134 Elliott and Broschat: Uptake, Movement, and Persistence of Fungicides in Mature Coconut Palms continued to increase over time in the youngest, unopened leaf, the initial target site of the patho- gen, and persisted in this tissue for at least one year. There have also been numerous studies exam- ining the uptake, movement, and persistence of insecticides in palms (e.g., Kaakeh 2006; Ali and Caldwell 2010; Dembilio et al. 2014), but very few examining systemic fungicides. Fungicide stud- ies have been conducted with some of the benz- imidazole compounds, but with products that are no longer available in the United States (benomyl and carbendazim phosphate). Benomyl was used as a soil drench on adult Canary Island date palm (Phoenix canariensis), and residue was detected in the leaves via a bioassay (Surico 1977). Carben- dazim phosphate was used as a trunk injection of adult Canary Island date palms, with the fungicide detected at 48–56 hours aſter the injection, which was the only time samples were obtained (Feather 1982). This fungicide was detected in the trunk and apical meristem, but not leaf tissue. A recent methods paper examined detection of multiple fungicides, including the benzimidazole fungi- cides carbendazim, thiabendazole, and thiophanate methyl aſter coconut trunk injection (Ferreira et al. 2015). However, samples were again obtained only once, at 45 hours aſter injection, and then only from the trunk. Carbendazim and thiabendazole were detected, but thiophanate methyl was not. While fungicide uptake, movement, and per- sistence has been examined in eudicotyledonous trees, this information is difficult to extrapolate to palms, as palms are arborescent monocotyledons. Palm trunks are composed of thousands of vascular bundles, with each bundle containing xylem vessels, phloem sieve tube cells, and fibers, the numbers of which are essentially predetermined as palms have no vascular cambium (Tomlinson 1990; Tomlinson et al. 2011; Renninger et al. 2013). The directional orientation of vascular bundles is referred to as the “Rhapis principle,” and means that bundles start- ing near the outside of the trunk near the soil line curve into the middle of the stem and then curve to the outside of the stem, and so forth up to the leaf canopy (Zimmerman and Tomlinson 1965). Furthermore, vascular bundles produce short branches that connect to other vascular bundles (Zimmerman 1973). This means that only one trunk injection site is needed to disperse pesticides ©2017 International Society of Arboriculture uniformly throughout the palm canopy versus the multiple injection sites needed for eudicot trees. Another feature of palms that may influence pesticide persistence is the fact that they are not deciduous, but are constantly shedding old leaves as new leaves emerge from the apical meristem. How- ever, the anatomical structure of the palm ensures vascular connections are made with new leaves, because tyloses are formed to plug the protoxylem of abscising leaves (Zimmerman and Tomlinson 1965; Zimmerman 1973; Zimmerman and Tom- linson 1972). Thus, if products are taken up by the palm, the product may continue to move into new leaves, but will eventually become diluted as more new leaves emerge from the apical meri- stem and older leaves abscise. This would espe- cially be expected with xylem-mobile pesticides. The objectives of the study described herein were twofold: i) determine uptake, movement, and per- sistence of four systemic fungicides—propiconazole, tebuconazole, thiabendazole, and thiophanate methyl—in mature coconut palms; and ii) compare uptake, movement, and persistence of root drench versus trunk injection of propiconazole and thiaben- dazole in mature coconut palms. This information could then be used to plan landscape experiments with mature, tall palms affected by diseases such as Fusarium wilt, petiole blight, and rachis blight. Thiophanate methyl was evaluated, as it is a standard product used as a root drench on palms in the landscape in Florida. It is not an inject- able fungicide. For large palms (those with more than 1.5 to 2 m of clear trunk), foliar applica- tion of fungicides is impractical and spray driſt a major hazard in the landscape. Injectable fun- gicides are desirable for palms as they limit exposure of the fungicide to humans and the environment. Propiconazole, tebuconazole, and thiabendazole were selected because they are cur- rently registered for use as trunk injections in the U.S., although not necessarily for use in palms. MATERIALS AND METHODS Palms The 18 coconut palms selected for the experi- ments were located at the University of Florida’s Fort Lauderdale Research and Education Cen- ter (Davie, Florida, U.S.). The palms were not a
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