Two previously identified triazine-resistant annual bluegrass (Poa annua) biotypes in Mississippi were evaluated to determine the level as well as the mode of resistance compared to a triazine-susceptible biotype. Annual bluegrass seeds were collected at two sites (Meridian and Tupelo, MS) from field plots previously treated with 11.2 kg ai/ha simazine. Seeds were planted on agar containing simazine (0, 0.001, 0.01, 0.1, 1, 10, or 100 mM) or diuron (0, 0.01, 0.1, 1, 10, 100, or 1,000 μM), placed in the growth chamber with an 8 h daylength and alternating temperatures of 15 and 20 C, and allowed to grow for 3 wk, after which fresh weight was determined. Data were subjected to regression analyses and the amount of simazine or diuron required to reduce fresh weight to 50% of the untreated plants was determined. These data indicated a greater than 1,000-fold level of resistance to simazine with either biotype. Diuron concentrations of 5.0, 3.0, and 3.2 μM were required to reduce fresh weight to 50% of the untreated for the triazine-susceptible, Meridian, and Tupelo biotypes, respectively. Sequencing a portion of the chloroplast psbA gene revealed a serine 264 to glycine mutation, which reduced the affinity of the triazine herbicides for the QB-binding niche on the D1 protein in photosystem II.

Various populations of common lambsquarters (Chenopodium album L. # CHEAL) and late-flowering goosefoot (Chenopodium strictum Roth. # CHESG) plants were studied at the eight-to ten-leaf stage. Prior to measurement of fluorescence induction, the detached leaves were vacuum infiltrated with unformulated atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine], diuron [N′-(3,4-dichlorophenyl)-N,N-dimethylurea], and pyrazon [5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone]. The fluorescence induction measurements were then made 1 h after adaptation in the dark. The fluorescence induction curves of intact leaves were recorded with a laboratorybuilt apparatus. A xenon lamp was used to produce the actinic beam. Cytological tests were done on meristematic cells of the root tips of the seedlings treated in 2 nM 8-hydroxyquinoline. The cytotypes were hexaploids (among which were atrazine-susceptible, pyrazon-susceptible, atrazine-resistant, and pyrazon-tolerant populations), and tetraploids (among which were atrazine-susceptible and atrazine-tolerant populations).

Photosynthetic growth of 9 of the 11 known species of the family Rhodospirillaceae was strongly inhibited by atrazine. Growth of a wild-type strain of the bacterium Rhodopseudomonas sphaeroides van Niel was almost completely inhibited by 100 μM concentrations of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] and diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. These herbicides gave 50% inhibition of growth at concentrations of 35 and 25 μM, respectively. Through continuous culture of the wild-type R. sphaeroides in the presence of 100 μM atrazine, strains of the organism were selected in which photosynthetic growth was highly resistant to both atrazine and diuron. Members of the genus Rhodopseudomonas isolated from two natural ecosystems showed various levels of resistance to 100 μM atrazine, but Rhodopseudomonas showing a high level of resistance were more common among the isolates from a pond-mud sample which had been exposed to runoff from atrazine-treated cropland than from a sample of a lake mud never exposed to triazines.

Herbicide-resistant populations have evolved only in monoculture and/or monoherbicide conditions at predictable rates for each compound and weed. No populations of triazine-resistant weeds have appeared in corn where rotations of crops and herbicides or herbicide mixtures were used. This is due to the greatly reduced competitive fitness of the resistant individuals, which could be expressed only during rotational cycles, and also to the greater sensitivity of resistant individuals to other herbicides, pests, and control practices (“negative cross-resistance”). The model presented here describes how an understanding of all of these factors can provide strategies to decrease the frequency of the resistant individuals during rotation. Rotations or mixtures may not delay the rate of appearance of resistance to inhibitors of acetolactate synthase (ALS), where the fitness of resistant biotypes is claimed to be near normal. The best way to delay resistance to ALS inhibitors is to use those compounds with less persistence so that the selection pressure will be lowered. Too little is known about the frequency of resistance to other herbicides with target-site resistance–to dinitroanilines, to acetyl CoA carboxylase inhibitors, or to those situations where a single enzyme system confers resistance to a broad spectrum of seemingly unrelated herbicides.

