©2023 International Society of Arboriculture 12 in the deeper xylem coupled with relatively lower rates of growth and transpiration in such a tree would result in poor distribution of the fungicide within the tree. Despite the encouraging results of field inocula- tion trials, the findings in both trials that bioassays of samples collected within the first 16 weeks of inocu- lation from PZOL + HL trees were less likely to inhibit H. lauricola growth than those collected more than 1 year later is difficult to explain. Mayfield et al. (2008a) reported very little change in retention within stem samples over time following propiconazole injections in redbay, with H. lauricola inhibition observed in 100% of samples (n = 12) collected at 4.5 months, and in 92% of samples collected at 7.5 months post-inoculation with H. lauricola. It is unknown whether our results were related to slowed movement of the fungicide due to the trees’ response (creation of tyloses, etc.) to H. lauricola, an artifact of sampling intensity and location, or some other unknown factors. These vagaries reinforce the guidance that preven- tive treatments should be applied in advance of the spring dispersal flights of redbay ambrosia beetles to allow for sufficient product distribution within trees, and trees should be retreated annually to ensure ade- quate protection in sassafras against LWD. Future research should be conducted on the value and poten- tial for therapeutic treatment of LWD-infected sassa- fras, sampling intensity and methods for the purpose of product retention analyses, and on the duration of biologically active propiconazole concentrations in sassafras as it relates to tree protection. LITERATURE CITED Appel DN, Kurdyla T. 1992. Intravascular injection with propi- conazole in live oak for oak wilt control. Plant Disease. 76:1120-1124. https://doi.org/10.1094/PD-76-1120 Bates CA, Fraedrich SW, Harrington TC, Cameron RS, Menard RD, Best GS. 2013. First report of laurel wilt, caused by Raf- faelea lauricola, on sassafras (Sassafras albidum) in Alabama. Plant Disease. 97(5):688. https://doi.org/10.1094/PDIS-09-12 -0866-PDN Blaedow RA. 2009. Use of the systemic fungicide propiconazole for oak wilt management: An assessment of uncharacterized host-pathogen-fungicide interactions [PhD dissertation]. Saint Paul (MN, USA): University of Minnesota. 131 p. https://hdl.handle.net/11299/50778 Brar GS, Capinera JL, Kendra PE, McLean S, Peña JE. 2013. Life cycle, development, and culture of Xyleborus glabratus (Coleoptera: Curculionidae: Scolytinae). Florida Entomolo- gist. 96(3):1158-1167. https://doi.org/10.1653/024.096.0357 Cameron RS, Hanula J, Fraedrich SW, Bates C. 2015. Progression and impact of laurel wilt disease within redbay and sassafras propiconazole (using Alamo® [Syngenta International AG, Basel, Switzerland], at the recommended high rate of 1.2-g active ingredient per cm DBH; a rate twice that recommended on the Propizol® label used in this evaluation for LWD prevention) prevented LWD symptoms in more than one-third of the crown for at least 30 weeks post-inoculation with H. lauri- cola but did not monitor treated trees beyond this point. Protection of Quercus sp. and Ulmus sp. with propiconazole may last 2 years or more against oak wilt (Wilson and Lester 1996; Ward et al. 2004; Eggers et al. 2005) and possibly up to 2 years against Dutch elm disease (Haugen and Stennes 1999; but see Ploetz et al. 2011). In each of our trials, results suggest propi- conazole at the treatment rate of 0.61 g/cm DBH may prevent expression of disease symptoms in ≥ 50% of the crown beyond 16 weeks in some trees, but further study is necessary to determine when such results can be consistently expected. Our results suggest the use of micro-infusion techniques may provide more con- sistent results beyond this point. The higher rates of H. lauricola inhibition observed in bioassays and higher/longer survival times of PZOL + HL trees in the 2020–2021 TREE I.V. Micro-InfusionTM system trial may be due to better product translocation by trees using this treatment method. Micro-infusion of propiconazole in Ulmus sp. results in better transloca- tion and distribution of propiconazole than that observed following micro-injection (Haugen and Stennes 1999; Stipes 2017). It is also likely this method resulted in less product lost to compromised xylem within the study trees used in the micro-infusion trial, since the treatment holes drilled for this method were not as deep as those used in trees treated using the QUIK-jet® Micro-InjectionTM system. If this method is used and internal decay is suspected, we recom- mend care in placing product injection sites at the tree base or by increasing the number of treatment holes (but decreasing the depth) to avoid internal decay. Another advantage to delivering the required dose in a shallower hole is that product is more likely to be located in more physiologically active layers of the xylem. In examinations of sanded cross sections of PZOL + HL trees treated with the micro-injection system in 2019–2020, the smallest tree in this group included in the analyses (DBH 14.9 cm) was 46 years old (at injection plane 15 to 30 cm above ground) and had 16 growth rings in the outer 2.5 cm of xylem. It is likely that a lower rate of translocation of propiconazole Johnson et al: Propiconazole for Prevention of Laurel Wilt Disease in Sassafras
January 2023
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