Arboriculture & Urban Forestry 38(3): May 2012 Arboriculture & Urban Forestry 2012. 38(3): 99–104 99 Rapid Removal of Symptomatic Trees Reduces Dutch Elm Disease Infection Rates J. Veilleux, J. Leferink, and N.J. Holliday Abstract. In the province of Manitoba, Canada, 14 communities were paired on the basis of size and location. From 2004 to 2010 in one member of each pair, rapid removal of American elm (Ulmus americana) trees that displayed symptoms of Dutch elm disease was practiced: newly symptomatic trees were removed within six weeks of symptom detection. The remaining member of each pair continued with autumn or winter removal of symp- tomatic trees. Treed urban study areas were selected in each community, and in 2008 an inventory of U. americana was taken in these areas. From this inventory and from records of tree removals, estimates of the number of living U. americana and prevalence of Dutch elm disease were made for each year from 2004 to 2010. Following the switch to rapid removal, the annual prevalence of Dutch elm disease in rapid removal communities was 1.5 ± 0.2%, significantly lower than in communities with autumn/winter removal (3.1 ± 0.4%). The study authors estimate that in similar areas the value of the elm resource after 10 years would be almost CAD $600,000/km2 greater if rapid removal rather than autumn/winter removal were practiced. There- fore, under conditions similar to those in these communities, rapid removal should be a component of management programs for Dutch elm disease. Key Words. American elm; Hylurgopinus rufipes; Ophiostoma novo-ulmi; Sanitation; Tree Removal; Ulmus americana. Dutch elm disease (DED) is a wilt disease of many species of Ulmus, and the two pandemics of this disease that have swept North America (Brasier 2000) have been responsible for the de- mise of the majority of American elm (U. americana) trees in many North American towns and cities. The causative agent of the second and more destructive pandemic is the fungal patho- gen Ophiostoma novo-ulmi (Hintz et al. 1993; Brasier 2000). Al- though transmission of O. novo-ulmi between neighboring trees can occur through root grafts (Miller et al. 1969), the majority of transmission is the result of transfer of fungal spores by bark bee- tle vectors of the sub-family Scolytinae (Webber 2000). In North America, Scolytus multistriatus and Hylurgopinus rufipes are the two most important vector species (Strobel and Lanier 1981). In Manitoba, although S. multistriatus has been collect- ed (Buth and Ellis 1981), H. rufipes is by far the most impor- tant vector of DED (Hildahl and Wong 1965). Adults of H. rufipes overwinter in tunnels in the bark at the base of healthy U. americana (Anderson and Holliday 2003), and there is no evidence that larvae can survive the winter in Manitoba (An- derson 1996; Oghiakhe and Holliday, unpublished data). Brood galleries are constructed in recently dead or dying U. ameri- cana (Hildahl and Wong 1965; Bright 1976), which frequently are trees that are infected with, or have succumbed to, DED. In Manitoba, DED presence was first confirmed in 1975 (Hil- dahl 1977), and since then a provincial integrated DED manage- ment program (Jeffrey 1982) has been active. In 2010, the pro- gram had a budget of CAD $1.3 million and operated in 31 urban communities and in buffer zones surrounding the Cities of Win- nipeg and Brandon, both of which have their own management programs. Main elements of the programs are area-wide sanita- tion of diseased and declining elms, regular insecticide applica- tions to the basal part of healthy elm trees to control overwin- tering H. rufipes (Jin et al. 1996; Anderson and Holliday 2003; Oghiakhe and Holliday 2011), and regular pruning of dead wood. More than 70% of the provincial DED management budget is dedicated to the sanitation program in which removals aver- aged 6,710 trees annually from 2004 to 2010. The program in- volves the removal of all trees showing DED symptoms as well as hazard elm trees, defined as those with ≥40% deadwood. Yearly, between the end of June and late August, each community in the program was surveyed twice for elms displaying the DED symptoms of wilting and browning of foliage. Diagnosis of DED was confirmed if brown streaking of sapwood were seen in an infected branch. Infected trees were tagged and entered into a GIS-based database system so that they could be removed before April 1 of the following year (Manitoba Natural Resources 1981). The effectiveness of sanitation for reducing DED prevalence (Miller et al. 1969; Van Sickle and Sterner 1976; Barger 1977) and its value for minimizing DED-related costs (Cannon et al. 1977) have been demonstrated, and some of these publications emphasize that tree removal should be prompt. Landwehr et al. (1982) found that the percentages of H. rufipes that were spore- bearing were higher in Minnesota, U.S., cities where the inter- val from DED symptom detection to tree removal was longer, and suggested that trees showing symptoms in June should be removed within 90 days. In Manitoba, however, most removals are conducted during autumn and winter as fewer competing ur- ban forestry operations are occurring at this time, and freezing of the ground reduces damage by heavy equipment used in removal operations. In Manitoba, DED symptoms normally first become ©2012 International Society of Arboriculture
May 2012
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