Sensitivity of Phytophthora infestans to mandipropamid and the effect of enforced selection pressure in the field

Sensitivity to the new carboxylic acid amide (CAA) fungicide mandipropamid (MPD) in Phytophthora infestans was measured for isolates collected between 1989 and 2002 in Israel prior to the commercial use of MPD (baseline sensitivity, 44 isolates), and from MPD-treated (25 isolates) and untreated fields (215 isolates) in nine European countries and Israel between 2001 and 2005. All isolates were sensitive to MPD, with EC₅₀ values ranging between 0.02 and 2.98 μg mL^-1. Plastic-tunnel (UK), shade-house (Israel) and field experiments (Israel) conducted during 2001-05 showed that enforced selection pressure, applied preventively or curatively, imposed by repeated sublethal (5 μg mL^-1) or excessive (500-1000 μg mL ^-1) doses of MPD on mixed isolates of P. infestans produced no isolates resistant to the compound. The results of this study indicate that the probability of a buildup of resistant sub-populations of P. infestans to mandipropamid in the field is low.