Reduced sensitivity of Venturia inaequalis to strobilurins and anilinopyrimidines has again changed apple scab control in Italy

Owing to suspected activity reduction of strobilurins and anilinopyrimidines in control of apple scab, a survey to verify the sensitivity of Venturia inaequalis to these fungicides was carried out in some apple growing areas of Northern Italy over a number of years (2003-2010). Infected leaves were sampled, mainly in commercial orchards where scab control issues were reported about programmes based on strobilurins and anilinopyrimidines. For each sample a bulk population of V. inaequalis was isolated and evaluated for its sensitivity by biological tests based on spore germination (DE50) on water agar for strobilurins and on curative activity (% relative efficacy) on apple seedlings for anilinopyrimidines. The assessments were made by comparison with three levels of sensitivity (normal, moderately reduced, reduced), preliminarily defined with similar tests on reference populations of V. inaequalis sampled in known situations (apple trees never treated = wild types, commercial orchards with good scab control by the aforementioned fungicides, experimental plots of an experimental orchard where an efficacy decrease was observed for the same products). For populations sampled in the present survey two sensitivity classes were defined: 1. normal-moderately reduced, likely corresponding to a normal field activity of strobilurins and anilinopyrimidines; 2. reduced, probably corresponding to field control issues. Concerning strobilurins, 66 on about 100 tested populations, collected in all sampled areas (Emilia Romagna, Lombardy, Friuli-Venezia Giulia, Trentino), showed a reduced sensitivity (table 1). As regards anilinopyrimidines, all 41 populations sampled in two areas, similarly to all reference populations preliminarily tested, exerted a normal or almost normal preventive activity. On the contrary, for more than half of 31 samples collected in Emilia Romagna a reduced curative activity was observed, while those coming from Lombardy (10) were almost all evaluated as normally sensitive (table 2). The reduced activity shown by the two fungicide groups on populations tested in this survey can be related to field control issues pointed out in some Italian areas, and to the occurrence of cases of practical resistance for strobilurins. On the contrary, for anilinopyrimidines the resistance situation is less evident, but the reduced curative activity observed on some populations, including those collected in our experimental centre where these fungicides have shown a loss of effectiveness, seems to indicate the occurrence of a sensitivity shift and of a reduced control. Nevertheless, the new situation led to reconsider the use of these fungicides, which in a few years had become the key products in control of apple scab in Italy. This is a further change in history of V. inaequalis control, that had already been modified by fungicide resistance development, in particular to benzimidazoles and DMIs. In practice, strobilurins have been now abandoned in some areas and are no longer utilized in high risk periods in other ones, while anilinopyrimidines continue to be used in several areas but only mixed with other products. In recent years this has led to an increasing use of other known fungicides, mainly dithianon among contact products and difenoconazole among systemic ones. The consequent repeated utilization of just a few active ingredients could again create conditions favourable to selection of resistant strains, especially concerning difenoconazole, that was not so much involved in resistance, unlike other former DMIs.