Understanding and combating the threat posed by Lolium multiflorum as a weed of arable crops. PhD Thesis

Lolium multiflorum (Italian rye-grass) is an increasing problem as a weed of arable crops in the UK and many other countries. However, there exists only limited knowledge on many basic aspects of this weed‟s agro-ecology and how it interacts with cultural and chemical control methods. Therefore, the aim of this project was to characterise better both the agroecology and the basis of resistance to ACCase inhibitors in this species.

Resistance in L. multiflorum to at least one ACCase-inhibiting herbicide was found to be widespread in England and was detected on 70% of the 50 farms surveyed. At least one ACCase mutation (Asp-2078-Gly (in 24.5% of 384 resistant plants assayed), Ile-1781-Leu (13.3%), Ile-2041-Asn (2.1%), Cys-2088-Arg (1.8%), Trp-2027-Cys (1.0%) and Trp-1999-Cys (0.3%)) was found in 40% of the resistant plants analysed. However, at present, the most common mechanism of resistance to ACCase-inhibiting herbicides in UK L. multiflorum populations appears to be non-target site resistance, most likely enhanced metabolism.

Field studies showed that the majority (94%) of L. multiflorum plants in winter wheat fields emerged between October and December, with only 6% emerging in spring. Autumn emerging plants were much more competitive and produced on average 23 times as many seeds per plant as spring emerging cohorts. The success of L. multiflorum as a weed of winter cereals is due to its ability to produce high numbers of heads per plant (mean = 20) and seeds per head (mean = 295), even at low weed densities. L. multiflorum also had a highly detrimental effect on wheat yield with losses of up to 89%. L. multiflorum seed dormancy is short and determined by both genetics and weather conditions during seed maturation. By delaying the sowing date based on predictions of dormancy status and using resistance testing to help design an effective chemical control strategy in autumn, L. multiflorum infestations could be significantly reduced.