Characterisation and management of Phalaris paradoxa resistant to ACCase-inhibitors.

Phalaris paradoxa is a competitive grass commonly found in durum wheat crops of central and southern Italy. Among the 85 populations screened from 1998 to 2008 for resistance to ACCase-inhibitors and graminicide sulfonylureas, 17 resulted as being resistant to at least one ACCase inhibitor while none of the populations showed resistance to sulfonylureas. ACCase resistance in hood canary-grass seems to be spreading rather slowly in Italy. Out of the 17 populations, seven were characterised through outdoor dose-response pot experiments to investigate resistance levels and cross-resistance patterns to ACCase-inhibitors and multiple resistance to other mode of action. Molecular bases of resistance to the recently introduced DEN herbicide pinoxaden were also investigated. Six populations were confirmed to be ACCase-resistant with various cross-resistance patterns. Two populations were resistant to all tested ACCase herbicides, with pinoxaden resistance indexes (RI) based on survival ranging from 22 to 50. The two populations have been molecularly characterised for resistance to pinoxaden. A single point-mutation in the ACCase gene was identified in each population, causing the amino-acid substitutions of Ile1781Val and Asp2078Gly in 0478L and 0025, respectively. The results suggest that resistance of P. paradoxa to pinoxaden is due to an altered target site and different mutations cause different resistance levels. The biological characteristics of the species, mainly self-pollinated, and the absence of multiple resistance allow herbicides with different modes of action to be used for controlling ACCase-resistant populations. Chemical tools should be carefully used within integrated weed management strategies.