In 2006 and 2007, farmers from two counties in Illinois reported failure to control waterhemp with glyphosate. Subsequent onsite field experiments revealed that the populations might be resistant to multiple herbicides. Greenhouse experiments therefore were conducted to confirm glyphosate resistance, and to test for multiple resistance to other herbicides, including atrazine, acifluorfen, lactofen, and imazamox. In glyphosate dose-response experiments, both populations responded similarly to a previously characterized glyphosate-resistant population (MO1). Both Illinois populations also demonstrated high frequencies of resistance to the acetolactate synthase (ALS) inhibitor, imazamox. Additionally, one of the populations demonstrated high frequencies of resistance to both atrazine and the protoporphyrinogen oxidase (PPO) inhibitor, lactofen. Furthermore, using combinations of sequential and tank-mix herbicide applications, individual plants resistant to herbicides spanning all four site-of-action groups were identified from one population. Molecular experiments were performed to provide an initial characterization of the resistance mechanisms and to provide confirmation of the presence of multiple resistance traits within the two populations. Both populations contained the W574L ALS mutation and the ΔG210 PPO mutation, previously shown to confer resistance to ALS and PPO inhibitors, respectively. Atrazine resistance in both populations is suspected to be metabolism-based, because a triazine target-site mutation was not identified. A P106S EPSPS mutation, previously reported to confer glyphosate resistance, was identified in one population. This mutation was identified in both resistant and sensitive plants from the population; however, and so more research is needed to determine the glyphosate-resistance mechanism(s). This is the first known case of a weed population in the United States possessing multiple resistance to herbicides from four site-of-action groups.

Research was conducted to determine the competitiveness and fitness of a protoporphyrinogen oxidase (protox)-resistant common waterhemp biotype. Protox-resistant and protox-susceptible biotypes were grown under noncompetitive and competitive arrangements in the greenhouse. In the noncompetitive study, a single plant of each biotype was planted separately in 15-cm-diam pots. Photosynthesis, leaf area, and plant biomass were measured 10, 20, 30, and 40 d after transplanting (DATP). In general, photosynthesis rate and plant biomass were similar between biotypes. However, the protox-resistant biotype had higher leaf area than the susceptible biotype at 20, 30, and 40 DATP. A replacement series study was conducted in the greenhouse to evaluate the relative competitiveness of protox-resistant and protox-susceptible common waterhemp. Photosynthesis, leaf area, plant height, and plant biomass were measured 7, 14, 21, and 28 DATP. Protox-resistant and -susceptible common waterhemp were equally competitive 28 DATP. Relative crowding coefficient values 28 DATP were 0.86, 0.89, 1.09, and 1.13 for photosynthesis, leaf area, plant height, and plant biomass, respectively. This suggests protox-resistant and -susceptible common waterhemp were equally competitive and the frequency of protox-resistant biotype is unlikely to decrease in the absence of protox–herbicide selection pressure.

Applications of metribuzin plus MSMA have been used to control goosegrass in bermudagrass turf for over 20 yr. In 2003, two goosegrass biotypes on the island of Kauai, Hawaii were found to be resistant to applications of metribuzin plus MSMA. Metribuzin plus MSMA applied at rates of 0.28 kg ai/ha plus 2.2 kg/ha, respectively, followed by MSMA at 2.2 kg/ha 7 d later, provided 100% control of two susceptible goosegrass biotypes, but no control of two resistant biotypes. At the flowering growth stage, metribuzin applied at a rate of 0.28 kg/ha controlled both susceptible biotypes (> 92%), but did not control the two resistant biotypes. Two applications of MSMA applied at a rate of 2.2 kg/ha (7 d apart) did not control any of the four biotypes at the flowering growth stage. The two resistant biotypes were approximately 100 to 200 times less sensitive to increasing rates of metribuzin than the two susceptible biotypes. The two biotypes that were resistant to metribuzin plus MSMA were also resistant to applications of simazine plus MSMA. However, all four biotypes were susceptible to applications of glyphosate and foramsulfuron.

