Resistance in Smallflower Umbrella Sedge (Cyperus difformis L.) to Acetolactate Synthase-Inhibiting Herbicides in Rice - First Case in India

Smallflower umbrella sedge is one of the problematic weeds in direct-seeded rice in India. Bispyribac-sodium (acetolactate synthase-inhibiting herbicide) is a commonly used in rice, but recently growers have reported lack of smallflower umbrella sedge control with this herbicide. An extensive survey was carried out in two rice growing states, Chhattisgarh and Kerala, where 53 putative bispyribac-sodium resistant (BR) biotypes were collected. Studies were conducted to confirm resistance to bispyribac-sodium and to test the efficacy of newly developed synthetic auxin herbicide florpyrauxifen-benzyl on putative BR biotypes. Whole-plant bioassay revealed that bispyribac-sodium is no longer effective. Of 53 putative BR biotypes, 17 biotypes survived recommended label rate of 25 g ai ha−1. Effective bispyribac-sodium rate required to control 50% of the plants in most of the BR biotypes (ED50) ranged from 19 to 96 g ha−1 whereas it was 10 g ha−1 in susceptible biotype. In two highly resistant biotypes, ED50 was beyond the maximum tested rate, 200 g ha−1. This suggests 2 to >20-fold resistance in BR biotypes. Acetolactate synthase (ALS) enzyme activity assay suggests altered target site as mechanism of resistance to bispyribac-sodium. This study confirms the first case of evolved resistance in smallflower umbrella sedge for bispyribac-sodium in India. However, the newly developed synthetic auxin, florpyrauxifen-benzyl effectively controlled all BR biotypes at the field use rate 31.25 g ae ha−1.

Environmental diagnosis of water quality in situ and ex situ in streams in Upper Paraná River Basin, Brazil

The aim of this study was to (i) evaluate water quality using limnological parameters in three streams belonging to the Ivinhema River basin, (ii) investigate the composition and structure of the landscape around the locations sampled and their possible contribution to changes in water quality, (iii) investigate genotoxicity in fish in situ as environmental bioindicators, and (iv) evaluate the cytotoxic and genotoxic potential of water through bioassays in animal and plant models. A total of seven fish species were sampled in situ as bioindicators of environmental toxicogenetic damage: Astyanax fasciatus, Astyanax lacustris, Cyphocharax modestus, Moenkhausia forestii, Piabina argentea, Psellogrammus kennedyi, and Hypostomus ancistroides. Four types of nuclear and one cytoplasmic alteration were observed, with nuclear invagination being the most frequent. Animal bioassays showed five types of nuclear and one cytoplasmic alteration. In the plant bioassay, a reduction in germination and cell division was observed, indicating the cytotoxic and genotoxic effects of the water for Allium cepa cells. Thus, genotoxic alterations in fish and plant bioassays may reflect the different types of land use and cover around streams and the conversion of forest fragments into agricultural areas, mainly due to the expansion of sugarcane crops. The evaluation of the toxicogenetic status of the three streams is essential to emphasize the need to create projects to recover the native vegetation cover, which would improve the quality of the habitat and ensure the permanence of native species.

Herbicide Residues in Manure, Compost, or Hay

Some manure-based products can contain herbicide residues that can affect the growth of sensitive plants. A simple plant bioassay can be conducted to test for herbicide contamination. Written by J. A. Ferrell, P. J. Dittmar, B. A. Sellers, and P. Devkota, and published by the UF/IFAS Agronomy Department, revised May 2020.

Differential response of horseweed (Conyza canadensis) to halauxifen-methyl, 2,4-D, and dicamba

Halauxifen-methyl is an auxin herbicide for broadleaf weed control in preplant applications to corn and soybean. Our objective for this research was to characterize the phytotoxicity of halauxifen-methyl on horseweed, relative to 2,4-D and dicamba, in terms of weed height, the response to an auxin synergist, and root activity. The 50% reduction in plant growth (GR50) value for halauxifen-methyl on rosette-sized plants was 0.05 g ae ha−1, 100 times less than the labeled use rate of 5 g ae ha−1, compared with 36 and 31 g ha−1 for 2,4-D and dicamba, respectively. In a whole-plant bioassay, 240 g ae ha−1 of 2,4-D was calculated as the GR50 value on horseweed 20-cm tall, whereas applications of only 53 and 0.40 g ae ha−1 were necessary for dicamba and halauxifen-methyl, respectively, to achieve the same response. As weed size decreased, there was a concomitant reduction in the estimated herbicide dose for the GR50 with similar differences observed between halauxifen-methyl and the other two auxin herbicides. The addition of diflufenzopyr, an auxin synergist, to 2,4-D and dicamba resulted in a synergistic response on horseweed. However, the addition of diflufenzopyr to halauxifen-methyl resulted in an additive or antagonistic effect, depending on rate of diflufenzopyr, demonstrating a distinctive physiological pathway for halauxifen-methyl compared with 2,4-D and dicamba. In the agar-based bioassays, GR50 values for horseweed root length for 2,4-D and dicamba were 0.16 and 0.19 µM, respectively, whereas only 0.004 µM halauxifen-methyl was required for a comparable root response. These results indicate that horseweed exhibits a high level of sensitivity to halauxifen-methyl and suggest the activity of halauxifen-methyl is different from that of 2,4-D and dicamba. These differences in herbicide activity may reflect differential absorption, translocation, metabolism, or targeting of auxin receptors found in horseweed.

The First Case of Short-Spiked Canarygrass (Phalaris brachystachys) with Cross-Resistance to ACCase-Inhibiting Herbicides in Iran

The weed Phalaris brachystachys Link. severely affects winter cereal production. Acetyle-CoA Carboxylase (ACCase)-inhibiting herbicides are commonly used to control this weed in wheat fields. Thirty-six populations with suspected resistance to ACCase-inhibiting herbicides were collected from wheat fields in the Golestan Province in Iran. A rapid test performed in Petri dishes and whole-plant dose–response experiments were conducted to confirm and investigate the resistance level of P. brachystachys to ACCase-inhibiting herbicides. The seed bioassay results showed that 0.02 mg ai L−1 clodinafop-propargyl (CP) and 1.36 mg ai L−1 of the diclofop-methyl (DM) solution were the optimal amounts for reliably screening resistant and susceptible P. brachystachys populations. In the whole plant bioassay, all populations were found to be resistant to CP, resistance ratios ranging from 2.7 to 11.6, and all of the CP-resistant populations exhibited resistance to DM. Fourteen populations showed low resistance to cycloxydim, and thirteen of these populations were also 2-fold resistant to pinoxaden. The results showed that DM resistance in some P. brachystachys populations is likely due to their enhanced herbicide metabolism, which involves Cytochrome P450 monooxygenases, as demonstrated by the indirect assay. This is the first report confirming the cross-resistance of ACCase-inhibiting herbicides in P. brachystachys in Iran.