322 Mayfield et al.: Propiconazole for Laurel Wilt Disease Table 4. Results of bioassay testing for fungicide retention (indicated by inhibition of Raffaelea spp. growth) in redbay stem plugs and branch discs plated on spore-seeded, cycloheximide streptomycin malt agar (CSMA). Sample Tree injection treatment Months since fungicide injection Stem plugs Propiconazole 7.5 Propiconazole 4.5 None N/A Branch discs Propiconazole 7.5 Propiconazole 4.5 None Mean and range are based only on data from trees in which inhibition of mycelial growth was exhibited. Within columns, means followed by the same letter are not significantly different (P > 0.05) as determined by a two-sample t-test. Four stem plugs and six branch samples were collected per tree. N/A not applicable. z Uptake of fungicide solution by redbay trees in this study was relatively slow compared with the authors’ experience with pre- ventive root-flare injections on ring-porous species such as elms and oaks. An accurate uptake record was maintained for 12 redbay trees and indicated that the uptake process lasted an av- erage of 53 min/L (0.26 gal) of solution, or approximately 156 min per tree (range, 30 to 360 min). The pronounced drought conditions at the time of injection may have contributed to the slow rate of uptake. At the start of injections in late March, the study site had received only 0.3 cm (0.12 in) of rain in the preceding 25 days, and the average monthly Palmer Drought Severity Index in northern Florida from July 2006 to June 2007 remained continuously negative and averaged –3.78 through the year leading up to the completion of the injections (National Climatic Data Center 2008). Thus, uptake times on redbay may have been better under conditions of more normal rainfall. The diffuse–porous nature of redbay xylem and the twisting growth habit of many redbay trunks may also have contributed to the slow uptake times relative to ring-porous and typically straighter-trunked tree species such as elms and oaks. The rela- tively slow rate of uptake may have also limited the distribution of propiconazole into the small branches of the crown, which exhibited rather limited fungicide retention. The macroinfusion technique used in this study delivers a relatively large volume of pressurized fungicide solution into the xylem through holes drilled in the root flare. Macroinfusion provides more adequate and even distribution of fungicide into the canopy of treated trees than do alternative delivery methods such as microinjection (Haugen and Stennes 1999). In general, injection methods result in bark and sapwood injury when drill- ing the injection holes; such wounds can limit the number of times injections can be repeated and may cause sapwood discol- oration or decay (Haugen and Stennes 1999). The authors are unaware, however, of less injurious delivery methods that would be as effective for treating wilt disease in mature trees. Propiconazole has been used successfully in therapeutic (i.e., curative) treatments for oak wilt in red oaks exhibiting less than 25% crown wilt symptoms (Ward et al. 2005) and in combina- tion with pruning in white oaks with 5% to 45% crown wilt symptoms (Eggers et al. 2005). Although this study did not evaluate therapeutic treatments, we suspect that such treatments on redbays would have to be implemented very early during laurel wilt symptom development as a result of the extremely rapid nature of disease progression once trees begin showing signs of wilt. Because wounding increases the attractiveness of ©2008 International Society of Arboriculture the redbay trees to the redbay ambrosia beetle (Hanula et al., in press), the use of pruning in combination with injection treat- ments is probably not an effective option for treating laurel wilt. As a result of the devastating effects of laurel wilt on popu- lations of mature redbays and the rapid rate of spread of this disease geographically (estimated to be at least 32 km [19.2 mi] per year), we consider it important to report these results now rather than to wait for multiple years of efficacy data. In this study, propiconazole was toxic to the pathogen at low concen- trations in vitro, prevented development of wilt symptoms in inoculated redbays, and was retained in the sapwood (primarily in the trunk) for at least 7.5 months after injection. Propiconazole injections could be potentially useful to protect large, high-value redbay shade trees in parks, residential neighborhood yards, and historic sites that are currently succumbing to this disease and for which no other preventive management options currently exist. Although the results in this study demonstrate that the systemic fungicide propiconazole could be useful for the protection of redbay trees from laurel wilt, additional research is clearly needed to evaluate delivery methods, rate and season of appli- cation, and the effect of various levels of inoculation pressure (i.e., number and frequency of inoculation events or beetle at- tacks) on the long-term efficacy of propiconazole and other fun- gicides. Acknowledgments. Steve Fraedrich (USDA Forest Service) is grate- fully acknowledged for providing isolates of the laurel wilt pathogen, professional advice, and informal review of the manuscript in draft form. We also thank Peter Scalco, Heath Alboher, and Clifford Joyce (Florida Department of Environmental Conservation, Fort Clinch State Park) for permission to use the study site and field assistance. We are grateful to Marc Hughes and David Nolletti (University of Florida) for laboratory pathogen isolations and to Carol Scoates (Florida DACS Division of Plant Industry) for preparation of media and other laboratory assistance. This project was funded by the USDA Forest Service–Forest Health Protection–Southern Region and supported by materials and equipment donated by Rainbow Treecare Scientific Advancement. LITERATURE CITED Appel, D.N., and T. Kurdyla. 1992. Intravascular injection with propi- conazole in live oak for oak wilt control. Plant Disease 76: 1120–1124. Difco Laboratories. 1953. Difco Manual of Dehydrated Culture Media and Reagents for Microbial and Clinical Laboratory Procedures. 9th ed. Difco Laboratories, Inc., Detroit, MI. 350 pp. N/A No. of trees 12 12 No. of trees with samples exhibiting inhibition 11 55 50 5 54 50 Percentage of samples per tree exhibiting inhibitionz Mean 84 a 95 a N/A 33 a 29 a N/A Range 25–100 75–100 N/A 17–83 17–50 N/A Percentage of samples per tree exhibiting inhibition zone in the agarz Mean 27 a 80 b N/A 0 0 N/A Range 0–75 50–100 N/A 0 0 N/A
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