Whole-plant, negative cross-resistance was studied in Conyza canadensis and Echinochloa crus-galli, important global weeds. Negative cross-resistance can be a most useful preemptive, cost-effective tool for delaying the evolution of resistance, as well as for resistance management, after resistant populations evolve. Seeds of triazine-resistant and -susceptible biotypes were collected in or near orchards that had been continuously treated with atrazine for more than 10 yr. Plants grown from the seeds were treated, in a greenhouse, with herbicides from the following chemical families: triazine, benzothiadiazole, phenyl-pyridazine, arylophenoxy-propionate, cyclohexanedione, phenoxycarboxylic acid, pyridine carboxylic acid, phosphinic acid, glycine phosphate, chloroacetamide, sulfonylurea, and bipyridylium. Eleven of the 18 herbicides tested exerted significant negative cross-resistance against atrazine-resistant weeds, ranging from 0.03 to 0.67 of the concentration required to affect the triazine-sensitive type. No synergism was found between bentazon and fluroxypyr in mixture on Conyza, even though both separately exerted negative cross-resistance. Using a mixture with half the amount of each component lowers the environmental effect of each component while controlling a broader spectrum of other weeds.

Joint action of the effects of atrazine and mesotrione can lead to synergistic herbicidal activity in broadleaf weed species. The objective of these experiments was to determine if specific rates are required to provide synergistic joint activity between mesotrione and atrazine in both triazine-sensitive (TS) and triazine-resistant (TR) redroot pigweed. Herbicide rates were evaluated in TS and TR redroot pigweed in two experiments: a dose response of mesotrione alone and in mixture with a constant rate of atrazine and a dose response of atrazine alone and in mixture with a constant rate of mesotrione. Results from these experiments revealed that synergism was detected in the TS pigweed when 56 g ai ha−1 mesotrione was mixed with 126 g ai ha−1 atrazine. In the TR pigweed, synergism was detected when mesotrione rate at 10 to 56 g ha−1 was mixed with a constant rate of atrazine at 126 g ha−1. Additionally, when mesotrione was held constant at 10 g ha−1, synergism was detected in mixture with atrazine from 31 to 3556 g ha−1 in TR pigweed. Furthermore, in TR pigweed, analysis of slope deviation across the dose-response curves of mesotrione with and without atrazine revealed a divergence that increased in magnitude as the rate of mesotrione increased. In other words, increased synergism was observed with increased mesotrione rate in the TR pigweed, which was also supported by biomass reduction and atrazine-like injury to the leaves. An additional experiment investigated synergism between mesotrione and bromoxynil in both TS and TR pigweed. Mesotrione at 10 g ha−1 was synergistic when paired with bromoxynil from 70 to 210 g ha−1 in both the TS and the TR pigweed.

Laboratory and greenhouse studies were conducted to determine if the reported red morningglory control failures with atrazine in sugarcane are caused by triazine-resistant mutants. Plants were grown from seeds collected at 24 locations in Louisiana and Arkansas where atrazine had never been used, and where it had been used in sugarcane with poor results. Terminal fluorescence of leaf material from all locations increased after treatment with 10−3 M atrazine, indicating electron transport inhibition and, hence, triazine susceptibility. However, small differences in the magnitude of fluorescence increase were observed among populations, possibly indicating the existence of biotypes with slightly different inherent tolerances to atrazine. Some phenotypic differences were observed among the red morningglory populations. In a separate study, postemergence application of atrazine at 1.1 kg ai/ha plus nonionic surfactant controlled greenhouse-grown plants from all populations at least 99%, which supports the findings of the fluorescence assay. This research was unable to verify that reduced red morningglory control with atrazine was the result of an altered binding site mutation, even in populations exposed to atrazine annually for more than 10 yr. Other factors should be evaluated to determine their impact on atrazine performance.

This study documents the physiology and genetics of evolved atrazine resistance in a wild radish population from Western Australia. Plant response to atrazine treatment confirmed a high level of resistance in population WARR5. At 0.25 kg atrazine/ha, all plants from a susceptible population were killed, whereas resistant WARR5 was unaffected at the highest dose tested (4 kg atrazine/ha). Leaf photosynthesis in susceptible plants was inhibited after 1 kg atrazine/ha treatment, whereas leaf photosynthesis in WARR5 plants was unaffected. Furthermore, atrazine resistance was maternally inherited. Sequencing of a psbA gene fragment in resistant WARR5 and susceptible plants revealed a single point mutation resulting in a coding change from Ser264 to Gly of the D1 protein in resistant plants. We are confident that this mutation is the basis of resistance to the photosystem II inhibitors in this wild radish population.