Search

In Europe, glyphosate-resistant weeds have so far only been reported in perennial crops. Following farmers' complaints of poor herbicide efficacy, resistance to glyphosate as well as to ACCase and ALS inhibitors was investigated in 11 populations of Lolium spp. collected from annual arable cropping systems in central Italy. Field histories highlighted that farmers had relied heavily on glyphosate, often at low rates, as well as in a non-registered crop. The research aimed at elucidating the resistance status, including multiple resistance, of Lolium spp. populations through glasshouse screenings and an outdoor dose-response experiment. Target-site resistance mechanism was also investigated for the substitutions already reported for EPSPs, ALS and ACCase genes. Three different resistant patterns were identified: glyphosate resistant only, multiple resistant to glyphosate and ACCase inhibitors and multiple resistant to glyphosate and ALS inhibitors. Amino acid substitutions were found at position 106 of the EPSPs gene, at position 1781, 2088 and 2096 of the ACCase gene and at position 197 and 574 of the ALS gene. Not all populations displayed amino acid substitutions, suggesting the presence of non-target-site-mediated resistance mechanisms. After 39 years of commercial availability of glyphosate, this is the first report of multiple resistance involving glyphosate selected in annual arable crops in Europe. Management implications and options are discussed. [ABSTRACT FROM AUTHOR]

A comprehensive research project was set up to characterize by different approaches some Italian populations of Lolium spp. susceptible and resistant to diclofopmethyl. The present study was aimed at the taxonomic identification of these Lolium populations on the basis of the electrophoretic analysis of seed reserve proteins. Electrophoresis confirmed the great range of variation detected by preliminary morphological analysis. Approximately half the individuals in each population could be classified as belonging to Lolium multiflorum or Lolium rigidum, the most common Lolium species present in the area. However, electrophoretic patterns revealed a significant number (40-60%) of hybrid individuals in all populations. The origin of these off-types was postulated as resulting from both intrageneric hybridization among different Lolium species and intergeneric hybridization between Lolium and Festuca genera. No significant differences were detected in the taxonomy of herbicide-susceptible and -resistant populations. [ABSTRACT FROM AUTHOR]

The mechanisms of herbicide resistance were investigated in two diclofop-methyl-resistant Lolium spp. populations from central Italy, Roma ’94 and Tuscania ’97. These two populations were compared with two susceptible Italian populations (Vetralla ’94, Tarquinia ’97) and a resistant and a susceptible population from Australia, SLR31 and VLR1. The activity of acetyl Co-A carboxylase (ACCase) extracted from susceptible (S) or resistant (R) individuals from the Italian populations was inhibited by both aryloxyphenoxypropanoate (diclofop acid and fluazifop acid) and cyclohexanedione (sethoxydim) herbicides. Diclofop-methyl was rapidly de-esterified to diclofop acid at a similar rate in both R and S populations. In all populations, diclofop acid was subsequently degraded to other metabolites. The rate of degradation of diclofop acid was not significantly faster in R than in S populations; however, diclofop acid was degraded more completely in Roma ’94 and Tuscania ’97 compared with the S populations. Application of the mixed-function oxidase inhibitor 1-aminobenzotriazole (ABT) significantly enhanced diclofop-methyl toxicity towards both R populations, but not in S populations. However, enhanced herbicide metabolism does not completely account for the measured resistance level. A mechanism other than an altered ACCase and enhanced herbicide metabolism appears to be responsible for resistance to diclofop-methyl in Roma ’94 and Tuscania ’97. [ABSTRACT FROM AUTHOR]

Diclofop-methyl resistance was evaluated in populations of Italian ryegrass (Lolium multiflorum Lam.) infected and uninfected with fungal endophyte (Neotyphodium). Survival was tested in susceptible populations using herbicide screening. The results served as inputs to a model for investigating the role of endophyte infection in the evolution of L. multiflorum resistance to herbicide. The tolerance of infected plants varied depending on the origin of the population and the herbicide dose. Only in some populations and at some diclofop-methyl doses did plants infected with these endophytes have higher survivorship than endophyte-free plants. The model demonstrated that endophytes might indeed play an important role in the evolution of herbicide-resistant weeds, delaying the appearance of herbicide resistance. [ABSTRACT FROM AUTHOR]

Populations of Lolium spp. collected in central Italy were screened for resistance to acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides and compared with known susceptible and resistant Lolium rigidum (Gaud.) populations from Australia. Populations Roma’94 and Tuscania’97 were up to 8- and 7.5-fold more resistant to diclofop-methyl, respectively, than susceptible populations in pot experiments. However, populations Tarquinia’97 and Vetralla’94 were not resistant. Diclofop-methyl resistance levels in the Italian populations were lower than in the Australian populations SLR31 and WLR96 (16.5 and > 64 times more resistant than S respectively). In an agar germination test, Tuscania’97 showed low levels of cross-resistance to fluazifop-p-butyl, whereas no cross-resistance was found in the Roma’94 population. [ABSTRACT FROM AUTHOR]