Stemphylium vesicarium resistance to fungicides on pear in Italy

Since the late ‘70s the most important fungal disease of pear in Italy has been brown spot caused by Stemphylium vesicarium, an anamorphic fungus (teleomorph Pleospora allii) that affects all green parts of pear tree, especially fruits. Many fungicide applications are required from petal fall to fruit ripening to control this pathogen. The most used compounds have been dithiocarbamates (mainly thiram), dichlofluanid, captan and dicarboximides (procymidone and iprodione) and more recently tebuconazole among SBIs, strobilurins kresoxim-methyl and trifloxystrobin, the ready mixtures fludioxonil+cyprodinil and pyraclostrobin+boscalid. In the early 1990s, problems in brown spot control were reported in some areas of northern Italy with schedules based on procymidone. Four phenotypes with different degrees of sensitivity to dicarboximides were pointed out by a monitoring study started in 1995. The present study presents an overview on the sensitivity of many S. vesicarium populations collected in the Po valley towards dicarboximides, phenylpyrroles and strobilurins. Dicarboximide resistant phenotypes were identified and quantified: the most frequent one was the R1 phenotype, which is highly resistant to procymidone and moderately resistant to iprodione, vinclozolin and chlozolinate. The R2 phenotype (highly resistant to all dicarboximides) and S+ phenotype (slightly resistant to procymidone and iprodione) were very rarely found. Cross resistance between dicarboximides and phenylpyrroles was noted in the R2 phenotype. All populations collected between 2002-2004 showed EC50s towards strobilurins comparable to baseline values. The minimum inhibitory concentration (MIC) towards these fungicides was lower than 0.5 mg/l for 323 populations considered up to 2006. Only one population collected in 2006 showed an MIC greater than 0.5 mg/l both for kresoxim-methyl and trifloxystrobin.