Arboriculture & Urban Forestry 36(4): July 2010 Arboriculture & Urban Forestry 2010. 36(4): 171–178 171 Controlling Spread of the Oak Wilt Pathogen (Ceratocystis fagacearum) in a Minnesota Urban Forest Park Reserve Jennifer Juzwik, Joseph O’Brien, Charles Evenson, Paul Castillo, and Graham Mahal Abstract. Effectiveness of oak wilt control actions taken between 1997 and 1999 were evaluated for an urban forest park reserve in Minnesota, U.S. A high level of success (84% of evaluated disease centers) was achieved in controlling belowground spread of the vascular pathogen for four to six years by mechanically disrupting inter-tree root connections with the blade of a cable plow (vibratory plow, VP). Placements of the outer- most (i.e., primary) VP treatment lines were based on a modified, rule-of-thumb model. Plausible scenarios based on two protocols for preventing pathogen spore production, and thus aboveground insect-mediated spread, were explored in conjunction with alternative, root treatment models us- ing a geographical information system. For the 95% confidence level of a statistical model, the numbers of red oaks inside primary lines were 2.5 times greater than those inside the primary, installed lines and represents the difference in tree losses if all red oaks were removed to the primary lines [i.e., a “cut-to-the-line” (CTL) protocol]. Alternatively, a “monitor and remove” (MR) option (i.e., annual removal of wilting red oaks), would have resulted in 64% fewer removals than CTL. The park’s forestry division subsequently added the MR protocol to its oak wilt control program. Key Words: Cultural Control; Insect Vector Spread; Integrated Pest Management; Plant Health Care. Oak wilt, caused by the fungus Ceratocystis fagacearum (Bretz) Hunt, has been considered the forest disease of great- est concern in four of the twenty-three U.S. states in which it occurs (Billings 2000). It is estimated that tens of thou- sands of oaks are killed annually in the north central United States. In Anoka County, Minnesota, U.S., 990 active in- fection centers were documented in 2008; however, no es- timates of tree mortality were given (Haight et al. 2009). Operational management of oak wilt in portions of states with organized disease suppression programs includes tools or activities that disrupt or prevent the spread of the pathogen from diseased to healthy trees. In Minnesota, belowground or root-graft spread is commonly controlled through the use of a vibratory plow equipped with a 152 cm long blade that mechani- cally severs roots and, thus, breaks root connections between oaks joined by root grafts. Different models exist for directing the placement of the vibratory plow lines (VPLs) (French and Stienstra 1978; Bruhn et al. 1991). A modified version of the model developed by French and Stienstra (1978) is most widely used in Minnesota. In general, root grafting frequency is highest between individuals of the same species and lower between dif- ferent oak species. Root grafting between a red oak (Section Lo- batae, Subgenus Quercus) and a white oak species tree (Section Quercus, Subgenus Quercus) occurs infrequently to rarely (Juz- wik 2009). Other factors within a stand that affect grafting and the frequency of pathogen spread frequency include basal area of oaks, topographic relief, soil depth, soil texture, and occur- rence of non-oak species (Bruhn and Heyd 1992; Juzwik 2009). Aboveground spread of C. fagacearum in Minnesota oc- curs when a sap beetle vector (Coleoptera: Nitidulidae) visits a fresh, xylem-penetrating wound on a healthy oak after acqui- sition of the oak wilt fungus from sporulation mats formed on oaks that died from oak wilt the previous growing season (Juz- wik et al. 2004b). Sporulation is most common on red oak spe- cies, less common on bur oak, and insignificant on white oak. The primary strategies used to control insect spread of oak wilt in Minnesota are 1) avoidance of wounding during the critical spring period (e.g., not pruning or harvesting oaks unless the exposed xylem is immediately covered by paint), and 2) elim- ination of inoculum by removing all potential spore mat pro- ducing trees (PSPTs) before mats form. At least two different options have been used in Minnesota to implement the second strategy. “Monitor and remove” involves annual inspection and subsequent removal of any wilting red oaks inside the outer- most VPLs (USDA Forest Service, NA S&PF 2009). A modi- fication of this option involves removal of a red oak only if its cambial condition indicates sporulation is likely to occur (Cook 2001). The “cut-to-the-line” option involves removal of all red oaks within the outermost VPLs within the same year of vibra- tory plow treatment (USDA Forest Service, NA S&PF 2009). The oak wilt control program for Murphy-Hanrehan Park Reserve (44°42’34”N; 93°20’08”W), northeastern Scott County, MN, was initiated in 1978 and is administered through the forestry division of the Three Rivers Park District. This program represents one of the longest-running, continuous oak wilt suppression ef- forts in the state. The 2,400 acre reserve is undeveloped and largely forested land managed for recreation, species diversity, wildlife habitat, and the preservation of existing forest vegetation in the larger Minneapolis-St. Paul metropolitan area. Privately-owned, suburban properties populate the park boundaries, particularly on the northern and eastern edges. Northern red oak (Quercus rubra), bur oak (Q. macrocarpa), and white oak (Q. alba) are the predom- ©2010 International Society of Arboriculture
July 2010
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