal distribution of ED values. The ED means were 0.525 μg/ml and 1.735 μg/ml for the wild type and managed orchards populations, respectively. In managed orchard, 31.4% of the isolates were resistant to dodine (ED > 1.0 μg/ml). Cross-resistance with myclobutanil and with kresoxim-methyl was tested and found not to be significant. The results of this study suggest that resistance to dodine is still present in the populations of V. inaequalis from Quebec and that reintroduction of dodine should only be done along with an appropriate resistance management strategy. Introduction Dodine (n-dodecylguanidine acetate) was introduced in the USA and Canada in the 1960s as a protective and eradicant fungicide for the control of apple scab, caused by Venturia inaequalis (Cke) Wint. Dodine was intensively used and provided satisfactory control for about nine years. In 1968, reduced control of apple scab with dodine was reported, and failure in controlling apple scab was reported 1 year later (22,23). Initially, resistance to dodine was reported for the Lake Ontario fruit belt counties and in the Geneva area of New York State. Resistance to dodine was then reported for other apple production areas of the USA (4,7,11,25), Canada (15,17,19), New Zealand (1), and Poland (16). In apple-growing areas of Eastern Canada, apple scab is the key disease for scheduling fungicide sprays. Most current strategies for managing apple scab are based on controlling primary infections caused by ascospores, in order to avoid epidemic build-up caused by secondary infections and, thus, reduce or eliminate the need for subsequent fungicide applications during the summer months. To achieve this goal, fungicide applications generally begin in the spring at the green tip phenological stage and continue throughout the spring to early summer, for a total of approximately 8 to 10 applications (13). The number of fungicide sprays made each year in individual orchards is related to the cultivar susceptibility and to the intended market (fresh or processed fruits). In some orchards, fungicides are applied routinely following predetermined fixedinterval; in other orchards, fungicides are applied based on risk estimated from various forecasting systems (14). In Quebec, sterol demethylation inhibitor fungicides (DMIs) and strobilurin-based fungicides (QoIs) are the two main classes of fungicides used in post infection management of apple scab (6). These fungicides are prone to selecting for resistance in V. inaequalis, and resistance to these compounds has been reported for most major apple production areas (2,3,5,6,8,12,18). 50