Target-site and non–target site mechanisms of pronamide resistance in annual bluegrass (Poa annua) populations from Mississippi golf courses

Abstract

The mitotic-inhibiting herbicide pronamide controls susceptible annual bluegrass (Poa annua L.) pre- and post-emergence, but in some resistant populations, post-emergence activity is compromised, hypothetically due to a target-site mutation, lack of root uptake, or an unknown resistance mechanism. Three suspected pronamide-resistant (LH-R, SC-R, and SL-R) and two pronamide-susceptible (BS-S and HH-S) populations were collected from Mississippi golf courses. Dose-response experiments were conducted to confirm and quantify pronamide resistance, as well as resistance to flazasulfuron and simazine. Target-sites known to confer resistance to mitotic-inhibiting herbicides were sequenced, as were target-sites for herbicides inhibiting acetolactate synthase (ALS) and photosystem II (PSII). Pronamide absorption, and translocation was investigated following foliar and soil applications. Dose-response experiments confirmed pronamide resistance of LH-R, SC-R, and SL-R populations, as well as instances of multiple resistance to ALS and PSII inhibiting herbicides. Sequencing of the α-tubulin gene confirmed the presence of a mutation that substituted isoleucine for threonine at position 239 (Thr239-Ile) in LH-R, SC-R, SL-R, and BS-S populations. Foliar application experiments failed to identify differences in pronamide absorption and translocation between the five populations, regardless of harvest time. All populations had limited basipetal translocation—only 3–13% of the absorbed pronamide—across harvest times. Soil application experiments revealed that pronamide translocation was similar between SC-R, SL-R, and both susceptible populations across harvest times. The LH-R population translocated less soil-applied pronamide than susceptible populations, 24, 72, and 168 HAT, suggesting that reduced acropetal translocation may contribute to pronamide resistance. This study reports three new pronamide-resistant populations, two of which are resistant to two modes-of-action (MOA), and one of which is resistant to three MOA. Results suggest that both target-site- and translocation-based mechanisms may be associated with pronamide resistance. Further research is needed to confirm the link between pronamide resistance and the Thr239-Ile mutation of the α-tubulin gene.

Effect of spray droplet spectra on control of Poa annua with pronamide

“…The only exception was at the 1,000 μm VMD applied at the 0.5× rate, which resulted in greater control than the 400 μm VMD in the LH-R and HH-S populations. Effects of droplet spectrum and flazasulfuron rate on SL-R may have been tempered due to the population being resistant to ALSinhibiting herbicides [target-site mutation Trp-574-Leu characterized in Ignes et al (2023)]. This result was also observed with the ALS-inhibiting herbicide imazethapyr, whose droplet spectrum did not affect weed control (Ferreira et al 2020).…”

“…Application rates represented 0.5×, 1×, and 2× (fold) doses for annual bluegrass suggested on the product label (1.160 kg ai ha −1 for pronamide and 0.044 kg ai ha −1 for flazasulfuron). Treatments were tested on plants from four annual bluegrass populations: two known pronamide-S populations from Battle Sod Farm (BS-S) in Tunica, MS and Humphreys High School (HH-S) in Belzoni, MS (33.186879°N, 90.483539°W); and two populations resistant to POST pronamide applications from Lion Hills Golf Club (LH-R) in Columbus, MS (33.522983°N, 88.404055°W) and Shell Landing Golf Club (SL-R) in Gautier, MS (30.388181°N, 88.676874°W), both of which have previously been characterized for resistance by Ignes et al (2023).…”

“…Differential control among resistant and susceptible populations was attributed to varying absorption and translocation. More recently, Ignes et al (2023) reported three pronamideresistant annual bluegrass populations from golf courses in Mississippi, with resistance attributed to target-site-based (Thr239-Ile mutation of the α-tubulin gene in three populations) and translocation-based mechanisms (reduced translocation of soil-applied pronamide in one population).…”

Developing and implementing a sustainable, integrated weed management program for herbicide‐resistant Poa annua in turfgrass

“…There are 49 herbicide resistant P. annua cases reported worldwide and there are 18 cases reported in the United States (Heap, 2023). Cases are likely severely under‐reported based upon ongoing research (Ignes et al., 2023; Rutland et al., unpublished data, 2023; Singh et al., 2021). The worrying trend is that for some turfgrass scenarios, there are no longer effective means of chemical control (Allen et al., 2022).…”

“…The process of repeated selection using the same herbicide SOA expands this trait across an entire population, leading to population‐wide resistance. Target site resistance in P. annua in the United States is reported to acetolactate synthase (ALS) Inhibitors (Group 2) (Brosnan et al., 2015, 2016; Cross et al., 2013; McElroy et al., 2013; Rutland et al., unpublished data, 2023), microtubule assembly inhibitors (Group 3) Ignes et al., 2023; Rutland et al., 2022; Rutland et al., unpublished data, 2023), photosystem II (PSII) inhibitors (Group 5) (Kelly et al., 1999; Perry et al., 2012; Rutland et al., unpublished data, 2023), and enolpyruvyl shikimate‐3‐phosphate (EPSP) synthase inhibitors (Group 9) (Rutland et al., unpublished data, 2023). In some cases, the P. annua is resistant to more than one SOA (Brosnan et al., 2015; Ignes et al., 2023; Singh et al., 2021).…”