172 Juzwik et al.: Controlling Spread of the Oak Wilt Pathogen (Ceratocystis fagacearum) inant species in the forested areas. Other tree species present in- clude American elm, hackberry, red maple, and trembling aspen. Oak wilt is a major disease affecting oaks in the reserve. Mortality of 57 red and bur oaks was documented in 2008 (C. Evenson, unpublished data). Park staff believe annual mortality would be much greater if not for the long-term operational con- trol efforts. Oak wilt is also common in forested lots bordering the park lands, but uniform control efforts are lacking in these areas. The objectives of the park’s oak wilt control program are to preserve as many oaks as possible while reducing probability of pathogen spread to a low level. In general, eradication of oak wilt from the park is not considered achievable largely due to the inability to control pathogen spread from bordering private lands. The primary tools or activities used to control oak wilt in the park have been aerial and ground surveys for detecting and mon- itoring the disease, the vibratory plow to disrupt belowground spread of the fungus in established infection centers, and avoid- ance of wounding during spring months when risk of above- ground spread is highest. The park staff maintains detailed records of annual occurrence of wilt in existing and new infection centers, potential for oak wilt spore mat occurrence on wilted oaks, and of vibratory plow lines established each year. These records as well as annual assessments of the status of infection centers are used to evaluate the effectiveness of vibratory plow lines in controlling belowground pathogen spread and the need for re-treatment. A more formal, but unpublished, evaluation of VPL effectiveness in the park was conducted in 1988 (D. Shelstad, pers. comm.; copy on file with USDA Forest Service). Based on evaluation of ten vibratory plow-treated oak wilt sites per year between 1978 and 1982, treatments failed to contain the fungus on 30% to 40% of the sites four years after plowing on each site. However, the lines were established using a vibratory plow with a 122 cm long blade as this was before the availability of the current 152 cm long blade. This paper focuses on an evaluation of oak wilt control activi- ties in the park reserve between 1997 and 2006 and the results of spatial analyses used to consider adoption of new and/or refined control measures to the park’s program. The goal of the latter was to consider ways to improve vibratory plow effectiveness and additional measures designed to reduce probability of above- ground pathogen spread by insect vectors. The specific objectives of this evaluation were to 1) determine the success rate of VPLs in preventing oak wilt spread beyond the established disease cen- ter over a six to eight-year period, and 2) using a geographical information system, a) compare actual VPL placement to hypo- thetical line placement had a different model been used, and b) compare numbers of oaks that would have been removed within outermost VPLs under different disease sanitation scenarios de- signed to reduce pathogen inoculum availability for insect spread. MATERIALS AND METHODS Study Area and Site Descriptions Field data used in this analysis were collected from actively man- aged oak wilt infection centers in Murphy-Hanrehan Park Re- serve. The area is characterized by glacial ridges and hilly terrain occurring on glacial moraines. The mixed hardwood forests of the reserve lie within the Big Woods ecological subsection of the Minneapolis-St. Paul metropolitan area (Keys et al. 1995; Min- nesota DNR 2006). Twenty-five active oak wilt centers in which ©2010 International Society of Arboriculture VPLs had been installed between 1997 and 1999 were selected for this study. The criteria for selection of a study site stipulated that the candidate center was not coalescing with an adjacent cen- ter and the center had only been treated one time for oak wilt, i.e. had not and would not be re-treated with the vibratory plow during the study time period. Detailed site records, maps of VPL placement, and electronic geographic information system (GIS) files collected by park staff were available for use in the study. Soils in each of the VPL treatment sites were classified in one of the following: Kingsley, Mahtomedi and Hayden soil series complex; Kingsley-Mahtomedi complex; Hayden loam; and Hayden sandy loam arising from glacial till or outwash/till soil parent material (USDA NRCS 2007). Moderate slopes (15% to 25%) were characteristic of most (21) of the disease sites. The estimated densities of red oaks in the sites ranged from 30 to 178 stems per acre. The average diameter of red oak stems in treatment sites ranged from 23 to 51 cm at 1.4 m height (dbh). White oak presence on the sites varied from two sites where the white oaks out-numbered the red oaks to three sites where red oaks were ten times more numerous than the white oaks. Vibratory Plow Line Treatment The park forester marked lines for VPL treatment using a modi- fication of a model initially developed by French and Stienstra (1978). The outermost lines established, called primary lines, are those considered highly likely to arrest belowground spread of the pathogen outside the existing disease center. Lines es- tablished within the primary line, called secondary lines, were placed between the wilting tree(s) and the primary VPL, usually just beyond the last wilting tree or one tier of healthy trees toward the primary line. On most sites, both primary and secondary lines were marked. A tracked cable plow (Ditch Witch HT-100, The Charles Machine Works, Inc., Perry, OK) or an articulated-steer- ing, rubber-tired cable plow (Vermeer Corp., Pella, IA), both with a vibrating unit operating a 152 cm blade, was used to sever any connected roots. The tracked plow was preferred in the study area due to its superior ability to climb and maneuver on the steep slopes present in the park. However, this machine was not available for use in 1999. The plow blade was inserted into the ground to the maximum operating depth and pulled through soil along flagged lines. The plowing on the sites was performed between late Au- gust and early November during 1997, 1998, and 1999. The field crews documented VPL placement by tagging the base of “wit- ness” trees that were generally on the outside of the plowed lines. Site Mapping Field site and tree data were collected to enable mapping of all oak trees (dead or alive) on each study site. The data were collected between July and mid-September 2001. In general, the plot center was near the original, diseased oak or within the cluster of originally infected trees. Coordinates for the plot center were obtained using a geographical positioning system (GPS) with sub-meter accuracy (Ashtech BR2G GPS/Beacon Receiver, Magellan Corp., Santa Clara, CA). A portable field computer (Ranger 133R, Tripod Data Systems, Corvallis, OR) connected to a digital compass (Laser Technology Map Star Compass, Module I, Centennial, CO) and a laser range finder (Laser Technology Impulse 200 Laser Range Finder, Centen- nial, CO) was used to collect spatial location data for each oak
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