ABSTRACT
The American willowherb, Epilobium ciliatum Rafin (syn.: E. adenocaulon Hausskn.), is rapidly spreading in Belgium and may occur as a weed in tree nurseries, fruit plantations, maize and at sites with total weed control. This species also occurs in long-term herbicide experiments in fruit plantations where 2-chloro-triazines, alone or in combination with paraquat or aminotriazole, have been applied each year since more than 20 years. These cases concern triazine-resistant biotypes of this species.
In whole plant laboratory bioassay studies a triazine-resistant biotype was about 30–40 times less susceptible to 2-chloro-triazines than a susceptible one. Metribuzine, lenacil and certain urea herbicides were also clearly less active against the triazine-resistant biotype; some other ureas and cell division inhibitors however displayed about equal activity against resistant and susceptible origins.
A paraquat-tolerant biotype was supporting higher rates of paraquat applied post-emergence and higher concentrations during germination.
A comparison of 12 biotypes (resistant and susceptible) revealed a high variability between biotypes concerning their leaf colour and dimensions, plant height and fresh matter production although these differences could not always be related to the reaction to s-tria-zines. Seeds of neither biotype showed any dormancy.
