Genetic Variations among Fleabane (Conyza bonariensis (L.) Cronquist) Populations in Jordan and Their Susceptibility Levels to Contact Herbicides

A field demonstration and pot experiments were implemented to assess the effect of paraquat, oxadiazon, and oxyfluorfen herbicides in controlling selected populations of fleabane Conyza bonariensis (L.), grown in the central valley of Jordan. Conyza mature seeds were collected from six investigated sites (five from Jordan valley named P1, P2, P3, P4, P5, and one from the University of Jordan Campus named P6). Only populations proved to be C. bonariensis via ITS assessment were involved in the glasshouse experiments at the University of Jordan in 2017 and 2019. Results showed that recommended or two-fold higher rates (2.5 and 5 kg ha−1) of paraquat failed to affect weed plants in a date palm orchard located at Tal-al-Ramel in the Central Jordan Valley. Paraquat, oxyfluorfen, and oxadiazon (2.5, 3.3, and 5 kg ha−1, respectively), failed to control plants of the same weed population grown in pot experiments. Treated plants at Tal-al-Ramel grew similarly to untreated control, mostly due to different genetic backgrounds. The other C. bonariensis populations (University Research Station, al-Twal, and University Campus) were effectively controlled with all herbicides. The application of recommended or 10-fold higher rates of herbicides failed to control or slightly injured the resistant population. Seed DNA analysis of the ITS region showed genetic differences among the investigated populations. It indicated that four populations are C. bonariensis (P1, P3, P4, and P6). At the same time, two are C. canadensis (a closely related species) collected from the University Research Station (P2) and al-Twal sites (P5), and also that the population of C. bonariensis in the date palm orchard was genetically distinct from the other C. bonariensis populations. It is concluded that C. bonariensis population in the Tal-al-Ramel site developed resistance to paraquat, oxadiazon, and oxyfluorfen herbicides. Thus, novel alternative practices in controlling the resistant weed population are necessary to prevent its possible spread to other regions in the country and obstruct the development of new herbicide-resistance weed populations.