First Identification of P230L and H134R Mutations Conferring SDHIs Resistance in Stemphylium vesicarium Isolated from an Italian Experimental Pear Orchard

Since the late 1970s, brown spot of pear (BSP), a fungal disease caused by Stemphylium vesicarium (Wallr.) Simmons, has been one of the most important pear fungal diseases in Italy. To protect orchards from BSP, frequent fungicide application is essential throughout the period spanning petal fall to the onset of fruit maturation. In Italy, boscalid was the first succinate dehydrogenase inhibitor (SDHIs) fungicide authorised against BSP; subsequently, penthiopyrad and fluxapyroxad were authorised against the disease. In 2016 and 2017, SDHI compounds were applied against BSP as solo products at the University of Bologna’s experimental farm, showing a reduction in efficacy. Stemphylium vesicarium strains were isolated from leaves and fruit, and sensitivity assays and molecular analyses were performed. In vitro tests confirmed resistance to SDHIs, and two specific single-nucleotide polymorphisms were discovered, SDHB P230L and SDHC H134R, both leading to amino acid substitutions in succinate dehydrogenase subunits and confirming the resistant phenotype.