Methiozolin is a new herbicide with an unknown mechanism of action (MOA) for control of annual bluegrass (Poa annua L.) in several warm- and cool-season turfgrasses. In the literature, methiozolin was proposed to be a pigment inhibitor via inhibition of tyrosine aminotransferases (TATs) or a cellulose biosynthesis inhibitor (CBI). Here, exploratory research was conducted to characterize the herbicide symptomology and MOA of methiozolin. Arabidopsis (Arabidopsis thaliana L.) and P. annua exhibited a similar level of susceptibility to methiozolin, and arrest of meristematic growth was the most characteristic symptomology. For example, methiozolin inhibited A. thaliana root growth (GR50 8 nM) and shoot emergence (GR80 ˜50 nM), and apical meristem growth was completely arrested at rates greater than 500 nM. We concluded that methiozolin was neither a TAT nor a CBI inhibitor. Methiozolin had a minor effect on chlorophyll and alpha-tocopherol content in treated seedlings (<500 nM), and supplements in the proposed TAT pathway could not lessen phytotoxicity. Examination of microscopic images of roots revealed that methiozolin-treated (100 nM) and untreated seedlings had similar root cell lengths. Thus, methiozolin inhibits cell proliferation and not elongation from meristematic tissue. Subsequently, we suspected methiozolin was an inhibitor of the mevalonic acid (MVA) pathway, because its herbicidal symptomologies were nearly indistinguishable from those caused by lovastatin. However, methiozolin did not inhibit phytosterol production, and MVA pathway metabolites did not rescue treated seedlings. Further experiments showed that methiozolin produced a physiological profile very similar to cinmethylin across a number of assays, a known inhibitor of fatty-acid synthesis through inhibition of thioesterases (FATs). Experiments with lesser duckweed (Lemna aequinoctialis Welw.; syn. Lemna paucicostata Hegelm.) showed that methiozolin also reduced fatty-acid content in Lemna with a profile similar, but not identical, to cinmethylin. However, there was no difference in fatty-acid content between treated (1 µM) and untreated A. thaliana seedlings. Methiozolin also bound to both A, thaliana and L. aequinoctialis FATs in vitro. Modeling suggested that methiozolin and cinmethylin have comparable and overlapping FAT binding sites. While there was a discrepancy in the effect of methiozolin on fatty-acid content between L. aequinoctialis and A. thaliana, the overall evidence indicates that methiozolin is a FAT inhibitor and acts in a similar manner as cinmethylin.

Continued reliance on chemical methods for controlling annual bluegrass has resulted in many populations evolving resistance to PRE and POST herbicides, particularly in warm-season turfgrass species such as zoysiagrass. Soil seedbank management is critically important when managing herbicide-resistant weeds. Fraise mowing (also spelled fraze, frase, and fraize) is a new turfgrass cultivation practice designed to remove aboveground biomass while allowing turf to regrow vegetatively. We hypothesized that this process would remove annual bluegrass seed and therefore be a mechanical means of controlling annual bluegrass in turfgrass. Zoysiagrass field plots were fraise-mowed in June 2015 only, June 2016 only, June 2015 and June 2016, or left untreated. The fraise mower was configured to remove the uppermost 25 mm of plot surface (i.e., 15-mm verdure and 10-mm soil). Annual bluegrass infestation was quantified in April following fraise mowing via grid count. Soil cores (10.8 cm diameter) were extracted from each plot after grid count data were collected to assess effects of fraise mowing on the soil seedbank. Moreover, replicated subsamples (7.6 L) of debris generated during fraise mowing were collected to better understand weed seed content removed during the fraise mowing process. Fraise mowing in June offered a slight reduction (24%) in annual bluegrass cover the following April. Whereas 28% of the seed in fraise-mowing debris consisted of annual bluegrass, there was no difference in the quantity of annual bluegrass seed remaining in the soil seedbank among fraise-mowed and non–fraise-mowed plots. Although fraise mowing may help to temporarily reduce existing annual bluegrass infestations via mechanical removal, the frequency and depth we studied did not effectively reduce the seedbank. Fraise mowing is a useful tool for providing mechanical suppression of annual bluegrass but it is not a replacement for properly timed herbicide applications.

