Rotala Rotundifolia)
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J. Aquat. Plant Manage. 55: 13–18 Efficacy of aquatic herbicides on dwarf rotala (Rotala rotundifolia) CARL J. DELLA TORRE III, LYN A. GETTYS, WILLIAM T. HALLER, JASON A. FERRELL, AND RAMON LEON* ABSTRACT the rapid and reliable movement of water to protect people and property during periods of heavy rainfall. Several Rotala is an invasive aquatic plant that was introduced to submersed aquatic weeds have invaded flood-control canals, the United States through the aquarium and water garden which can drastically lower the volume of water that can be industries. The species has become a significant weed moved through these systems. One of the most recent problem in southern Florida and is particularly trouble- invaders is dwarf rotala [Rotala rotundifolia (Buch.-Ham. ex some in flood-control canals, where its dense growth Roxb.) Koehne], which forms dense monocultures that impedes water movement. Previous research revealed that greatly reduce ecosystem services because oxygen produc- the organo-auxin herbicides 2,4-D and triclopyr provided tion and light penetration are hampered (Gettys and Della good control of rotala, but the use of these products in canal Torre 2014). Importantly, water flow is restricted because of systems can be problematic because of the risk of damage to the excessive growth of this weed, so the ability of infested nontarget species, such as crop and ornamental plants. canals to function properly in flood-control systems is Therefore, broad screening trials of aquatic herbicides on hindered. As such, management of this aquatic invader is a rotala were used to identify products that reduced biomass major concern. by 50% compared with untreated controls. Expanded Rotala was first discovered in Broward County in 1996 testing was then conducted to determine the EC90 val- and has since established large, but mostly isolated, ues—the concentration of herbicide required to reduce populations throughout the southern regions of Florida biomass by 90% compared with untreated controls. Most (Jacono and Vandiver 2007). It has red stems, bright green herbicides had EC90 values that were above the maximum leaves, and spikes of fuchsia flowers (Gettys and Della label rate, but three may have utility for controlling rotala Torre 2014). Rotala is easy to cultivate and grows well as in aquatic systems. Foliar applications of bispyribac or À1 either an emergent or submersed plant. Rotala generally imazamox at 44 or 469 g ai ha , respectively, and establishes in the moist soils along the shoreline, then À1 submersed applications of fluridone at 37 lgaiL reduced readily moves down into the water, where its growth biomass of rotala by 90% compared with untreated rapidly fills the water column (C. J. Della Torre and L. A. controls. These EC90 values are less than half the maximum Gettys, pers. obs.). application rate allowed on the product labels. This The genus Rotala comprises about 40 species, three of research demonstrates that bispyribac, imazamox, and which are present in the United States. Lowland rotala fluridone may be useful in rotation programs and/or as [Rotala ramosior (L.) Koehne] is a native herb that is broadly alternatives to the organo-auxins. distributed throughout the county and is considered Key words: amphibious weeds, canal management, foliar endangered or threatened in New England and Washington application, herbicide screening. State (USDA NRCS 2016b). A quick Internet search revealed that the introduced species Indian toothcup [Rotala indica INTRODUCTION (Willd.) Koehne and dwarf rotala (sometimes called round- leaf toothcup; hereafter, ‘‘rotala’’) are referred to inter- Florida’s aquatic systems are constantly challenged by the changeably and synonymously by many aquarium plant introduction of nonnative species, many of which become dealers, but the U.S. Department of Agriculture (USDA) invasive. The highest elevation in Florida is 60 m, but coastal classifies them as distinct species with geographically regions and most of southern Florida are , 10 m above sea discrete invasion patterns in the United States (USDA level (NASA 2004), so flood-control canals are an integral NRCS 2016a,c). Indian toothcup has been reported in component of south Florida’s rural/urban landscape. These California and Louisiana, whereas rotala has been intro- channels must be kept clear of aquatic vegetation to ensure duced to Florida and Alabama (USDA NRCS 2016a,c). Mechanical harvesting is the primary method currently *First and second authors: Former Graduate Research Assistant and used to manage rotala in flood-control canals, but that can Assistant Professor of Agronomy, University of Florida, Institute of Food and Agricultural Sciences (IFAS), Fort Lauderdale Research and be impractical for several reasons. Firstly, equipment Education Center, Davie, FL 33314; third author: Professor of Agron- expense and transport of harvested material can be cost omy, University of Florida, IFAS, Center for Aquatic and Invasive Plants, prohibitive, and many sites are too remote for proper Gainesville, FL 32653; fourth author: Professor of Agronomy, University offloading of the harvested material. Secondly, mechanical of Florida, IFAS, Gainesville, FL 32611; fifth author: Assistant Professor of Agronomy, West Florida Research and Education Center, Jay, FL harvesting can provide short-term results because regrowth 32565. Corresponding author’s E-mail: lgettys@ufl.edu. Received for from roots often occurs. In addition, the harvesting process publication March 9, 2016 and in revised form October 11, 2016. often causes plant fragmentation; fragments are transport- J. Aquat. Plant Manage. 