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- Author or Editor: William T. Haller x
‘Cocktail Whiskey’ begonia (Begonia semperflorens), ‘Sun Devil Extreme’ vinca (Catharanthus roseus), ‘Million Gold’ melampodium (Melampodium paludosum), and ‘Super Elfin’ impatiens (Impatiens walleriana) plants were irrigated with water treated with quinclorac, topramezone, imazamox, and penoxsulam to identify herbicide concentrations that cause phytotoxic effects. Plants were irrigated four times over a 10-day period with the equivalent of 0.5 inch of treated water during each irrigation and were then irrigated with tap water until they were harvested 28 days after the first herbicide treatment. Visual quality and dry weight data revealed that melampodium was the most sensitive of the bedding plants to quinclorac, imazamox, and penoxsulam, whereas vinca was the most sensitive species to topramezone. Noticeable reductions in visual quality and dry weight of melampodium were evident after exposure to 240, 580, and 10 ppb of quinclorac, imazamox, and penoxsulam, respectively, while dry weight of vinca was reduced after exposure to 110 ppb of topramezone. Current irrigation restrictions on imazamox, penoxsulam, and topramezone are adequate to minimize damage to these bedding plants if herbicide-treated waters are used for four irrigation events. However, irrigation restrictions should be established for quinclorac to prevent damage to sensitive bedding plants such as melampodium.
‘Miami Beauty’ anthurium (Anthurium andreanum), ‘Frieda Hemple’ caladium (Caladium ×hortulanum), ‘Debbie’ spathiphyllum (Spathiphyllum), and ‘Regina Red’ syngonium (Syngonium podophyllum) were irrigated with water treated with bispyribac-sodium, quinclorac, topramezone, and trifloxysulfuron to identify herbicide concentrations that cause phytotoxic effects. Plants were irrigated four times over a 11-day period with the equivalent of 0.5 inch of treated water during each irrigation and were then irrigated with well water until they were harvested 43 days after the first herbicide treatment. Visual quality and dry weight data revealed that caladium was the most sensitive of the foliage plants, regardless of herbicide mode of action. Noticeable reductions in visual quality and dry weight of caladium were evident after exposure to 182, 144, 186, and 1135 ppb of bispyribac-sodium, quinclorac, topramezone, and trifloxysulfuron, respectively. Of the four herbicides evaluated in these experiments, only quinclorac caused noticeable damage to plants when applied at a concentration similar to the proposed use rate.
Two formulations of the contact herbicide endothall are used to control submersed aquatic weeds. Waters treated with the amine or dipotassium salt formulations have irrigation restrictions varying from 7 to 25 days depending on the concentration of endothall applied. These water-use restrictions may be reduced for turfgrass if studies conclude there is no phytotoxicity to turf species irrigated with concentrations of endothall that may exist after an aquatic application. Two separate experiments were conducted to determine turfgrass tolerance to endothall in irrigation water on five species of grass: annual ryegrass (Lolium multiflorum), annual bluegrass (Poa annua), centipedegrass (Eremochloa ophiuroides), `Floratam' st. augustinegrass (Stenotaphrum secundatum), and `Tifton 419' bermudagrass (Cynodon dactylon). Expt. 1 used constant concentrations of endothall; Expt. 2 used decreasing concentrations of endothall over time. Annual turf species (bluegrass and ryegrass) were generally more susceptible than perennial turfgrasses. Concentrations resulting in a 10% reduction in total dry weight harvested compared to control plants [effective concentration (EC10)] for the amine and dipotassium salt formulations were 10 and 14 mg·L–1 (ppm) a.i. on annual ryegrass, 10 and 16 mg·L–1 a.i. on annual bluegrass, 50 and 54 mg·L–1 a.i. on centipedegrass, 47 and 72 mg·L–1 a.i. for st. augustinegrass, and for bermudagrass 1301 and 908 mg·L–1 a.i. in Expt. 1. Expt. 2 resulted in EC10 values of 31 and 35 mg·L–1 a.i. on annual ryegrass, 7 and 12 mg·L–1 a.i. on annual bluegrass, 32 and 99 mg·L–1 a.i. on centipedegrass, 27 and 20 mg·L–1 a.i. on st. augustinegrass for the amine and dipotassium formulations of endothall respectively, and 958 mg·L–1 a.i. for the dipotassium formulation on bermudagrass. There was no effect on bermudagrass dry weights when exposed to the amine formulation of endothall in Expt. 2 at concentrations up to 1600 mg·L–1 a.i. There is a low risk of inhibiting growth of turf species at endothall concentrations used for aquatic weed control considering the maximum use concentrations, typical uses of the products, and decomposition rates.
