BASELINE SENSITIVITY OF STEMPHYLIUM VESICARIUM OF PEAR TO SDHIs AND FLUAZINAM

Brown spot of pear (BSP), a fungal disease caused by Stemphylium vesicarium (Wallr.) Simmons, is the most important pear fungal pathogen in Italy since late seventies. Many fungicide applications are required from petal fall to fruit ripening to protect orchards from BSP. The fungus showed field resistance against key products as dicarboximides and strobilurins. The introduction in field of new fungicides with different mode of action is thus fundamental. Boscalid was authorized in Italy as first SDHI (inhibitor of the succinate dehydrogenase in complex II) against BSP in 2007. The aim of this study was to evaluate the baseline sensitivity of 43 isolates of S. vesicarium, collected before 2007 from pear orchards located in Po Valley area, to recent and broadspectrum fungicide SDHIs (fluxapyroxad, fluopyram, penthiopyrad, bixafen, isopyrazam) and fluazinam, an uncoupler of oxidative phosphorylation. Sensitivity assays were carried out on spore suspension (YBA liquid medium; final density 2 x 104/mL) in microtiter plates using the low time consuming and validated spectrophotometric method. For each compound 0-0.02- 0.05-0.5-1-2.5 mg/L concentrations were tested on each isolate, in four replicates. After two days of growth, absorbance (450nm) was evaluate and EC50 values were calculated by probit analysis. Overall baseline sensitivity data, carried out on 43 isolates showed for tested fungicides EC50 values ranging from 0.01 to 0.52 mg/L. These data describe the initial sensitivity level prior to the introduction of the fungicides and are essential information in resistance monitoring program to detect potential shifts in pathogen sensitivity.