55: 2017 13 ed downstream and often root at previously uninfested MATERIALS AND METHODS areas. Based on these limitations, alternatives to mechanical control are required. Rotala plant material was collected as needed during Puri and Haller (2010) reported that endothall and 2013 and 2014 from existing stocks in Davie, FL, and Gainesville, FL. Apical tip cuttings (10 cm long) were flumioxazin did not cause measurable damage to rotala, À1 whereas diquat (at 400 lgaiLÀ1) provided about 80% collected and planted 10 pot in 2-L pots without holes, which were filled with coarse builder’s sand amended with 5 control. Puri and Haller also found that submersed À1 1 applications of triclopyr or 2,4-D at 2 mg LÀ1 could be g pot of a controlled-release fertilizer. Rotala cultures were initiated in unheated greenhouses under terrestrial/ expected to result in total or near-total control of rotala. shallow water conditions and transferred to submersed Unfortunately, both of these herbicides are organo-auxins, conditions when new growth was evident. Once established, which are known to cause significant damage at low plants were selected for uniformity and randomly moved concentrations to sensitive species, such as crop and into mesocosms (46 cm in diameter by 46 cm deep; total ornamental plants. Although products with 2,4-D as the volume 77.6 L), filled with 65 L of well water to begin active ingredient have irrigation restrictions that are as herbicide testing. Three replicates (mesocosms) were pre- short as 7 d, most herbicide labels for products containing pared for each treatment in all experiments. Water levels triclopyr require that treated water not be used for crop were maintained at 38 cm throughout the course of the irrigation for 120 d after the application or until herbicide experiments. Initial screening trials were conducted sepa- , concentrations are 1 ppbv. rately on emergent and submersed rotala using foliar Herbicide resistance or tolerance in the genus Rotala is (emergent) or water column (submersed) applications to not unknown because the phenomenon was first docu- identify which products merited further evaluation. All mented in a biotype of Indian toothcup in Asia in 1997 herbicides that were evaluated in the initial screening trials (Uchino et al 2016). That biotype was resistant to the were tested at maximum and half-maximum label rates and acetolactate synthase (ALS)-inhibiting sulfonylurea (SU) concentrations (Table 1), and all herbicides used for foliar herbicides bensulfuron methyl, pyrazosulfuron ethyl, and (emergent) testing included a nonionic surfactant2 at a imazosulfuron (Blancaver et al. 2001). Further testing concentration of 0.25%. revealed that the biotype was cross-resistant to a number of other SU herbicides as well (Kuk et al. 2002). Blancaver et al. (2002) evaluated whole-plant response of SU-resistant Foliar application trials and SU-susceptible biotypes of Indian toothcup to the Herbicides (Table 1) were applied to emergent rotala non-SU ALS-inhibiting herbicides bispyribac sodium (mesocosm coverage . 90%) using aerosol spray units3 (pyrimidinyl-carboxy [PC]) and imazamox (imidazolinone fitted with 50-ml glass jars. Each suction straw and spray [IM]) and found that SU resistance status had no nozzle was used to apply a single herbicide, then replaced measurable impact on resistance to the PC or IM with a new straw and nozzle before applying the next herbicides tested. Both biotypes were susceptible to treatment to minimize contamination. The surface area of bispyribac sodium at a rate lower than that recommended 2 À1 each mesocosm was 1,688 cm , and an application rate of (200 g ai ha ) for weed control in direct-seeded rice fields 935 L haÀ1 (16 ml solution mesocosmÀ1) was used to ensure and resistant to imazamox at higher than the recommend- À1 adequate coverage of all emergent plant material in each ed rate (17 g ai ha ) for weed control in adzuki bean [Vigna mesocosm. Treatments and untreated controls (applications angularis (Willd.) Ohwi & H. Ohashi] fields (Blancaver et al. of water and surfactant only) were arranged in a random- 2002). ized complete-block design. Each mesocosm was treated Puri and Haller (2010) suggested that of the five aquatic once on 18 December 2013. Plants were maintained under herbicides evaluated (diquat, endothall, flumioxazin, triclo- static conditions and were destructively harvested during pyr, and 2,4-D), only the organo-auxins provided . 90% the week of 14 March 2014 (3 mo after treatment [MAT]). All control of rotala.