Resistance of Phytophthora capsici to metalaxyl in plastic‐house capsicum crops in southern Italy*

In Calabria (southern Italy), control of crown and root rot of capsicum caused by Phytophthora capsici has relied primarily on soil drenches of metalaxyl. However, severe outbreaks occur every year in glasshouse crops, in which the practice of using plastic mulch and furrow irrigation favours the disease. Single-hypha isolates of P. capsici collected in Calabria in 1992/1998 were tested in vitro for their level of sensitivity to metalaxyl. Isolates of other species of Phytophthora were used as reference. Fungicide sensitivity was determined by plating mycelial plugs onto potato dextrose agar amended with metalaxyl, at final concentrations ranging from 0.1 to 1000μg mL−1 a.s. Inhibition of radial growth (%) was determined when colonies on unamended medium had covered approximately two-thirds of the plate. The ED50 values for inhibition of mycelial growth of P. capsici isolates ranged from 1.41 to44.6μg mL−1 a.s. More than 80% of the P. capsici isolates from commercial plastic-house crops in Calabria showed a moderate level of resistance as they were inhibited less than 60% at 5 μg mL−1 but more than 60% at 100μg mL−1