Reduced sensitivity of the mushroom pathogen Verticillium fungicola to prochloraz-manganese in vitro

105 isolates of Verticillium fungicola from Spanish mushroom crops collected between 1992 and 1999 were tested in vitro for their sensitivities to prochloraz-manganese. Dose response relationships for inhibition of mycelial growth by the fungicide were assessed in radial growth experiments on fungicide-amended malt extract agar. The ED50 values recorded for all 105 isolates studied ranged between 0.8 ppm in 1992 and 8.8 in 1998, with an average of 2.9. 86% of the isolates tested were more sensitive to prochloraz-manganese and had ED50 values below 5 ppm, while the other 14% were slightly tolerant with ED50 values equal or above 5 ppm. Of those tested from 1999, 60% (21 isolates) grew with 50 ppm and 40% (14) also at 100 ppm, although mycelial growth was inhibited at least by 82 and 87%, respectively. The resistance factor calculated ranged from low fungicide resistance (RF = 3.0) in 1992 to moderate resistance (RF = 12.6) in 1998. These data confirm that the sensitivity of V. fungicola to the prochloraz-manganese gradually diminishes.