Annual bluegrass is a problematic weed of Kentucky bluegrass turf that can be selectively controlled with POST applications of primisulfuron-methyl. The objective of this research was to evaluate physiological behavior of primisulfuron-methyl attributed to selectivity in these species. In application placement experiments, annual bluegrass shoot weight reductions from the nontreated from high to low were treatments including: foliar + soil ≥ soil only ≥ foliar. Annual bluegrass averaged 33 and 52% shoot weight reductions from the nontreated after 4 wk from primisulfuron-methyl at 40 and 80 g ha−1, respectively. Kentucky bluegrass shoot weight was not reduced from the nontreated, and application placements were similar. From five harvests ranging 1 to 168 h after treatment (HAT), annual and Kentucky bluegrass absorbed up to 25 and 32% of foliar applied 14C-primisulfuron-methyl, respectively. Both grasses distributed 15% of foliar absorbed 14C to nontreated shoots with minimal translocation (≤ 2%) to roots after 168 h. Annual bluegrass translocated 2 times more root-absorbed 14C to shoots than Kentucky bluegrass at 24, 72, and 168 HAT. From foliar uptake, the time required for annual and Kentucky bluegrass to degrade 50% of the absorbed herbicide to the major metabolite detected (Rf 0.1) measured > 168 and 93 h, respectively. In root metabolism experiments, annual bluegrass had ≈ 3 times and 2 times more primisulfuron acid in roots and shoots, respectively, than Kentucky bluegrass at 24, 72, and 168 HAT. The isolated acetolactate synthase (ALS) enzymes from the two grasses were equally susceptible to inhibition by primisulfuron-methyl. Overall, selectivity of primisulfuron-methyl for annual bluegrass control in Kentucky bluegrass is attributed to differential translocation and metabolism between species.

Annual bluegrass resistance to inhibitors of acetolactate synthase (ALS) and photosystem II (PSII) in managed turf has been confirmed in the southeastern United States. A biotype of annual bluegrass that had developed resistance (R) to the PSII inhibitor simazine was not controlled by POST applications of foramsulfuron or trifloxysulfuron in 2011 or 2012. In whole plant dose-response experiments, trifloxysulfuron, simazine, and indaziflam controlled a susceptible (S) population of annual bluegrass > 91% when applied POST to nontillering plants. However, trifloxysulfuron applications at 3.5 to 223 g ai ha−1 only controlled R annual bluegrass ≤ 40%. Similarly, simazine at 140 to 9,000 g ai ha−1 only controlled R annual bluegrass ≤ 20%. R annual bluegrass plants were more tolerant to indaziflam applied POST to leaf stage plants prior to tillering, as rates > 100 g ai ha−1 were needed to control R annual bluegrass ≥ 96%. No differences in the activity of ALS in R and S plants exposed to increasing foramsulfuron concentrations from 0 to 100 µM were detected suggesting that nontarget mechanisms could explain reduced efficacy of POST herbicide applications in whole plant dose-response experiments. Applications of indaziflam (35 to 70 g ha−1) and oxadiazon (2,240 to 4,500 g ai ha−1) effectively controlled R annual bluegrass when applied PRE. This biotype of R annual bluegrass is the first reported instance of a weed developing resistance to multiple modes of action in managed turf. Education is needed among turf managers regarding the consequences of exclusive use of the same herbicides for annual bluegrass control leading to the onset of herbicide resistance.

Nicosulfuron provides POST weed control in corn, pastures, and grassy roadsides, and has potential for use in fine turfgrass. The objective of this research was to evaluate tolerance, absorption, translocation, and metabolism of nicosulfuron in annual bluegrass and four turfgrass species. In greenhouse experiments, relative tolerance of grasses to nicosulfuron (35, 70, or 140 g ai ha−1) from high to low was bermudagrass = zoysiagrass > tall fescue > creeping bentgrass > annual bluegrass. In laboratory experiments, grasses had similar foliar and root absorption of 14C-nicosulfuon. Annual bluegrass and creeping bentgrass averaged 80% greater radioactivity per unit dry mass in shoots than bermudagrass following root uptake of 14C-nicosulfuron, but other species were similar to these grasses. At 72 h after treatment (HAT), annual bluegrass metabolized 36% of absorbed 14C-nicosulfuron, which was less than bermudagrass, tall fescue, and zoysiagrass that metabolized 47 to 58%. Creeping bentgrass metabolism of nicosulfuron was similar to annual bluegrass. Tall fescue had similar levels of metabolism to bermudagrass and zoysiagrass, averaging 67%, at 168 HAT but produced fewer metabolites. Overall, turfgrass tolerance to nicosulfuron is associated with relative herbicide concentrations in shoots and differential species metabolism.