132 Camilli et al.: Pruning Cuts and Wound Dressings for Oak Wilt Arboriculture & Urban Forestry 2007. 33(2):132–139. Studies on Pruning Cuts and Wound Dressings for Oak Wilt Control Kim Camilli, David N. Appel, and W. Todd Watson Abstract. Ceratocystis fagacearum causes the destructive tree disease called oak wilt. One means of pathogen spread is by insect vectors (Nitidulidae) that transmit spores into fresh wounds on healthy trees. Experiments were conducted in central Texas on native live oaks (Quercus fusiformis) to test pruning methods and paints on disease development. Three treatment combinations were tested on 30 trees (10 trees/treatment): flush cut unpainted, flush cut painted, and unpainted pruning cuts made according to the Shigo method. Unpainted puncture wounds were made on the lower trunks of an additional 20 trees as controls. C. fagacearum spores were applied to the pruning cuts and half of the puncture wounds (positive controls) after treatment, whereas the other half of the punctures received distilled water as negative controls. Oak wilt symptoms first appeared in the flush cut unpainted treatment 31 days after inoculation. Infection rates, in decreasing order, were; positive control (70%), flush cut unpainted (60%), Shigo pruning method (40%), flush cut painted (20%), and negative control (10%). Pruning wounds, regardless of method, were effective infection courts for the oak wilt pathogen. Fewer trees became infected when pruning cuts were painted, but differences among infection rates for pruning cuts were not statistically significant. Tree diameters and stem aspect ratio had no bearing on infection rates. The Shigo method is recognized as a superior method for pruning, but there is no reason to change current recommendations to paint fresh wounds on susceptible oaks in high-hazard oak wilt areas. Key Words. Branch protection zone; natural target pruning; nitidulid beetles; oak wilt; pruning paints; Shigo. Ceratocystis fagacearum (Bretz) Hunt, the pathogen respon- sible for the highly destructive oak wilt disease, spreads in two ways (Gibbs and French 1980; MacDonald and Hindal 1981). Over relatively short distances, spores of the fungus are drawn from diseased to healthy trees through root con- nections. These connections arise from grafting or from com- mon root systems formed during vegetative propagation by root sprouts. Because root connections play an important role in oak mortality in Texas, U.S., considerable resources are expended on control in live oak (Quercus fusiformis Small) to prevent root transmission of the pathogen (Appel 2001). The second means of spread for C. fagacearum is over longer distances by sap-feeding beetles (Coleoptera: Nitidulidae). Inoculum sources called fungal mats form on diseased red oaks (gen. Quercus sub gen. Erythrobalanus) and provide spores for nitidulid beetles to spread to fresh wounds on healthy oaks (Norris 1953; Curl 1955; Jewell 1955; Rexrode 1976; Juzwik et al. 1985; Appel et al. 1987, 1990; Ambourn et al. 2005). From an epidemiologic perspective, the initiation of new disease centers by nititulids is a critical stage in the oak wilt disease cycle. This means of spread is also a con- troversial issue for arborists throughout the range of oak wilt because pruning wounds are implicated as important infec- ©2007 International Society of Arboriculture tion courts for nitidulids in the oak wilt syndrome. Much of the controversy involves the recommended oak wilt control measure of applying wound dressings to prevent nitidulids from inoculating pruning cuts on susceptible trees. Many other studies have also shown that wound dressings have some benefits when used to prevent infection from the fungal spores of various pathogens (May and Palmer 1959; Luepschen and Rohrbach 1969; Gupta and Agarwala 1972; Davis and Peterson 1973; Mercer 1982; Juzwik et al. 1985; Biggs 1990). Luepschen and Rohrbach (1969) demonstrated that wound susceptibility of Prunus spp. to Leucostoma spp., the pathogen causing a perennial canker disease of stone fruits, varied by time of year and that the application of shellac was beneficial in reducing infection. Similar benefits of pruning paints to control infection of Malus spp. with Cylindrocarpon mali, another canker disease of apples, have also been demonstrated (Gupta and Agarwala 1972). Not all studies, however, regarding wound dressings and their effect on disease control have been conclusive. Biggs (1990) found that wound susceptibility to infection decreases with increas- ing suberin and lignin formation after wounding. This varies considerably based on temperature, soil moisture, and spe- cies. After testing the effects of several postwounding treat-
March 2007
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