Economic Effectiveness of Operational Therapeutic Pruning for Control of Dutch Elm Disease

Therapeutic pruning for Dutch elm disease control has not been widely used in Minnesota because of fear of low success, operational difficulties, and difficulty in obtaining funds for such a program. We report a study in which trees were identified for pruning during routine tree inspections, and pruned by contractors. Of 32 pruned trees, 8 have died. Had trees not been pruned, removal costs would have been $5351. The pruning and removal of trees which died subsequently cost $1856. The pruning program reduced the Dutch elm disease program budget in participating communities, and prolonged the life of the pruned elms. Dutch elm disease, caused by the fungus Ceratocystls ulmi, is causing serious mortality of American elms (Ulmus americana) and red elms (U. rubra) in Minnesota. Many communities have maintained Dutch elm disease control programs, even when state funding decreased and was finally eliminated. The primary objective of most disease management programs is sanitation: removal of diseased trees and bark intact elm wood. Root graft disruption, elm injection and therapeutic pruning are considered secondary control measures. Hart (1970) reported that therapeutic pruning was costly and had only a 23% success rate. Several more recent studies have related the degree of symptoms and of vascular discoloration to the likelihood of survival after pruning (Campana, 1975, and Gregory and Allison, 1979). We now know that pruning can save more than 60% of pruned elms if more than 5 feet of wood can be removed between the main stem and the last detectable vascular discoloration. Himelick and Ceplecha (1976), reported that operational therapeutic pruning saved 62% of pruned trees, resulting in considerable cost savings. They attributed success to prompt pruning. In spite of these results, few Dutch elm disease programs include pruning as a component. Urban foresters in Minnesota have considered pruning ineffective, too expensive, or too difficult to administer. This study was done to evaluate the effectiveness of therapeutic pruning under operational constraints, in terms of dollar cost and the number of trees saved. Materials and Methods The communities participating in these studies were Falcon Heights, and Inver Grove Heights, Minnesota. In 1 983 Falcon Heights lost 68 of its 840 elms to Dutch elm disease; Inver Grove Heights lost 56 of its 2600 elms. Candidate trees were identified during routine biweekly inspections for Dutch elm disease. Trees were considered pruneable if, based on foliar symptoms, the fungus was not yet thought to be in the main stem of the tree. The locations of candidate trees were given to tree removal firms under contract to the appropriate city. Contractors were selected by lowest bids for a projected number of hours of pruning, and a projected number of trees to be removed. Trees were pruned as soon as possible after detection, often within 24 hours. Contractors were instructed to remove the diseased branch at the main stem of the tree. When possible, a sample was collected for confirmation of the presence of the Dutch elm disease fungus, and the distance between the main stem and the vascular discoloration was measured. All samples processed were positive for Dutch elm disease. Pruning cost was determined from contractor billing for pruning. Removal cost was determined from the contractors bid price, based on tree diameter at 1.4m (dbh). Pruned trees were examined during routine inspections, and at least once annually for the purpose of this study. Results During this study, 32 trees were pruned (Table 1). Of these, 25 are still alive. Pruning costs averaged $33.60 per tree, and removal cost for 1. Paper number 14,287, Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, MN 55108. 248 Baker and French: DED Therapeutic Pruning the 7 trees that died after pruning was $781 (Table 2). The removal cost of the trees that had been pruned would have been $5351. The average distance between the main stem and the last detectable stain was 8.5 feet. One tree with 5 feet of stain-free wood did not survive. Discussion This study reports the effectiveness of a pruning component within an operational Dutch elm disease control program. The trees pruned were selected during routine inspections for Dutch elm disease. Pruning was done by commercial contractors, at competitively bid prices. Under these constraints, it was usually possible to prune trees within 24 hours of detection. By pruning trees, 25 trees were saved, and $3495 that would have been spent on tree removal was saved. Although several more trees may die, with an average removal cost of $167 each, and savings may decrease somewhat, pruning will still provide substantial savings. Our study, and the results are quite similar to those reported by Himelick and Ceplecha (1976). Several urban foresters have suggested that administrative costs of a pruning program are much greater. This is in fact the opposite. By securing contractor services based on hourly rates for pruning and dbh class for tree removal, there is Table 1. Effectiveness of pruning to remove Dutch elm disease from American elms. Year Number of trees pruned