112 Elliott et al.: Evaluation of Electrical Resistance Tomography for Imaging Palm Trunks trees (Nicolotti et al. 2003; Brazee et al. 2011; Lin et al. 2012; Guyot et al. 2013). While one disadvantage of ERT is the difficulty in characterizing defects or cavities in dry heartwood (al Hagrey 2007), this is not an issue for palm trunks as they have no heartwood. To date, tomography techniques reported for detection of wood decay or insect damage within palm trunks have used ultrasonic and stress waves (Lin et al. 2011; Najmie et al. 2011; Huang et al. 2013), gamma rays (Abdullah et al. 2013), sound (Mazliham et al. 2008; Al-Sulaiman and Hawwa 2012), X-rays (Ma et al. 2012), cone beam neu- tron (Alghamdi 2012), and infrared heat (Al- Sulaiman and Hawwa 2012), but not ERT. One of the current authors (Göcke) has used ERT on various palms in multiple locations world- wide but results have not been reported publicly. For a tool to be useful, baseline data is required. A preliminary study was conducted in March 2014 in southeastern Florida, U.S., on Syagrus romanzof- fiana (queen palm) to determine the usefulness of ERT in imaging palm trunks for assessment and comparison of healthy and diseased palms. Two distinctly different urban sites were selected, but at both sites, queen palms were dying from Gano- derma butt rot caused by Ganoderma zonatum, a typical white rot fungus (Elliott and Broschat 2001). One site was also affected by Fusarium wilt caused by Fusarium oxysporum f. sp. palmarum, a vascular wilt pathogen (Elliott et al. 2010). MATERIALS AND METHODS Study Sites and Palm Selection The palms selected at the two urban landscape sites for ERT were evaluated 19–20 March 2014, in the middle of the six-month dry season for southeastern Florida. The first site was a residential community located in Palm Beach County. The landscape was intensively maintained and well-irrigated, and over one-third of the queen palms had died from either Ganoderma butt rot or Fusarium wilt. The four palms selected had trunks with approximately 5 m of gray wood and diameters of approximately 26 cm. The second site was a minimally maintained, non- irrigated site in Broward County at the University of Florida’s Fort Lauderdale Research and Education Center. This site has a concentration of queen palms in one location on the property, and Ganoderma ©2016 International Society of Arboriculture butt rot has affected some of these palms. The three palms selected had trunks with approximately 6 m of gray wood and diameters of approximately 30 cm. At each location, a palm was selected that was severely wilted, with G. zonatum basidiocarps emerging from the trunk. At the Palm Beach County site, a queen palm with typical and severe Fusarium wilt symptoms was also selected. Two apparently healthy queen palms were selected at each site for comparison with the diseased palms. Electrical Resistance Tomography The instrument used to measure electrical conduc- tivity within the palm trunks was a PiCUS Treetron- ic® (Argus Electronic GmBH, Rostock, Germany), a multichannel, multielectrode resistivity system. Electrical resistance of the wood was measured by inserting 12 zinc-galvanized nails (3 mm diameter) equidistant into the trunk at each cross section mea- sured (three to five sections per palm). The nails were only tapped until firmly in place. Electrical cables were attached to each nail and to the base unit. Be- tween two of the nails, an electric DC voltage (12 to 50 volts) with changing polarity was applied, which caused an electrical current of 0.003 to 0.01 amps to flow. The duration of the impulses was less than one second and was applied to all nails in sequence. Data was collected, processed, and analyzed using PiCUS soſtware to obtain color tomographs, which illus- trate the apparent electrical resistance of the wood. Aſter the tomographs were obtained, diseased queen palms at both sites were felled and cross sec- tions cut at the height of sampling with the instru- ment. One healthy queen palm was felled at the Palm Beach County site for comparison of internal cross sections with diseased queen palm cross sections. Fungal Isolation from Trunk Tissue Cross sections from diseased queen palm trunks were brushed free of debris and transferred to the University of Florida plant pathology laboratory in Broward County, where they were placed in large plastic containers and covered loosely with plastic bags to encourage fungal growth. Ganoderma-like growth grew out of the trunk sections associated with the palm with Ganoderma butt rot. Fusarium oxysporum grew out of the trunk associated with Fusarium wilt. For both fungi, DNA was extract- ed and used for the polymerase chain reaction,
March 2016
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