Reduced sensitivity of Venturia inaequalis to strobilurins and anilinopyrimidines in Italy

Abstract: A survey to verify the sensitivity of Venturia inaequalis to strobilurins and anilinopyrimidines was carried out in Northern Italy over many years (2003-2010) in more than 100 orchards where difficulties in scab control with these fungicides were reported. A sample of 30-40 scabby leaves was collected in each orchard treated with anilinopyrimidines and/or strobilurins. Bulk populations of V. inaequalis (one isolate for each sample) were evaluated by biological tests in vitro and in vivo for their sensitivity towards strobilurins and/or anilinopyrimidines. Concerning strobilurins, EC50 values in spore germination on water agar were calculated. Afterwards, through comparison with EC50s of previously characterized “reference populations”, the sensitivity of each population was assessed as ‘normal-moderately reduced’ or ‘reduced’. Sixty-six out of ninety-nine tested populations showed a reduced sensitivity, probably corresponding to field control issues. As regards anilinopyrimidines, % relative efficacy with a preventive or curative application on apple seedlings was assessed and the evaluation of population sensitivity was based on curative activity. Similarly to strobilurins, through comparison with “reference populations”, the isolates were ranked as ‘normally (or moderately less) sensitive’ and with ‘reduced sensitivity’, this latter case likely corresponding to field control issues. For more than half of thirty samples collected in Emilia Romagna, a reduced curative activity was observed, while samples from Lombardy (10) were almost all normally sensitive. The reduced sensitivity to the two fungicide groups shown by sampled populations can be related to field control issues pointed out in some Italian areas. In particular, for strobilurins this shows the occurrence of many cases of practical resistance. For anilinopyrimidines the resistance situation is less clear, but the reduced curative activity observed on some populations seems to indicate an ongoing sensitivity shift, with a moderate practical activity reduction. In any case, this new situation has led to reconsider the use of strobilurins and anilinopyrimidines, which since the late 1990s have become the key products against apple scab in Italy.