Restrictions on the endothall aquatic herbicide label include the use of endothall treated water for irrigating plants from 7 to 25 days after application. This interval was established to allow sufficient time for endothall to dissipate to levels that were considered safe for irrigation to prevent phytotoxicity to desirable plants. The affects of endothall on begonias (Begonia semperflorens `Vodka Cocktail'), pansies (Viola × wittrockiana `Atlas Purple'), petunias (Petunia hybrida), and impatiens (Impatiens wallerana `Lipstick') were determined by comparing aboveground dry weight to control plants after exposure to endothall in irrigation water. The objective was to determine if endothall concentrations used for aquatic weed control were phytotoxic to ornamental plants. Plants were irrigated every other day for 6 days in Expt. 1 with constant concentrations of endothall, and for 8 days in Expt. 2 with decreasing concentrations of endothall. Concentrations causing a 10% reduction in dry weight (effective concentration: EC10) compared to control plants showed there was a wide range of tolerance among plants exposed to endothall in irrigation water in the two experiments. Begonias, pansies, and impatiens had the lowest tolerance to two formulations of endothall in irrigation water with EC10 values ranging from 2 to 4 mg·L–1 (ppm) a.i. (a.i. based on acid equivalence) in both experiments. Petunias, which were more tolerant to endothall, had EC10 values of 15 and 34 mg·L–1 a.i. in Expt. 1 and 11 and 20 mg·L–1 a.i. in Expt. 2 for the amine and dipotassium formulations of endothall, respectively. Endothall did abscise flowers on impatiens after irrigation with endothall at concentrations of 10 mg·L–1 a.i. and higher. Effects of endothall on begonias at concentrations close to the EC10 values (2 to 3 mg·L–1 a.i.) were limited to growth inhibition with no apparent desiccation or necrosis of plant tissue.
Topramezone and bispyribac-sodium were registered for aquatic weed control in the last decade. A primary target for these products is fluridone-resistant hydrilla (Hydrilla verticillata), which is one of the most invasive submersed weeds in the southeastern United States. Both products have water use restrictions that prohibit irrigation of turfgrasses with treated waters until the herbicides have degraded to very low concentrations. The objective of these studies was to identify the concentrations of topramezone and bispyribac-sodium that are phytotoxic to turfgrasses that are commonly planted in Florida. Three species of turfgrass were irrigated twice weekly with 0.5 inch of treated water for 4 weeks (eight irrigations total). Cumulative EC10 values (the herbicide concentration that caused a 10% reduction in biomass compared with untreated control plants) after eight irrigations with water containing topramezone were 3.5, 4.3, and 17 ppb for ‘Palmetto’ st. augustinegrass (Stenotaphrum secundatum), ‘Pensacola’ bahiagrass (Paspalum notatum), and ‘Tifway 419’ hybrid bermudagrass (Cynodon dactylon × C. transvaalensis), respectively. Bispyribac-sodium was less toxic to all turfgrasses evaluated, with EC10 values of 56, 16, and >800 ppb for ‘Palmetto’ st. augustinegrass, ‘Pensacola’ bahiagrass, and ‘Tifway 419’ hybrid bermudagrass, respectively. These results support label instructions and highlight the need to comply with irrigation restrictions because the typical use concentrations for submersed weed control with topramezone and bispyribac-sodium are in the 20–40-ppb range.
Topramezone is a 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide that was labeled for aquatic use in Florida in 2013 with a maximum submersed application concentration of 50 µg·L−1. Preliminary greenhouse studies reported that the concentration of herbicide that reduces growth by 10% compared with untreated controls (EC10) of topramezone in irrigation water applied to st. augustinegrass (Stenotaphrum secundatum) grown in 100% sand was 3.5 ppb. The objective of these experiments was to determine whether substrate carbon content influenced the response of ‘Palmetto’ st. augustinegrass to irrigation with topramezone-treated water. The herbicide was applied at concentrations ranging from 0 to 120 ppb to mature plants grown in 7.5-inch-diameter nursery containers. Pots were filled with washed masonry sand amended with one of five carbon contents: 0%, 0.3%, 0.6%, 1.5%, and 4.0%. Plants were irrigated twice weekly for 4 weeks with topramezone-containing water and grown out for 12 weeks after the final topramezone treatment to evaluate possible recovery from any herbicide damage. Plant material was clipped as needed for a total of eight harvests and each harvest was dried and weighed. EC10 values for ‘Palmetto’ st. augustinegrass grown in substrates with 0%, 0.3%, 0.6%, 1.5%, and 4.0% carbon were 3.7, 7.3, 10.1, 28.1, and 25.7 ppb, respectively. These experiments revealed that substrate carbon content has a noteworthy effect on the susceptibility of ‘Palmetto’ st. augustinegrass to topramezone in irrigation water. However, regular irrigation with water containing high concentrations of topramezone is likely to cause damage to ‘Palmetto’ st. augustinegrass in Florida's sandy soils.