Relative In Vitro Wood Decay Resistance of Sapwood from Landscape Trees of Southern Temperate Regions

The development of wood decay caused by 12 major root-rot and trunk-rot fungi was investigated in vitro with sapwood extracted from nine ornamental and landscape hardwood and conifer species native to southern temperate regions of North America, Europe, and the lower Mississippi Delta. Wood decay rates based on dry weight loss for 108 host tree–wood decay fungi combinations were compared at 21 8C over 1-year and 2-year incubation periods in the absence of tree-resistance mechanisms. Strains of Armillaria mellea, Ganoderma lucidum, and Heterobasidion annosum exhibited the highest decay potential in most tree species tested. The order of fungi causing the greatest decay varied over time as a result of temporal changes in decay-rate curves. Relative wood durability or resistance to decay generally was greater in gymnosperm than in angiosperm wood types. Quercus nuttallii, Fraxinus pennsylvanica, and Quercus lyrata sustained the highest levels of decay by all fungi. Northern white cedar (Thuja occidentalis) sapwood was most resistant to decay by all rot-fungi tested, sustaining only limited weight loss after 1 and 2 years of decay, although sapwood of Pinus taeda, Liquidambar styraciflua, and Platanus occidentalis had relatively low levels of decay after 2 years. These results in combination with data from portable decay-detection devices provide useful information for the management of tree breakages or failures resulting from wood decay fungi in hazardous landscape trees. Some potential landscaping applications for tree evaluations, risk assessments, and selections for tree-replacement plantings are discussed. Landscape trees are becoming increasingly important in urban and suburban environments as a result of the values and benefits they provide by virtue of their aesthetic nature, ability to purify ambient air, use in providing shade, and service as effective wind breaks (Dwyer et al., 1992; McPherson and Biedenbender, 1991; McPherson and Rowntree, 1993). For these reasons, urban forestry is rapidly gaining in importance relative to commercial forestry as urban trees continue to appreciate in value. A single large urban tree can add tens of thousands of dollars to urban and residential property values in terms of aesthetic (social), environmental, utility, and monetary benefits (Nowak, 1993; Nowak et al., 2002; Scott and Betters, 2000). Thus, tree mortality resulting from diseases, wood decay, and insects causes significant economic losses in many different ways. Despite the importance and valuable roles that landscape trees play in the enhancement of our quality of life, urban trees are often subjected to considerable abuse and must endure adverse and inadequate growing conditions that preclude normal development and increase susceptibility to further damage by biotic and abiotic factors. Many anthropogenic activities such as wounding, soil compaction, tree planting in adverse locations, and water pollution cause serious damage to roots and the lower trunk; retard tree growth; and open up infection courts for pathogen entry. Damage to tree roots may reduce structural stability and result in an inadequate nutrient supply to sustain a healthy crown (Jokela et al., 1996). Many adverse conditions present in the urban setting reduce tree vigor and subject trees to stresses that predispose them to attack by various pathogens, particularly woodrotting fungi that decay living sapwood. Common root-rot fungal pathogens in the southern United States such as Armillaria and Ganoderma species are often particularly damaging in cities because they compromise the stability of urban trees resulting in potentially dangerous consequences when trees fail, i.e., when limb or trunk breakages occur (Guglielmo et al., 2007). Devastating injuries to people and damage to property may result from falling trees and limbs. Wood decay is the most crucial risk factor that increases the probability that urban trees will fail, but environmental stresses, competition, anthropogenic disturbance, and the activities of other organisms are contributing factors considered in risk assessments (Ossenbruggen et al., 1986). Nevertheless, wood decay fungi are the primary and most common cause of decreases in mechanical strength of wood in standing trees (Råberg et al., 2005). Studies of wood decay development in forest and urban trees have involved many different aspects of decay processes, including detailed descriptions of effects on wood microscopic anatomy (Barnett and Bonham, 2004; Schwarze and Fink, 1998), different decay types (Greig, 1989; Luna et al., 2004; Otjen and Blanchette, 1985; Schwarze et al., 2000, 2003), possible mechanisms of infection (Despot, 1998; Schwarze and Baum, 2000; Sturrock et al., 2007), and the role of enzymes involved in degradation (Cullen and Kersten, 2004; Eichlerová et al., 2000; Hatakka, 1994, 2001; Tuor et al., 1995). Investigations of host–pathogen interactions have focused on the capability of wood decay fungi to overcome host-tree defenses (Schwarze and Baum, 2000; Schwarze and Fink, 1997) and the ability of trees to form chemical and structural barriers to restrict wood colonization by various decay fungi (Shigo and Shortle, 1979; Shortle, 1979). The most comprehensive in vitro wood decay study on decay mechanisms was done by Worral et al. (1997), who reported mean weight losses for birch (Betula alleghaniensis Britton) and loblolly pine (Pinus taeda L.) artificially inoculated with 79 wood decay fungi. Nevertheless, there is a dearth of comprehensive studies on the relative decay potential of different wood decay fungi on diverse urban tree hosts, particularly the most commonly destructive pathogenic fungi on ornamental and landscape tree species, and the relative decay resistance of wood from these species. The objectives of this study were to investigate the in vitro development of decay caused by 12 major rot-rot and trunk-rot fungi in sapwood extracted from nine ornamental and landscape tree species native to southern temperate forests in the lower Mississippi alluvial valley (Mississippi Delta region), to compare the relative wood decay potential and host specificity of damage associated with these wood-rotting fungi, and to determine the relative in vitro susceptibility or resistance of sapwood from each tree species to decay over 1-year and 2-year incubation periods in the absence of active tree-resistance mechanisms. This information is prerequisite for the development of improved management guidelines for city foresters, urban arborists, and other tree-care specialists to facilitate urban forestry planning decisions and strategies. Urban tree management activities involve tree plantings, Received for publication 20 Oct. 2009. Accepted for publication 23 Jan. 2010. This research was funded by Comune di Milano, Settore Tecnico Arredo Urbano e Verde, and from Demetra Società Cooperativa Sociale ONLUS, Besana Brianza, MI, Italy. We give special thanks to Luigi Bonanomi, Gabriele Villa, Charisse Oberle, and Luca Maccabelli for their assistance in this study. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 45(3) MARCH 2010 401 PLANT PATHOLOGY