Three metalaxyl sensitivity levels in Australian isolates of Pseudoperonospora cubensis (Berk. et Curt.) Rost

Abstract

Isolates of Pseudoperonospora cubensis from the Murrumbidgee Irrigation Area (MIA), New South Wales, and the Burdekin district, Queensland, were compared for metalaxyl sensitivity with a known sensitive isolate. Several different fungicide application techniques were used (foliar sprays, soil drenches, floating leaf discs), but the relativity of metalaxyl sensitivity between isolates was maintained. Isolates from the Burdekin district were highly resistant and came from fields where control of downy mildew was poor. Isolates from the MIA showed intermediate sensitivity but there was no apparent loss of field control. In a floating disc experiment the EC50 values of sensitive, intermediate, and resistant isolates were in the ranges >0.01-<0.1, >1-<10, and >100 µg metalaxyl/mL, respectively.