176 Juzwik et al.: Controlling Spread of the Oak Wilt Pathogen (Ceratocystis fagacearum) Table 4. Numbers of red oaks inside primary vibratory plow lines (VPLs) installed 1997–1999 compared to numbers inside hypothetical modeled lines at 80% and 95% confidence lev- els (CL) per published mathematical equation.z Yr VPLs Site 1997 1997 1997 1997 1997 1997 1997 1997 1997 1998 1998 1998 1998 1998 1998 1998 1998 1998 1999 1999 1999 1999 1999 1999 Equation published in Bruhn et al. (1991) was used to calculate the required distance from each red oak in each site at each confidence level (CL). The soil factor for the Pemene Soil Series was used. All yrs z DISCUSSION Belowground Spread Control A high level (84% of infection sites) of local, or belowground, disease control over four to six years was achieved using primary VPLs based on modified use of French and Stienstra’s (1978) model installed with a vibratory plow with a 152 cm long blade. This represents an improvement in control within the park from 1978 to 1982, and from 1997 to 1999 [i.e., where 60% to 70% of belowground spread control was documented in 1988 by Sheldstad (unpublished University of Minnesota report; copy on file with USDA Forest Service)]. The suppression of local (i.e., root graft) spread for 118 oak wilt sites treated with a vi- bratory plow (generally multiple times) as part of the Minnesota Department of Natural Resources coordinated oak wilt suppres- sion program in east central Minnesota was evaluated in a re- cent study (R. Venette, pers. comm.; unpublished report on file with USDA Forest Service). Overall, 81% of the sites showed no evidence of spread up to 20.1 m beyond the primary VPL in- stalled around each infection center within three years of treat- ment. However, success of the overall oak wilt control effort was related to concurrent use of other treatment tools (i.e., PSPT removal and systemic fungicide injection of red oaks inside the primary lines). Based on evaluation of operational VPL lines established on deep sand soils within the town of Ham Lake, ©2010 International Society of Arboriculture 71 74 75 Inside installed Inside modeled Inside modeled installed number primary VPLs lines 80% CL lines 95% CL 1997 5 715 721 722 730 736 737 738 83 84 814 816 821 825 828 829 830 91 94 921 928 934 945 All sites 15 2 6 13 12 19 8 9 2 14 10 21 15 3 8 7 11 7 12 4 14 17 21 24 279 5 15 3 13 86 25 39 12 18 12 19 18 15 16 6 21 21 17 7 15 5 12 22 19 24 465 11 21 5 15 86 38 44 25 18 27 21 37 21 22 13 21 42 34 21 23 10 28 47 27 32 689 MN, between 1993 and 1996, primary VPLs were apparently successful in stopping belowground spread of the pathogen on 82% of 49 treated sites (O’Brien and Mahal, unpublished data). Secondary VPLs were of limited value in controlling below- ground spread within the primary lines. The main value of second- ary line use for the park staff is to preserve some of the red oaks within the primary line on the site based on the anecdotal data that only about 50% of the trees between secondary and primary lines succumb to oak wilt on deep sand soil sites (Juzwik et al. 2004a). Thus, by separating groups of asymptomatic oaks from each other within the primary barriers, additional trees may be saved. Based on this evaluation, between two and seventeen red oaks were preserved on 39% of the sites where oak wilt was ap- parently contained within the secondary VPLs (data not shown). In this study, primary VPLs successfully prevented below- ground spread of the oak wilt fungus on more than 80% of 25 sites for at least four to six years. Several explanations are offered for why failures occurred. Primary VPLs may have been placed too close to the original infections and the fungus may have already moved beyond the point of root severing at the time of plowing. Alternative VPL line placement models can be considered as a possible improvement for future treatments. Site and machine limitations may also result in insufficient depth of root severing. Buried boulders may prevent full insertion of the plow blade at one or more points along designated plow lines. Hard soil condi- tions due to drought may also limit blade insertion and require more machine power and traction to overcome. As of the year 2000, only a tracked cable plow has been used by the park district due to its superior traction. Theoretically, it is possible that new root connections may have formed across the zone of severed roots several years after plow treatment. No evidence exists, how- ever, for the occurrence of re-grafting across either a VPL or, in other states, across a filled-in trench following root disruption by a trenching machine. The occurrence and frequency of new grafts forming across primary VPLs, the length of time for such grafts to form, and the abundance of new root proliferation for Q. rubra and Q. ellipsoidalis following root severing should be considered in future studies, particularly in Michigan, Minnesota, and Wis- consin, where vibratory plows are used in oak wilt control efforts. Lastly, local aboveground spread of the fungus may have occurred via pathogen-contaminated nitidulid beetles. A high percentage (75%) of such contaminated beetles visiting fresh wounds (artifi- cially created) on healthy trees inside primary VPLs in the same park reserve was documented previously (Juzwik et al. 2004b). The frequency of local pathogen spread by insects is higher than frequency of longer distance spread (Menges and Loucks 1984). Improvement of the park reserve’s oak wilt control program could logically be accomplished through removal of PSPTs. Options for Aboveground Spread Control Spatial analysis using the geo-referenced maps allowed hypo- thetical consideration of two options for reducing probability of aboveground fungus spread. In particular, estimates of numbers of red oaks that would have been removed under each option were obtained. Such data is valuable when determining which option is most consistent with the park’s oak wilt management objectives and for cost comparisons. As expected, a much larger number of red oaks would have been removed under a “cut-to- the-line” approach than under a “monitor and remove” option
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