BACKGROUND Protoporphyrinogen oxidase (PPO) with two isoforms, chloroplast‐targeted (PPO1) and mitochondrial‐targeted (PPO2), catalyzes a step in the biosynthesis of chlorophyll and heme. PPO1 and PPO2 are herbicide target sites of PPO‐inhibiting herbicides. Target‐site mutations conferring resistance to PPO inhibitors have all thus far been in PPO2. Oxadiazon is a unique PPO inhibitor utilized for preemergence Eleusine indica control. In this research, we evaluated the response of two previously confirmed oxadiazon‐resistant and susceptible E. indica biotypes to other PPO inhibitors and identified the resistance mechanism in two oxadiazon‐resistant E. indica biotypes. RESULTS Two E. indica biotypes were resistant to oxadiazon, but not to other structurally unrelated PPO inhibitors, such as lactofen, flumioxazin and sulfentrazone. A novel mutation A212T was identified in the chloroplast‐targeted PPO1, conferring resistance to oxadiazon in a heterologous expression system. Computational structural modeling provided a mechanistic explanation for reduced herbicide binding to the variant protein: the presence of a methyl group of threonine 212 changes the PPO1 active site and produces repulsive electrostatic interactions that repel oxadiazon from the binding pocket. CONCLUSION The novel A212T mutation in PPO1 conferring resistance specifically to PPO inhibitor oxadiazon was characterized. This is the first evidence of the direct role of PPO1 in the PPO mode of action, and the first evidence of evolved resistance in PPO1. © 2019 Society of Chemical Industry

Applying trinexapac-ethyl (TE) is a common practice for growth suppression and quality improvement of ultradwarf bermudagrass [Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy] putting greens. Previous research suggested that growing degree-days (GDD) effectively predict the suppression of creeping bentgrass (Agrostis stolonifera L.) putting greens following a TE application, and additional research showed similar results on ultradwarf bermudagrass putting greens. Whereas temperature may be the main factor influencing turfgrass suppression following a TE application, it is likely that other environmental factors affect suppression as well. The objective of this research was to identify the optimal variable unit for predicting suppression, particularly the maximum suppression point (MSP), after a TE application on a 'MiniVerde' ultradwarf bermudagrass putting green. Tested variable units included calendar days (DAT), GDD (base temperatures of 0-12˚C), soil temperature (2.5 cm), global horizontal irradiance (GHI), and photosynthetically active radiation (PAR). The GDD with a base temperature of 0˚C (GDD 0) model predicted suppression better than the GDD with a base temperature of 10˚C (GDD 10) model, GHI model, and PAR model. According to the GDD 0 model (pseudo-R 2 , .564; SE: 0.195), the MSP occurred at 262 GDD 0 with a suppression magnitude of 61%. The models did not indicate a rebound period following the single TE application. 1 INTRODUCTION Applying trinexapac-ethyl (TE) to ultradwarf bermudagrass putting greens [Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy] is a common practice. Trinexapac-ethyl Abbreviations: DAT, calendar days after treatment; GA, gibberellic acid; GDD 0 , growing degree-days with a base temperature of 0˚C; GDD 10 , growing degree-days with a base temperature of 10˚C; GHI, global horizontal irradiance; MSP, maximum suppression point; PAR, photosynthetically active radiation; TA, trinexapac acid; TE, trinexapac-ethyl; WAIT, weeks after initial treatment.

Trinexapac-ethyl (TE) is commonly applied to ultradwarf bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy] putting greens for growth suppression and secondary benefits. Improperly timed reapplications will reduce the benefits of TE, but knowing when to reapply is difficult because suppression duration is affected by environmental conditions, especially temperature. In another experiment we determined that GDD with a base temperature of 0˚C (GDD 0) was the most precise unit for predicting the maximum suppression point (MSP) after a TE application on a 'MiniVerde' ultradwarf bermudagrass putting green. The model suggested that the MSP occurred at 262 GDD 0 after the TE application. The objective of this second experiment was to test GDD 0 reapplication intervals for an extended period of time. We included four GDD 0 intervals (100, 200, 400, and 600) and two TE rates (0.022 and 0.044 kg a.i. ha −1). We hypothesized that reapplying TE before the MSP would result in a consistent suppression magnitude from day to day (i.e., consistent daily growth), which should be an ideal growth pattern for turfgrass managers. The 100-and 200-GDD 0 intervals yielded consistent suppression magnitude throughout the experiment, and suppression magnitude increased with the higher TE rate. In contrast, the 400-and 600-GDD 0 intervals allowed fluctuation in suppression magnitude from day to day. Discoloration occurred after initial applications and was more severe for the higher TE rate. Abbreviations: DAIT, days after initial treatment; GA, gibberellic acid; GDD 0 , growing degree-days with a base temperature of 0˚C; GDD 10 , growing degree-days with a base temperature of 10˚C; MSP, maximum suppression point; TE, trinexapac-ethyl.

Dithiopyr and dinitroanilines are preemergence-applied, mitotic-inhibiting herbicides used to control goosegrass [Eleusine indica (L.) Gaertn.]) in turfgrass. A suspected resistant E. indica population was collected from a golf course putting green and was evaluated for possible resistance to dithiopyr and prodiamine. After dose-response evaluation, the α-tubulin gene was sequenced for known target-site mutations that have been reported to confer resistance to mitotic-inhibiting herbicides. A mutation was discovered that resulted in an amino acid substitution at position 136 from leucine to phenylalanine (Leu136-Phe). Previous research has indicated that Leu136-Phe does confer resistance to dinitroaniline herbicides. The level of resistance indicated by regression models and I50 values indicates that there is a 54.1-, 4.7-, >100-, and >100-fold resistance to dithiopyr, prodiamine, pendimethalin, and oryzalin, respectively when compared to the susceptible population based on seedling emergence response and 88.4-, 7.8-, >100-, and >100-fold resistance to dithiopyr, prodiamine, pendimethalin, and oryzalin, respectively when compared to the susceptible population based on biomass reduction response. This is the first report of less resistance to prodiamine compared to pendimethalin or oryzalin due to a target-site α-tubulin mutation and the first report of a target-site α-tubulin mutation associated with dithiopyr resistance.

I n t e g r a t e d Tu r f g r a s s M a n a g e m e n t : We e d B i o l o g y a n d C o n t r o l