Distribution of glyphosate-resistant and glyphosate-susceptible hairy fleabane (Conyza bonariensis) in central California and their phenological development

Abstract

Glyphosate-resistant (GR) hairy fleabane was first documented in California in 2007. A preliminary study comparing plants from this population to a glyphosate-susceptible (GS) population determined that they developed more rapidly than the GS plants. However, it was not known if this was a general characteristic of all GR populations. Therefore, seeds of hairy fleabane were collected from 122 locations in central California. Plants from each location were grown and tested for resistance using a shikimate enzyme assay. Seeds from 90 locations germinated of which 27 % were determined as GR, 21 % as GS, and 52 % as intermediate/mixed. Plants from five GR and five GS populations, as determined by the assay, were randomly selected for comparisons and were grown in 2010/2011. Time taken by each plant to reach key phenological stages was recorded and converted into growing degree day (GDD) using a base temperature of 4.2 °C. The biomass of each plant was recorded at seed set. The five GR and GS plants had similar phenological development rates and biomass. Therefore, the differences previously identified between the two populations were not present in the additionally tested populations. The phenological development was hence described by a common GDD model with the plants requiring 374, 711, 970, 1289, and 1551 GDD, to reach the rosette, bolting, first bud, first flower, and first seed stages, respectively. This study shows widespread distribution of GR hairy fleabane in central California, provides useful information on biology of this weed for management, and suggests possible label changes to herbicide manufacturers.