Resistance to glyphosate in Lolium rigidum selected in Italian perennial crops: bioevaluation, management and molecular bases of target-site resistance

The resistance status of two Lolium rigidum populations (IBAF code 332 and 336) collected in northern Italian vineyards and southern Italian olive groves was investigated. Glasshouse and outdoor pot experiments confirmed that both populations were resistant to
glyphosate, exhibiting a resistance index of 4 (pop. 332) and 16 (pop. 336) based on plant survival. Multiple resistance to ACCase and ALS inhibitors was also investigated, and it has been demonstrated that pop. 332 has a lower susceptibility to fluazifop but
could be controlled by cycloxydim and flazasulfuron, whereas the other populations tested did not survive herbicides other than glyphosate. The molecular bases of possible target-site resistance were also investigated. Four different patterns of substitution at position 106 were discovered only in the sequences of pop. 336: (i) heterozygous proline-leucine (CCA-CTA); (ii) heterozygous
proline-serine (CCA-TCA); (iii) heterozygous leucine-serine (CTA-TCA); and (iv) homozygous serine-serine. The high glyphosate resistance level of pop. 336 indicated that resistance mechanism(s) other than target site are likely to be present. Integrated weed
management based on rotation of herbicide mode of action, a careful use of glyphosate, as well as nonchemical weed control, like mowing the vegetation between crop rows, should be implemented to effectively control and limit the dissemination of resistant
populations.