Aspects of herbicide resistance in three New Zealand weed species : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Manawatu, New Zealand
Herbicide resistant weeds have become a challenge in agricultural systems globally. In
this thesis, aspects have been studied of three weed species which have evolved
resistance to herbicides from different chemical families within New Zealand.
Dicamba-resistant fathen (Chenopodium album) was recently reported by researchers in
Waikato. In this thesis, the level of resistance to dicamba in two of these populations of
fathen was investigated using a whole plant dose-response experiments and it ranged
from 5- to 20-fold. Also, a seed-test for rapidly and reliably detecting dicamba resistant
fathen has been developed. Seed tests have seldom been used for detecting resistance
within weeds to auxinic herbicides.
The thesis also investigated aspects of the first reported cases of glyphosate resistance in
New Zealand, found in both Italian ryegrass (Lolium multiflorum) and perennial
ryegrass (Lolium perenne) from vineyards. Resistance to glyphosate in two populations
of Italian ryegrass (Populations A and P) and two populations of perennial ryegrass
(Populations J and N) was found to be almost 10-fold, whereas it was almost 30-fold for
one perennial ryegrass population (Population O). Three different quick tests (seed
assays, excised tiller bioassays and shikimic acid assays) were developed for detecting
glyphosate resistance in Italian ryegrass and perennial ryegrass.
Of the five populations of ryegrass studied, only Population O had a target site
modification at Codon 106 of the enzyme 5-enolpyruvylshikimate-3-phosphate synthase
(EPSPS). Translocation of radiolabelled glyphosate was studied in four of the
populations (Populations A, J, O and P), and movement from treated leaves was
significantly reduced in them all compared with susceptible populations (non-target site
mechanism of resistance). Therefore, Population O had two mechanisms of resistance,
possibly explaining the 30-fold resistance.
The studied glyphosate-resistant ryegrass populations were all found to be resistant to
glufosinate. Populations A, J and O were also found to be resistant to amitrole. Genetic
studies showed that the restricted glyphosate translocation trait is incompletely
dominant and can be transmitted via pollen. The restricted herbicide translocation was
suppressed under cool conditions in experiments, suggesting that application of
glyphosate during winter might improve control of glyphosate-resistant Italian ryegrass
and perennial ryegrass infestations.
KEYWORDs: Chenopodium album, dicamba, glyphosate, Lolium multiflorum, Lolium
perenne, amitrole, glufosinate, glyphosate mechanisms of resistance, target site
mechanism of resistance, restricted herbicide translocation.