Horseweed is an increasing problem in perennial crops and noncrop areas of the Central Valley of California. Similar to the situation in glyphosate-tolerant crops in other regions, glyphosate-based weed-management strategies in perennial crops and noncrop areas have resulted in selection of a glyphosate-resistant horseweed biotype in California. Research was conducted to determine the level of resistance to glyphosate in horseweed using an in vivo enzyme assay and to determine the distribution of the resistant horseweed biotype in central California. The resistant biotype was 4.8-fold more resistant to in vivo glyphosate exposure compared with the susceptible biotype, although enzyme function was inhibited in both biotypes at high glyphosate concentrations. An intermediate in vivo glyphosate dose was used to discriminate between glyphosate-resistant and glyphosate-susceptible individuals in a roadside survey conducted in 2006 to 2007. Overall, 62% of the individuals tested from the Central Valley were classified as resistant to glyphosate. Resistant individuals were found at most locations throughout the Central Valley, although the proportion of resistant individuals was slightly lower in the northern-most area. No correlation could be made between proportion of resistant or susceptible individuals and land use patterns likely because of long-distance seed dispersal or different selection pressure for resistant biotypes on field margins compared with that within fields. Horseweed with an economically significant level of resistance to glyphosate is already widely distributed in the Central Valley of California. Grower awareness of the problem and adoption of best management practices are needed to minimize the effects of horseweed in this highly productive and diverse agricultural region.

Nomenclature: Glyphosate; horseweed, Conyza canadensis (L.) Cronq. ERICA