EVOLUZIONE DELLA SENSIBILITÀ DI STEMPHYLIUM VESICARIUM, AGENTE DELLA MACULATURA BRUNA DEL PERO AI FUNGICIDI IN ITALIA

Riassunto E’ stata osservata una diffusa resistenza di Stemphylium vesicarium ai dicarbossimidici e la sua regressione, in seguito all’interruzione dei trattamenti, in tempi variabili a seconda della frequenza di individui resistenti selezionati nel frutteto. Si è riscontrata una resistenza incrociata del patogeno tra dicarbossimidici e fludioxonil, presente però solo in un fenotipo raro e che pertanto non costituisce un reale problema in campo. La sensibilità di S. vesicarium agli analoghi delle strobilurine, dopo circa dieci anni dalla loro introduzione in campo, non ha subito modifiche sostanziali, anche se sono stati recentemente riscontrati sporadici isolati resistenti. I noti ditiocarbammati e il recente boscalid non hanno evidenziato variazioni di sensibilità. Evolution in Italy of the sensitivity to fungicides in Stemphylium vesicarium, the causal agent of pear brown spot Abstract The control of Stemphylium vesicarium, the causal agent of pear brown spot, needs many treatments from petal fall to fruit ripening, therefore a knowledge of the effectiveness of fungicides is important. After the occurrence of the first problems in pear brown spot control in the early 1990s with procymidone, in vitro studies have demonstrated the occurrence of pathogen resistance in Italy. Further studies identified four phenotypes of S. vesicarium with different degrees of sensitivity to dicarboximides: the sensitive (S), the S+ slightly resistant to procymidone and iprodione, the R1 highly resistant to procymidone and moderately resistant to the other dicarboximides and the R2 highly resistant to all dicarboximides. These phenotypes are spread in all pear growing areas of Po valley. Fortunately the sensitive phenotype is still the most frequent followed by the R1 phenotype. The other resistant phenotypes are very rarely found in the field. Another important aspect considered was the possibility of the resistance to regress. The frequency of the phenotypes inside the populations of each orchard influences the time of resistance regression. If the resistant frequency is high more years of interruption of treatments are needed. The cross resistance between dicarboximides and phenylpyrroles was observed only in R2 phenotype, therefore it does not represent a real problem in the field because of their rarity. A wide monitoring was carried out also to evaluate the sensitivity of S. vesicarium to strobilurins. Ten years after their introduction in the field, they are still active and with EC50 values comparable to baseline ones, albeit the first few resistant isolates have been found since 2006. Only one of these cases was linked to a failure of strobilurins in pear brown spot control and it has appeared after several years of intense use of these compounds. Nevertheless this is an interesting information, because it demonstrates that S. vesicarium is able to develop strobilurin resistance and therefore it underlines again the importance of following strictly anti-resistance strategies. No changes in sensitivity were noted in S. vesicarium to dithiocarbamates, multi-site fungicides with a low resistance risk but widely used against pear brown spot since its appearance in the late 1970s. The study of S. vesicarium sensitivity to boscalid had the same result, in fact two years after its registration on pear in Italy, the level of sensitivity is comparable to that of populations collected before its introduction in the field.