Few studies on herbicide resistance report data to establish unambiguously the correlation between genotype and phenotype. Here we report on the importance of the EPSPS prolyl106 point mutation to serine (P106S) in conferring resistance to glyphosate in a goosegrass population from Davao, Mindanao Island, the Philippines (Davao). Initial rate-response studies showed clear survivors within the Davao population at glyphosate rates that completely controlled the standard sensitive goosegrass population (STD1). Assessment of potential resistance mechanisms identified the presence of P106S mutant individuals in the Davao population. Polymerase chain reaction (PCR) amplification of specific alleles (PASA) analysis established that the mixed-resistant Davao population was comprised of 39.1% homozygous proline wild-type (PP106), 3.3% heterozygous serine mutant (PS106), and 57.6% homozygous serine mutant (SS106) genotypes. Further rate-response studies on plants with a predetermined genotype estimated the Davao SS106 individuals to be approximately 2-fold more resistant to glyphosate compared to Davao PP106 individuals. Extensive analysis at different goosegrass growth stages and glyphosate rates established strong correlation (P < 0.001) between presence of P106S in EPSPS and the resistant phenotype. Importantly, no differences in the pattern of absorbed or translocated 14C–glyphosate were observed between PP106 and SS106 Davao genotypes or Davao and STD1 individuals, suggesting that glyphosate resistance in the Davao population was attributable to an altered target site mechanism. This study demonstrates that whilst P106S in EPSPS confers a moderate resistance level to glyphosate, the mechanism is sufficient to endow glyphosate failure at the recommended field rates.

The implementation of a successful glyphosate resistance management strategyrequires a simple and cost-effective method for detecting resistance in keyweeds. To date, however, glyphosate resistance is still routinely confirmedvia laborious and time consuming whole-plant pot assays using seedscollected at the end of the growing season. Here, we describe a simple,early-season bioassay for detecting evolved glyphosate resistance in grassand broadleaf weeds. It involves transplanting suspected glyphosateresistant seedlings alongside known sensitive and resistant standards intoagar containing informative rates of herbicide and recording percentagesurvival 14 d after plating. The method was validated using sensitive andresistant populations of Lolium, Eleusine, Conyza, and Amaranthus speciesencompassing the main glyphosate resistance mechanisms, namely, impairedtranslocation, EPSPS gene duplication, and mutations. The whole plant potand agar-based seedling tests generated comparable resistance indices indose-response assays and percentage survival at discriminating glyphosaterates. The method was applied successfully to detect resistance in a rigidryegrass population collected from a French vineyard well before glyphosatewas applied in the field for the current season. Additionally, the test wasshown to be highly transferable to several other grass and broadleaf weedsthat have evolved resistance to glyphosate. One major attribute of themethod is that it is capable of detecting resistance regardless of themechanism involved. In addition to being very simple, quick and,cost-effective, it allows determination of glyphosate resistance in weedsprior to field application. It thus offers the opportunity for an informedchoice of herbicides for effective weed control.