Amaranthus palmeri in Spain
Amaranthus palmeri (Amaranthaceae - EPPO A2 pest) is a dioecious summer annual species native to North America. In its native range, it is a weed in agricultural fields and disturbed habitats. It has a high fecundity and a long-lived seed bank, which make management of the species difficult. In the EPPO region, it is established in a few countries and transient in several others. In Spain in 2007, three locations of A. palmeri were found in the north east of the country. These populations were on roadsides and field margins in Lleida and Menàrguens provinces, in industrial areas near the city of Lleida and on roadsides in Binéfar. Research was carried out to determine if these populations were independent introduction events and if there was herbicide resistance in the populations. Seed material was collected from all populations, and also from populations in Georgia (USA) with no history of herbicide use in the last 20 years. The latter populations were used as a control to measure susceptibility to herbicides. Seed was grown from all populations and dose-response experiments were conducted using a selection of herbicides, at different rates and different timings (e.g. pre-emergence and post-emergence). The dose response trials confirmed that all three populations were susceptible to glyphosate but resistant to acetolactate synthase inhibiting (ALS) herbicides. Molecular analysis of ALS gene sequence revealed target site mutations in the three A. palmeri populations that confer resistance to ALS inhibitors. All three populations were however, controlled with other herbicides with three different modes of action, highlighting that there is still an opportunity to control these populations using herbicides. Additionally, molecular analysis highlighted that the three populations had genetic differences which suggests the populations result from separate colonisation events. Preventing new introductions of A. palmeri in the EPPO region is important, especially as new herbicide resistance traits may arrive. In 2020, EPPO published a pest risk analysis for A. palmeri which includes recommended phytosanitary measures.
Sources
Torra J, Royo-Esnal A, Romano Y, Osuna MD, Leόn RG, Recasens J (2020) Amaranthus palmeri a new invasive weed in Spain with herbicide resistant biotypes. Agronomy 10(7), 993 https://doi.org/10.3390/agronomy10070993
EPPO (2020) Pest risk analysis for Amaranthus palmeri. EPPO, Paris. Available at: https://gd.eppo.int/taxon/AMAPA/documents