Arboriculture & Urban Forestry 33(2): March 2007 133 ments, Biggs (1990) demonstrated that wound dressings, de- pending on type, can either hasten or retard suberin and lignin formation and infection by Leucostoma spp. in wounded Prunus spp. A number of wound dressings have been shown to inhibit the growth of Ceratocystis fimbriata f. platani (May and Palmer 1959; David and Peterson 1973). In contrast to any benefits, several studies have also shown that wound dressings can be phytotoxic or nonbeneficial to trees (Neely 1970; Wilson and Shigo 1973; Shigo and Wilson 1977; Mercer 1979; Shigo and Shortle 1983; Hudler and Jensen-Tracy 2002). These studies were directed toward the use of wound dressings to prevent the ingress of decay fungi in trees. Neely (1970) showed that petrolatum, latex paint, shellac, and asphalt compounds do not promote wound clo- sure. Shigo and Shortle (1983) tested several wound treat- ments in long-term experiments. They found that the treat- ments did not inhibit wood discoloration and that some wound dressings could harm trees. As a result, Shigo and Shortle (1983) strongly recommended that arborists discon- tinue the use of wound dressings. As a result of the requirement of fresh wounds for infection by the oak wilt pathogen, wound treatments have long been a potential control measure of interest to researchers and prac- titioners (Drake et al. 1958; Gibbs 1980). Juzwik et al. (1985) wounded over 5,000 trees to study natural infection of oaks with C. fagacearum. In Minnesota, infection from wounding occurred from May to mid-June. Numerous wounds were treated with a variety of commercially available wound dress- ings including Leonard’s Tree Compound (A.M. Leonard and Sons, Inc., Piqua, OH), Cabots Tree Healing Paint (Samuel Cabot Mfg., Inc., Boston, MA), and Treekote (Walter C. Clark and Son, Orange, CT). Of the 322 wounded trees treated with wound dressings, none of the trees became in- fected nor did any unwounded trees contract the disease. Infection rates on untreated trees in different plots varied from 3% to 29% depending on location and time of year the tree was wounded. As a result of these and other related studies, most educational materials developed by state and federal agencies include wound paints as part of comprehen- sive oak wilt control programs (Appel et al. 1995; French and Juzwik 1999; O’Brien et al. 2000; Bonello 2001; Cummings- Carlson and Martin 2005). Wound closure has also been implicated as important to the status of oak wilt infection courts. Rates of closure have been found to be associated with how pruning cuts are made in relation to branch collars and branch attachments (Shigo 1984, 1985). In this model, branches stay separate from the parent stem from which they arise. As branches and stems increase in girth, a branch bark ridge forms at the top of the junction of the branch and stem. Many times, there will be a swollen ring of tissue at the bottom of the branch, indicating the branch collar. Proper pruning cuts are those that involve cutting outside the branch bark ridge and as close to the branch collar as possible without damaging the branch collar (Shigo 1984). In addition, branches have branch protection zones (BPZ) that limit infection in the parent stem after branch injury or removal by forming pathogen-resistant com- pounds within the branch tissue (Von Aufess 1975, 1984; Green et al. 1981; Shigo 1985). Improper, or flush, cuts dam- age the tissue of the parent stem and therefore bypass the inherent physical and chemical barriers present in the branch. Several studies have demonstrated that pruning cuts through branch collars result in increased discoloration in the parent stem outside of the branch tissue (Neely 1970; Solomon and Shigo 1976; Shigo 1984, 1985; Eisner et al. 2002a). These studies have convinced many arborists to abandon the use of pruning paints when pruning oaks in areas infested with oak wilt in favor of relying on the anatomic advantages of a proper pruning cut. An important point to note is that these studies of wound closure involve branches. Not all stem attachments comprise true branches with BPZs and branch collars. True branches, as opposed to codominant stems, can be difficult to define. Eisner et al. (2002a) looked at three different criteria to de- termine how well branches compartmentalize discoloration associated with pruning cuts on live oaks (C. virginiana). These were visible collars, pith connections between the branch and parent stems, and the aspect ratio (branch diam- eter to trunk diameter). Their research found that branches with visible branch collars had significantly less discolored wood after pruning. There was significantly less discoloration in pruning cuts where the piths of the branch and the parent stem did not connect. These morphologic features and their influence on discoloration support findings by Shigo (1985). Most branches (89%) with visible branch collars did not have connected piths. The extent of discoloration increased as as- pect ratios increased to 1 (codominant stems). In addition, branches with lower aspect ratios had fewer pith connections. Pruning branches with aspect ratios lower than the predicted ratio (0.39) resulted in relatively small amounts of discolored wood. In a related study, Eisner et al. (2002b) demonstrated that branches with lower aspect ratios, no pith connections, and visible branch collars had lower conductivity ratios, which means that these features are associated with restricted movement of water from the parent stem to the branch. They found that lower conductivity ratios result in a decrease in discoloration, and this restriction in water flow may also re- duce the infection potential of C. fagacearum. Studies mea- suring the impact of proper pruning to limit disease transmis- sion must ensure that true branches, rather than codominant stems that do not have branch collars or BPZs, are used to accurately assess the benefits of these inherent morphologic features. To address these concerns, a study was developed to de- termine the accuracy of current recommendations for apply- ing pruning paints to pruning wounds on susceptible live oaks ©2007 International Society of Arboriculture
March 2007
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